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[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >

<chapter id='extendpoky'>

<title>Common Tasks</title>
    <para>
        This chapter describes standard tasks such as adding new
        software packages, extending or customizing images, and porting work to
        new hardware (adding a new machine).
        The chapter also describes how to combine multiple
        versions of library files into a single image, how to handle a package name alias, and
        gives advice about how to make changes to the Yocto Project to achieve the best results.
    </para>

    <section id="understanding-and-creating-layers">
        <title>Understanding and Creating Layers</title>

        <para>
            The OpenEmbedded build system supports organizing <link linkend='metadata'>metadata</link>
            into multiple layers.
            Layers allow you to isolate different types of customizations from each other.
            You might find it tempting to keep everything in one layer when working on a single project.
            However, the more modular you organize your metadata, the easier it is to cope with future changes.
        </para>

        <para>
            To illustrate how layers are used to keep things modular, consider machine customizations.
            These types of customizations typically reside in a BSP Layer.
            Furthermore, the machine customizations should be isolated from recipes and metadata that support
            a new GUI environment, for example.
            This situation gives you a couple of layers: one for the machine configurations, and one for the
            GUI environment.
            It is important to understand, however, that the BSP layer can still make machine-specific
            additions to recipes within the GUI environment layer without polluting the GUI layer itself
            with those machine-specific changes.
            You can accomplish this through a recipe that is a BitBake append
            (<filename>.bbappend</filename>) file, which is described later in this section.
        </para>

        <para>
        </para>

        <section id='yocto-project-layers'>
            <title>Layers</title>

            <para>
                The Source Directory contains several layers right out of the
                box.
                You can easily identify layers that ship with a
                Yocto Project release in the Source Directory by their
                folder names.
                Folders that are layers begin with the string
                <filename>meta</filename>.
                <note>
                    It is not a requirement that a layer begins with the
                    string <filename>meta</filename>.
                </note>
                For example, when you set up the <link linkend='source-directory'>Source Directory</link>
                structure, you will see several layers: <filename>meta</filename>,
                <filename>meta-hob</filename>, <filename>meta-skeleton</filename>,
                <filename>meta-yocto</filename>, and <filename>meta-yocto-bsp</filename>.
                Each of these folders is a layer.
            </para>

            <para>
                Furthermore, if you set up a local copy of the <filename>meta-intel</filename> Git repository
                and then explore that folder, you will discover many BSP layers within the
                <filename>meta-intel</filename> layer.
                For more information on BSP layers, see the
                "<ulink url='&YOCTO_DOCS_BSP_URL;#bsp-layers'>BSP Layers</ulink>"
                section in the Yocto Project Board Support Package (BSP) Developer's Guide.
            </para>
        </section>

        <section id='creating-your-own-layer'>
            <title>Creating Your Own Layer</title>

            <para>
                It is very easy to create your own layer to use with the OpenEmbedded build system.
                Follow these general steps to create your layer:
                <orderedlist>
                    <listitem><para><emphasis>Check Existing Layers:</emphasis> Before creating a new layer,
                        you should be sure someone has not already created a layer containing the metadata
                        you need.
                        You can see the
                        <ulink url='&OE_HOME_URL;/wiki/LayerIndex'><filename>LayerIndex</filename></ulink>
                        for a list of layers from the OpenEmbedded community that can be used in the
                        Yocto Project.</para></listitem>
                    <listitem><para><emphasis>Create a Directory:</emphasis>
                        Create the directory for your layer.
                        While not strictly required, prepend the name of the
                        folder with the string <filename>meta</filename>.
                        For example:
                        <literallayout class='monospaced'>
     meta-mylayer
     meta-GUI_xyz
     meta-mymachine
                        </literallayout></para></listitem>
                    <listitem><para><emphasis>Create a Layer Configuration File:</emphasis> Inside your new
                       layer folder, you need to create a <filename>conf/layer.conf</filename> file.
                       It is easiest to take an existing layer configuration file and copy that to your
                       layer's <filename>conf</filename> directory and then modify the file as needed.</para>
                       <para>The <filename>meta-yocto/conf/layer.conf</filename> file demonstrates the
                       required syntax:
                       <literallayout class='monospaced'>
     # We have a conf and classes directory, add to BBPATH
     BBPATH := "${LAYERDIR}:${BBPATH}"

     # We have recipes-* directories, add to BBFILES
     BBFILES := "${BBFILES} ${LAYERDIR}/recipes-*/*/*.bb \
                 ${LAYERDIR}/recipes-*/*/*.bbappend"

     BBFILE_COLLECTIONS += "yocto"
     BBFILE_PATTERN_yocto := "^${LAYERDIR}/"
     BBFILE_PRIORITY_yocto = "5"
                        </literallayout></para>
                        <para>In the previous example, the recipes for the layers are added to
                        <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-BBFILES'>BBFILES</ulink></filename>.
                        The
                        <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-BBFILE_COLLECTIONS'>BBFILE_COLLECTIONS</ulink></filename>
                        variable is then appended with the layer name.
                        The
                        <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-BBFILE_PATTERN'>BBFILE_PATTERN</ulink></filename>
                        variable is set to a regular expression and is used to match files
                        from <filename>BBFILES</filename> into a particular layer.
                        In this case, immediate expansion of
                        <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-LAYERDIR'>LAYERDIR</ulink></filename>
                        sets <filename>BBFILE_PATTERN</filename> to the layer's path.
                        The
                        <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-BBFILE_PRIORITY'>BBFILE_PRIORITY</ulink></filename>
                        variable then assigns a priority to the layer.
                        Applying priorities is useful in situations where the same package might appear in multiple
                        layers and allows you to choose what layer should take precedence.</para>
                        <para>Note the use of the
                        <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-LAYERDIR'>LAYERDIR</ulink></filename>
                        variable with the immediate expansion operator.
                        The <filename>LAYERDIR</filename> variable expands to the directory of the current layer and
                        requires the immediate expansion operator so that BitBake does not wait to expand the variable
                        when it's parsing a different directory.</para>
                        <para>Through the use of the <filename>BBPATH</filename> variable,
                        BitBake locates <filename>.bbclass</filename> files, configuration
                        files, and files that are included with <filename>include</filename>
                        and <filename>require</filename> statements.
                        For these cases, BitBake uses the first file with the matching name found in
                        <filename>BBPATH</filename>.
                        This is similar to the way the <filename>PATH</filename> variable is used for binaries.
                        We recommend, therefore, that you use unique <filename>.bbclass</filename>
                        and configuration file names in your custom layer.</para></listitem>
                    <listitem><para><emphasis>Add Content:</emphasis> Depending on the type of layer,
                        add the content.
                        If the layer adds support for a machine, add the machine configuration in
                        a <filename>conf/machine/</filename> file within the layer.
                        If the layer adds distro policy, add the distro configuration in a
                        <filename>conf/distro/</filename> file with the layer.
                        If the layer introduces new recipes, put the recipes you need in
                        <filename>recipes-*</filename> subdirectories within the layer.
                        <note>In order to be compliant with the Yocto Project, a layer must contain
                            a <ulink url='&YOCTO_DOCS_BSP_URL;#bsp-filelayout-readme'>README file.</ulink>
                            </note></para></listitem>
                </orderedlist>
            </para>

            <para>
                To create layers that are easier to maintain, you should consider the following:
                <itemizedlist>
                    <listitem><para>Avoid "overlaying" entire recipes from other layers in your
                        configuration.
                        In other words, don't copy an entire recipe into your layer and then modify it.
                        Use <filename>.bbappend</filename> files to override the parts of the
                        recipe you need to modify.</para></listitem>
                    <listitem><para>Avoid duplicating include files.
                        Use <filename>.bbappend</filename> files for each recipe that uses an include
                        file.
                        Or, if you are introducing a new recipe that requires the included file, use the
                        path relative to the original layer directory to refer to the file.
                        For example, use <filename>require recipes-core/somepackage/somefile.inc</filename>
                        instead of <filename>require somefile.inc</filename>.
                        If you're finding you have to overlay the include file, it could indicate a
                        deficiency in the include file in the layer to which it originally belongs.
                        If this is the case, you need to address that deficiency instead of overlaying
                        the include file.
                        For example, consider how Qt 4 database support plug-ins are configured.
                        The Source Directory does not have
                        MySQL or PostgreSQL, however OpenEmbedded's
                        layer <filename>meta-oe</filename> does.
                        Consequently, <filename>meta-oe</filename> uses <filename>.bbappend</filename>
                        files to modify the <filename>QT_SQL_DRIVER_FLAGS</filename> variable to enable
                        the appropriate plugins.
                        This variable was added to the <filename>qt4.inc</filename> include file in
                        the Source Directory specifically to allow the <filename>meta-oe</filename> layer
                        to be able to control which plugins are built.</para></listitem>
                </itemizedlist>
            </para>

            <para>
                We also recommend the following:
                <itemizedlist>
                    <listitem><para>Store custom layers in a Git repository that uses the
                        <filename>meta-&lt;layer_name&gt;</filename> format.</para></listitem>
                    <listitem><para>Clone the repository alongside other <filename>meta</filename>
                        directories in the
                        <link linkend='source-directory'>Source Directory</link>.</para></listitem>
                 </itemizedlist>
                 Following these recommendations keeps your Source Directory and
                 its configuration entirely inside the Yocto Project's core base.
            </para>
        </section>

        <section id='enabling-your-layer'>
            <title>Enabling Your Layer</title>

            <para>
                Before the OpenEmbedded build system can use your new layer, you need to enable it.
                To enable your layer, simply add your layer's path to the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-BBLAYERS'>BBLAYERS</ulink></filename>
                variable in your <filename>conf/bblayers.conf</filename> file, which is found in the
                <link linkend='build-directory'>Build Directory</link>.
                The following example shows how to enable a layer named <filename>meta-mylayer</filename>:
                <literallayout class='monospaced'>
     LCONF_VERSION = "6"

     BBPATH = "${TOPDIR}"
     BBFILES ?= ""

     BBLAYERS ?= " \
       $HOME/poky/meta \
       $HOME/poky/meta-yocto \
       $HOME/poky/meta-yocto-bsp \
       $HOME/poky/meta-mylayer \
       "

     BBLAYERS_NON_REMOVABLE ?= " \
       $HOME/poky/meta \
       $HOME/poky/meta-yocto \
       "
                </literallayout>
            </para>

            <para>
                BitBake parses each <filename>conf/layer.conf</filename> file as specified in the
                <filename>BBLAYERS</filename> variable within the <filename>conf/bblayers.conf</filename>
                file.
                During the processing of each <filename>conf/layer.conf</filename> file, BitBake adds the
                recipes, classes and configurations contained within the particular layer to the source
                directory.
            </para>
        </section>

        <section id='using-bbappend-files'>
            <title>Using .bbappend Files</title>

            <para>
                Recipes used to append metadata to other recipes are called BitBake append files.
                BitBake append files use the <filename>.bbappend</filename> file type suffix, while
                the corresponding recipes to which metadata is being appended use the
                <filename>.bb</filename> file type suffix.
            </para>

            <para>
                A <filename>.bbappend</filename> file allows your layer to make additions or
                changes to the content of another layer's recipe without having to copy the other
                recipe into your layer.
                Your <filename>.bbappend</filename> file resides in your layer, while the underlying
                <filename>.bb</filename> recipe file to which you are appending metadata
                resides in a different layer.
            </para>

            <para>
                Append files files must have the same name as the corresponding recipe.
                For example, the append file <filename>someapp_&DISTRO;.bbappend</filename> must
                apply to <filename>someapp_&DISTRO;.bb</filename>.
                This means the original recipe and append file names are version number specific.
                If the corresponding recipe is renamed to update to a newer version, the
                underlying <filename>.bbappend</filename> file must be renamed as well.
                During the build process, BitBake displays an error on starting if it detects a
                <filename>.bbappend</filename> file that does not have a corresponding recipe
                with a matching name.
            </para>

            <para>
                Being able to append information to an existing recipe not only avoids duplication,
                but also automatically applies recipe changes in a different layer to your layer.
                If you were copying recipes, you would have to manually merge changes as they occur.
            </para>

            <para>
                As an example, consider the main formfactor recipe and a corresponding formfactor
                append file both from the
                <link linkend='source-directory'>Source Directory</link>.
                Here is the main formfactor recipe, which is named <filename>formfactor_0.0.bb</filename> and
                located in the meta layer at <filename>meta/recipes-bsp/formfactor</filename>:
                <literallayout class='monospaced'>
     DESCRIPTION = "Device formfactor information"
     SECTION = "base"
     LICENSE = "MIT"
     LIC_FILES_CHKSUM = "file://${COREBASE}/LICENSE;md5=3f40d7994397109285ec7b81fdeb3b58 \
                         file://${COREBASE}/meta/COPYING.MIT;md5=3da9cfbcb788c80a0384361b4de20420"
     PR = "r20"

     SRC_URI = "file://config file://machconfig"
     S = "${WORKDIR}"

     PACKAGE_ARCH = "${MACHINE_ARCH}"
     INHIBIT_DEFAULT_DEPS = "1"

     do_install() {
     	# Only install file if it has a contents
             install -d ${D}${sysconfdir}/formfactor/
             install -m 0644 ${S}/config ${D}${sysconfdir}/formfactor/
     	if [ -s "${S}/machconfig" ]; then
     	        install -m 0644 ${S}/machconfig ${D}${sysconfdir}/formfactor/
     	fi
     }
                </literallayout>
                Here is the append file, which is named <filename>formfactor_0.0.bbappend</filename> and is from the
                Crown Bay BSP Layer named <filename>meta-intel/meta-crownbay</filename>.
                The file is in <filename>recipes-bsp/formfactor</filename>:
                <literallayout class='monospaced'>
     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"

     PRINC := "${@int(PRINC) + 2}"
                </literallayout>
                This example adds or overrides files in
                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                within a <filename>.bbappend</filename> by extending the path BitBake uses to search for files.
                The most reliable way to do this is by prepending the
                <filename>FILESEXTRAPATHS</filename> variable.
                For example, if you have your files in a directory that is named the same as your package
                (<ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink>),
                you can add this directory by adding the following to your <filename>.bbappend</filename> file:
                <literallayout class='monospaced'>
     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
                </literallayout>
                Using the immediate expansion assignment operator <filename>:=</filename> is important because
                of the reference to <filename>THISDIR</filename>.
                The trailing colon character is important as it ensures that items in the list remain
                colon-separated.
                <note>BitBake automatically defines the <filename>THISDIR</filename> variable.
                    You should never set this variable yourself.
                    Using <filename>_prepend</filename> ensures your path will be searched prior to other
                    paths in the final list.
                </note>
            </para>
        </section>

        <section id='prioritizing-your-layer'>
            <title>Prioritizing Your Layer</title>

            <para>
                Each layer is assigned a priority value.
                Priority values control which layer takes precedence if there are recipe files with
                the same name in multiple layers.
                For these cases, the recipe file from the layer with a higher priority number taking precedence.
                Priority values also affect the order in which multiple <filename>.bbappend</filename> files
                for the same recipe are applied.
                You can either specify the priority manually, or allow the build system to calculate it
                based on the layer's dependencies.
            </para>

            <para>
                To specify the layer's priority manually, use the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-BBFILE_PRIORITY'><filename>BBFILE_PRIORITY</filename></ulink>
                variable.
                For example:
                <literallayout class='monospaced'>
     BBFILE_PRIORITY := "1"
                </literallayout>
            </para>

            <note>
                <para>It is possible for a recipe with a lower version number
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink>
                in a layer that has a higher priority to take precedence.</para>
                <para>Also, the layer priority does not currently affect the precedence order of
                <filename>.conf</filename> or <filename>.bbclass</filename> files.
                Future versions of BitBake might address this.</para>
            </note>
        </section>

        <section id='managing-layers'>
            <title>Managing Layers</title>

            <para>
                You can use the BitBake layer management tool to provide a view into the structure of
                recipes across a multi-layer project.
                Being able to generate output that reports on configured layers with their paths and
                priorities and on <filename>.bbappend</filename> files and their applicable recipes
                can help to reveal potential problems.
            </para>

            <para>
                Use the following form when running the layer management tool.
                <literallayout class='monospaced'>
     $ bitbake-layers &lt;command&gt; [arguments]
                </literallayout>
                The following list describes the available commands:
                <itemizedlist>
                    <listitem><para><filename><emphasis>help:</emphasis></filename>
                        Displays general help or help on a specified command.</para></listitem>
                    <listitem><para><filename><emphasis>show-layers:</emphasis></filename>
                        Show the current configured layers.</para></listitem>
                    <listitem><para><filename><emphasis>show-recipes:</emphasis></filename>
                        Lists available recipes and the layers that provide them.
                        </para></listitem>
                    <listitem><para><filename><emphasis>show-overlayed:</emphasis></filename>
                        Lists overlayed recipes.
                        A recipe is overlayed when a recipe with the same name exists in another layer
                        that has a higher layer priority.
                        </para></listitem>
                    <listitem><para><filename><emphasis>show-appends:</emphasis></filename>
                        Lists <filename>.bbappend</filename> files and the recipe files to which
                        they apply.</para></listitem>
                    <listitem><para><filename><emphasis>flatten:</emphasis></filename>
                        Flattens the layer configuration into a separate output directory.
                        Flattening your layer configuration builds a "flattened" directory that contains
                        the contents of all layers, with any overlayed recipes removed and any
                        <filename>.bbappend</filename> files appended to the corresponding recipes.
                        You might have to perform some manual cleanup of the flattened layer as follows:
                        <itemizedlist>
                            <listitem><para>Non-recipe files (such as patches) are overwritten.
                                The flatten command shows a warning for these files.</para></listitem>
                            <listitem><para>Anything beyond the normal layer setup has been added to
                                the <filename>layer.conf</filename> file.
                                Only the lowest priority layer's <filename>layer.conf</filename> is used.
                                </para></listitem>
                            <listitem><para>Overridden and appended items from <filename>.bbappend</filename>
                                files need to be cleaned up.
                                The contents of each <filename>.bbappend</filename> end up in the
                                flattened recipe.
                                However, if there are appended or changed variable values, you need to tidy
                                these up yourself.
                                Consider the following example.
                                Here, the <filename>bitbake-layers</filename> command adds the line
                                <filename>#### bbappended ...</filename> so that you know where the following
                                lines originate:
                                <literallayout class='monospaced'>
     ...
     DESCRIPTION = "A useful utility"
     ...
     EXTRA_OECONF = "--enable-something"
     ...

     #### bbappended from meta-anotherlayer ####

     DESCRIPTION = "Customized utility"
     EXTRA_OECONF += "--enable-somethingelse"
                                </literallayout>
                                Ideally, you would tidy up these utilities as follows:
                                <literallayout class='monospaced'>
     ...
     DESCRIPTION = "Customized utility"
     ...
     EXTRA_OECONF = "--enable-something --enable-somethingelse"
     ...
                                </literallayout></para></listitem>
                        </itemizedlist></para></listitem>
                </itemizedlist>
            </para>
        </section>
    </section>

    <section id='usingpoky-extend-customimage'>
        <title>Customizing Images</title>

        <para>
            You can customize images to satisfy particular requirements.
            This section describes several methods and provides guidelines for each.
        </para>

        <section id='usingpoky-extend-customimage-custombb'>
            <title>Customizing Images Using Custom .bb Files</title>

            <para>
                One way to get additional software into an image is to create a custom image.
                The following example shows the form for the two lines you need:
                <literallayout class='monospaced'>
     IMAGE_INSTALL = "packagegroup-core-x11-base package1 package2"

     inherit core-image
                </literallayout>
            </para>

            <para>
                By creating a custom image, a developer has total control
                over the contents of the image.
                It is important to use the correct names of packages in the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_INSTALL'>IMAGE_INSTALL</ulink></filename>
                variable.
                You must use the OpenEmbedded notation and not the Debian notation for the names
                (e.g. <filename>eglibc-dev</filename> instead of <filename>libc6-dev</filename>).
            </para>

            <para>
                The other method for creating a custom image is to base it on an existing image.
                For example, if you want to create an image based on <filename>core-image-sato</filename>
                but add the additional package <filename>strace</filename> to the image,
                copy the <filename>meta/recipes-sato/images/core-image-sato.bb</filename> to a
                new <filename>.bb</filename> and add the following line to the end of the copy:
                <literallayout class='monospaced'>
     IMAGE_INSTALL += "strace"
                </literallayout>
            </para>
        </section>

        <section id='usingpoky-extend-customimage-customtasks'>
            <title>Customizing Images Using Custom Package Groups</title>

            <para>
                For complex custom images, the best approach is to create a custom package group recipe
                that is used to build the image or images.
                A good example of a package group recipe is
                <filename>meta/recipes-core/packagegroups/packagegroup-core-boot.bb</filename>.
                The
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES'>PACKAGES</ulink></filename>
                variable lists the package group packages you wish to produce. <filename>inherit packagegroup</filename>
                sets appropriate default values and automatically adds <filename>-dev</filename>
                and <filename>-dbg</filename> complementary
                packages for every package specified in <filename>PACKAGES</filename>.
                Note that the inherit line should be towards
                the top of the recipe, certainly before you set <filename>PACKAGES</filename>.
                For each package you specify in <filename>PACKAGES</filename>, you can use
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-RDEPENDS'>RDEPENDS</ulink></filename>
                and
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-RRECOMMENDS'>RRECOMMENDS</ulink></filename>
                entries to provide a list of packages the parent task package should contain.
                Following is an example:
                <literallayout class='monospaced'>
     DESCRIPTION = "My Custom Package Groups"

     inherit packagegroup

     PACKAGES = "\
         packagegroup-custom-apps \
         packagegroup-custom-tools \
         "

     RDEPENDS_packagegroup-custom-apps = "\
         dropbear \
         portmap \
         psplash"

     RDEPENDS_packagegroup-custom-tools = "\
         oprofile \
         oprofileui-server \
         lttng-control \
         lttng-viewer"

     RRECOMMENDS_packagegroup-custom-tools = "\
         kernel-module-oprofile"
                </literallayout>
            </para>

            <para>
                In the previous example, two package group packages are created with their dependencies and their
                recommended package dependencies listed: <filename>packagegroup-custom-apps</filename>, and
                <filename>packagegroup-custom-tools</filename>.
                To build an image using these package group packages, you need to add
                <filename>packagegroup-custom-apps</filename> and/or
                <filename>packagegroup-custom-tools</filename> to
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_INSTALL'>IMAGE_INSTALL</ulink></filename>.
                For other forms of image dependencies see the other areas of this section.
            </para>
        </section>

        <section id='usingpoky-extend-customimage-imagefeatures'>
            <title>Customizing Images Using Custom <filename>IMAGE_FEATURES</filename> and
                <filename>EXTRA_IMAGE_FEATURES</filename></title>

            <para>
                Ultimately users might want to add extra image features to the set by using the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_FEATURES'>IMAGE_FEATURES</ulink></filename>
                variable.
                To create these features, the best reference is
                <filename>meta/classes/core-image.bbclass</filename>, which shows how this is
                achieved.
                In summary, the file looks at the contents of the
                <filename>IMAGE_FEATURES</filename>
                variable and then maps that into a set of tasks or packages.
                Based on this information, the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_INSTALL'> IMAGE_INSTALL</ulink></filename>
                variable is generated automatically.
                Users can add extra features by extending the class or creating a custom class for use
                with specialized image <filename>.bb</filename> files.
                You can also add more features by configuring the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-EXTRA_IMAGE_FEATURES'>EXTRA_IMAGE_FEATURES</ulink></filename>
                variable in the <filename>local.conf</filename> file found in the Source Directory
                located in the Build Directory.
            </para>

            <para>
                The Yocto Project ships with two SSH servers you can use in your images:
                Dropbear and OpenSSH.
                Dropbear is a minimal SSH server appropriate for resource-constrained environments,
                while OpenSSH is a well-known standard SSH server implementation.
                By default, the <filename>core-image-sato</filename> image is configured to use Dropbear.
                The <filename>core-image-basic</filename> and <filename>core-image-lsb</filename>
                images both include OpenSSH.
                The <filename>core-image-minimal</filename> image does not contain an SSH server.
                To change these defaults, edit the <filename>IMAGE_FEATURES</filename> variable
                so that it sets the image you are working with to include
                <filename>ssh-server-dropbear</filename> or <filename>ssh-server-openssh</filename>.
            </para>
        </section>

        <section id='usingpoky-extend-customimage-localconf'>
            <title>Customizing Images Using <filename>local.conf</filename></title>

            <para>
                It is possible to customize image contents by using variables from your
                local configuration in your <filename>conf/local.conf</filename> file.
                Because it is limited to local use, this method generally only allows you to
                add packages and is not as flexible as creating your own customized image.
                When you add packages using local variables this way, you need to realize that
                these variable changes affect all images at the same time and might not be
                what you require.
            </para>

            <para>
                The simplest way to add extra packages to all images is by using the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_INSTALL'>IMAGE_INSTALL</ulink></filename>
                variable with the <filename>_append</filename> operator:
                <literallayout class='monospaced'>
     IMAGE_INSTALL_append = " strace"
                </literallayout>
                Use of the syntax is important.
                Specifically, the space between the quote and the package name, which is
                <filename>strace</filename> in this example.
                This space is required since the <filename>_append</filename>
                operator does not add the space.
            </para>

            <para>
                Furthermore, you must use <filename>_append</filename> instead of the <filename>+=</filename>
                operator if you want to avoid ordering issues.
                The reason for this is because doing so unconditionally appends to the variable and
                avoids ordering problems due to the variable being set in image recipes and
                <filename>.bbclass</filename> files with operators like <filename>?=</filename>.
                Using <filename>_append</filename> ensures the operation takes affect.
            </para>

            <para>
                As shown in its simplest use, <filename>IMAGE_INSTALL_append</filename> affects
                all images.
                It is possible to extend the syntax so that the variable applies to a specific image only.
                Here is an example:
                <literallayout class='monospaced'>
     IMAGE_INSTALL_append_pn-core-image-minimal = " strace"
                </literallayout>
                This example adds <filename>strace</filename> to <filename>core-image-minimal</filename>
                only.
            </para>

            <para>
                You can add packages using a similar approach through the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-CORE_IMAGE_EXTRA_INSTALL'>CORE_IMAGE_EXTRA_INSTALL</ulink></filename>
                variable.
                If you use this variable, only <filename>core-image-*</filename> images are affected.
            </para>
        </section>
    </section>

    <section id='usingpoky-extend-addpkg'>
        <title>Adding a Package</title>

        <para>
            To add a package you need to write a recipe for it.
            Writing a recipe means creating a <filename>.bb</filename> file that sets some
            variables.
            For information on variables that are useful for recipes and for information about recipe naming
            issues, see the
            "<ulink url='&YOCTO_DOCS_REF_URL;#ref-varlocality-recipe-required'>Required</ulink>"
            section of the Yocto Project Reference Manual.
        </para>

        <para>
            Before writing a recipe from scratch, it is often useful to check
            whether someone else has written one already.
            OpenEmbedded is a good place to look as it has a wider scope and range of packages.
            Because the Yocto Project aims to be compatible with OpenEmbedded, most recipes
            you find there should work for you.
        </para>

        <para>
            For new packages, the simplest way to add a recipe is to base it on a similar
            pre-existing recipe.
            The sections that follow provide some examples that show how to add standard
            types of packages.
        </para>

        <section id='usingpoky-extend-addpkg-singlec'>
            <title>Single .c File Package (Hello World!)</title>

            <para>
                Building an application from a single file that is stored locally (e.g. under
                <filename>files/</filename>) requires a recipe that has the file listed in
                the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'>SRC_URI</ulink></filename>
                variable.
                Additionally, you need to manually write the <filename>do_compile</filename> and
                <filename>do_install</filename> tasks.
                The <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-S'>S</ulink></filename>
                variable defines the
                directory containing the source code, which is set to
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'>
                WORKDIR</ulink></filename> in this case - the directory BitBake uses for the build.
                <literallayout class='monospaced'>
     DESCRIPTION = "Simple helloworld application"
     SECTION = "examples"
     LICENSE = "MIT"
     LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/MIT;md5=0835ade698e0bcf8506ecda2f7b4f302"
     PR = "r0"

     SRC_URI = "file://helloworld.c"

     S = "${WORKDIR}"

     do_compile() {
     	${CC} helloworld.c -o helloworld
     }

     do_install() {
     	install -d ${D}${bindir}
     	install -m 0755 helloworld ${D}${bindir}
     }
                </literallayout>
            </para>

            <para>
                By default, the <filename>helloworld</filename>, <filename>helloworld-dbg</filename>,
                and <filename>helloworld-dev</filename> packages are built.
                For information on how to customize the packaging process, see the
                "<link linkend='splitting-an-application-into-multiple-packages'>Splitting an Application
                into Multiple Packages</link>" section.
            </para>
        </section>

        <section id='usingpoky-extend-addpkg-autotools'>
            <title>Autotooled Package</title>
            <para>
                Applications that use Autotools such as <filename>autoconf</filename> and
                <filename>automake</filename> require a recipe that has a source archive listed in
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'>SRC_URI</ulink></filename> and
                also inherits Autotools, which instructs BitBake to use the
                <filename>autotools.bbclass</filename> file, which contains the definitions of all the steps
                needed to build an Autotool-based application.
                The result of the build is automatically packaged.
                And, if the application uses NLS for localization, packages with local information are
                generated (one package per language).
                Following is one example: (<filename>hello_2.3.bb</filename>)
                <literallayout class='monospaced'>
     DESCRIPTION = "GNU Helloworld application"
     SECTION = "examples"
     LICENSE = "GPLv2+"
     LIC_FILES_CHKSUM = "file://COPYING;md5=751419260aa954499f7abaabaa882bbe"
     PR = "r0"

     SRC_URI = "${GNU_MIRROR}/hello/hello-${PV}.tar.gz"

     inherit autotools gettext
                 </literallayout>
            </para>

            <para>
                The variable
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-LIC_FILES_CHKSUM'>LIC_FILES_CHKSUM</ulink></filename>
                is used to track source license changes as described in the
                "<ulink url='&YOCTO_DOCS_REF_URL;#usingpoky-configuring-LIC_FILES_CHKSUM'>Track License Changes</ulink>" section.
                You can quickly create Autotool-based recipes in a manner similar to the previous example.
            </para>
        </section>

        <section id='usingpoky-extend-addpkg-makefile'>
            <title>Makefile-Based Package</title>

            <para>
                Applications that use GNU <filename>make</filename> also require a recipe that has
                the source archive listed in
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'>SRC_URI</ulink></filename>.
                You do not need to add a <filename>do_compile</filename> step since by default BitBake
                starts the <filename>make</filename> command to compile the application.
                If you need additional <filename>make</filename> options you should store them in the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-EXTRA_OEMAKE'>EXTRA_OEMAKE</ulink></filename>
                variable.
                BitBake passes these options into the <filename>make</filename> GNU invocation.
                Note that a <filename>do_install</filename> task is still required.
                Otherwise, BitBake runs an empty <filename>do_install</filename> task by default.
            </para>

            <para>
                Some applications might require extra parameters to be passed to the compiler.
                For example, the application might need an additional header path.
                You can accomplish this by adding to the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-CFLAGS'>CFLAGS</ulink></filename> variable.
                The following example shows this:
                <literallayout class='monospaced'>
     CFLAGS_prepend = "-I ${S}/include "
                </literallayout>
            </para>

            <para>
            In the following example, <filename>mtd-utils</filename> is a makefile-based package:
                <literallayout class='monospaced'>
     DESCRIPTION = "Tools for managing memory technology devices."
     SECTION = "base"
     DEPENDS = "zlib lzo e2fsprogs util-linux"
     HOMEPAGE = "http://www.linux-mtd.infradead.org/"
     LICENSE = "GPLv2+"
     LIC_FILES_CHKSUM = "file://COPYING;md5=0636e73ff0215e8d672dc4c32c317bb3 \
                    file://include/common.h;beginline=1;endline=17;md5=ba05b07912a44ea2bf81ce409380049c"

     SRC_URI = "git://git.infradead.org/mtd-utils.git;protocol=git;tag=995cfe51b0a3cf32f381c140bf72b21bf91cef1b \
	     	file://add-exclusion-to-mkfs-jffs2-git-2.patch"

     S = "${WORKDIR}/git/"

     PR = "r1"

     EXTRA_OEMAKE = "'CC=${CC}' 'RANLIB=${RANLIB}' 'AR=${AR}' \
        'CFLAGS=${CFLAGS} -I${S}/include -DWITHOUT_XATTR' 'BUILDDIR=${S}'"

     do_install () {
	     oe_runmake install DESTDIR=${D} SBINDIR=${sbindir} MANDIR=${mandir} \
            INCLUDEDIR=${includedir}
	     install -d ${D}${includedir}/mtd/
	     for f in ${S}/include/mtd/*.h; do
	     	install -m 0644 $f ${D}${includedir}/mtd/
	     done
     }

     PARALLEL_MAKE = ""

     BBCLASSEXTEND = "native"
                </literallayout>
            </para>

            <para>
                If your sources are available as a tarball instead of a Git repository, you
                will need to provide the URL to the tarball as well as an
                <filename>md5</filename> or <filename>sha256</filename> sum of
                the download.
                Here is an example:
                <literallayout class='monospaced'>
     SRC_URI="ftp://ftp.infradead.org/pub/mtd-utils/mtd-utils-1.4.9.tar.bz2"
     SRC_URI[md5sum]="82b8e714b90674896570968f70ca778b"
                </literallayout>
                You can generate the <filename>md5</filename> or <filename>sha256</filename> sums
                by using the <filename>md5sum</filename> or <filename>sha256sum</filename> commands
                with the target file as the only argument.
                Here is an example:
                <literallayout class='monospaced'>
     $ md5sum mtd-utils-1.4.9.tar.bz2
     82b8e714b90674896570968f70ca778b mtd-utils-1.4.9.tar.bz2
                </literallayout>
            </para>
        </section>

        <section id='splitting-an-application-into-multiple-packages'>
            <title>Splitting an Application into Multiple Packages</title>

            <para>
                You can use the variables
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES'>PACKAGES</ulink></filename> and
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-FILES'>FILES</ulink></filename>
                to split an application into multiple packages.
            </para>

            <para>
                Following is an example that uses the <filename>libXpm</filename> recipe.
                By default, this recipe generates a single package that contains the library along
                with a few binaries.
                You can modify the recipe to split the binaries into separate packages:
                <literallayout class='monospaced'>
     require xorg-lib-common.inc

     DESCRIPTION = "X11 Pixmap library"
     LICENSE = "X-BSD"
     LIC_FILES_CHKSUM = "file://COPYING;md5=3e07763d16963c3af12db271a31abaa5"
     DEPENDS += "libxext libsm libxt"
     PR = "r3"
     PE = "1"

     XORG_PN = "libXpm"

     PACKAGES =+ "sxpm cxpm"
     FILES_cxpm = "${bindir}/cxpm"
     FILES_sxpm = "${bindir}/sxpm"
                </literallayout>
            </para>

            <para>
                In the previous example, we want to ship the <filename>sxpm</filename>
                and <filename>cxpm</filename> binaries in separate packages.
                Since <filename>bindir</filename> would be packaged into the main
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PN'>PN</ulink></filename>
                package by default, we prepend the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES'>PACKAGES</ulink>
                </filename> variable so additional package names are added to the start of list.
                This results in the extra
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-FILES'>FILES</ulink>_*</filename>
                variables then containing information that define which files and
                directories go into which packages.
                Files included by earlier packages are skipped by latter packages.
                Thus, the main
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PN'>PN</ulink></filename> package
                does not include the above listed files.
            </para>
        </section>

        <section id='including-static-library-files'>
            <title>Including Static Library Files</title>

            <para>
                If you are building a library and the library offers static linking, you can control
                which static library files (<filename>*.a</filename> files) get included in the
                built library.
            </para>

            <para>
                The <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES'><filename>PACKAGES</filename></ulink>
                and <ulink url='&YOCTO_DOCS_REF_URL;#var-FILES'><filename>FILES_*</filename></ulink>
                variables in the
                <filename>meta/conf/bitbake.conf</filename> configuration file define how files installed
                by the <filename>do_install</filename> task are packaged.
                By default, the <filename>PACKAGES</filename> variable contains
                <filename>${PN}-staticdev</filename>, which includes all static library files.
                <note>
                    Previously released versions of the Yocto Project defined the static library files
                    through <filename>${PN}-dev</filename>.
                </note>
                Following, is part of the BitBake configuration file.
                You can see where the static library files are defined:
                <literallayout class='monospaced'>
     PACKAGES = "${PN}-dbg ${PN} ${PN}-doc ${PN}-dev ${PN}-staticdev ${PN}-locale"
     PACKAGES_DYNAMIC = "${PN}-locale-*"
     FILES = ""

     FILES_${PN} = "${bindir}/* ${sbindir}/* ${libexecdir}/* ${libdir}/lib*${SOLIBS} \
                 ${sysconfdir} ${sharedstatedir} ${localstatedir} \
                 ${base_bindir}/* ${base_sbindir}/* \
                 ${base_libdir}/*${SOLIBS} \
                 ${datadir}/${BPN} ${libdir}/${BPN}/* \
                 ${datadir}/pixmaps ${datadir}/applications \
                 ${datadir}/idl ${datadir}/omf ${datadir}/sounds \
                 ${libdir}/bonobo/servers"

     FILES_${PN}-doc = "${docdir} ${mandir} ${infodir} ${datadir}/gtk-doc \
                 ${datadir}/gnome/help"
     SECTION_${PN}-doc = "doc"

     FILES_${PN}-dev = "${includedir} ${libdir}/lib*${SOLIBSDEV} ${libdir}/*.la \
                     ${libdir}/*.o ${libdir}/pkgconfig ${datadir}/pkgconfig \
                     ${datadir}/aclocal ${base_libdir}/*.o"
     SECTION_${PN}-dev = "devel"
     ALLOW_EMPTY_${PN}-dev = "1"
     RDEPENDS_${PN}-dev = "${PN} (= ${EXTENDPKGV})"

     FILES_${PN}-staticdev = "${libdir}/*.a ${base_libdir}/*.a"
     SECTION_${PN}-staticdev = "devel"
     RDEPENDS_${PN}-staticdev = "${PN}-dev (= ${EXTENDPKGV})"
                </literallayout>
            </para>
        </section>

        <section id='usingpoky-extend-addpkg-postinstalls'>
            <title>Post Install Scripts</title>

            <para>
                To add a post-installation script to a package, add a <filename>pkg_postinst_PACKAGENAME()
                </filename> function to the <filename>.bb</filename> file and use
                <filename>PACKAGENAME</filename> as the name of the package you want to attach to the
                <filename>postinst</filename> script.
                Normally
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PN'>PN</ulink></filename>
                can be used, which automatically expands to <filename>PACKAGENAME</filename>.
                A post-installation function has the following structure:
                <literallayout class='monospaced'>
     pkg_postinst_PACKAGENAME () {
     #!/bin/sh -e
     # Commands to carry out
     }
                </literallayout>
            </para>

            <para>
                The script defined in the post-installation function is called when the
                root filesystem is created.
                If the script succeeds, the package is marked as installed.
                If the script fails, the package is marked as unpacked and the script is
                executed when the image boots again.
            </para>

            <para>
                Sometimes it is necessary for the execution of a post-installation
                script to be delayed until the first boot.
                For example, the script might need to be executed on the device itself.
                To delay script execution until boot time, use the following structure in the
                post-installation script:
                <literallayout class='monospaced'>
     pkg_postinst_PACKAGENAME () {
     #!/bin/sh -e
     if [ x"$D" = "x" ]; then
          # Actions to carry out on the device go here
     else
          exit 1
     fi
     }
                </literallayout>
            </para>

            <para>
                The previous example delays execution until the image boots again because the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-D'>D</ulink></filename>
                variable points
                to the directory containing the image when the root filesystem is created at build time but
                is unset when executed on the first boot.
            </para>
        </section>
    </section>

    <section id="platdev-newmachine">
        <title>Adding a New Machine</title>

        <para>
            Adding a new machine to the Yocto Project is a straightforward process.
            This section provides information that gives you an idea of the changes you must make.
            The information covers adding machines similar to those the Yocto Project already supports.
            Although well within the capabilities of the Yocto Project, adding a totally new architecture
            might require
            changes to <filename>gcc/eglibc</filename> and to the site information, which is
            beyond the scope of this manual.
        </para>

        <para>
            For a complete example that shows how to add a new machine,
            see the
            "<ulink url='&YOCTO_DOCS_BSP_URL;#creating-a-new-bsp-layer-using-the-yocto-bsp-script'>Creating a New BSP Layer Using the yocto-bsp Script</ulink>"
            in the Yocto Project Board Support Package (BSP) Developer's Guide.
        </para>

        <section id="platdev-newmachine-conffile">
            <title>Adding the Machine Configuration File</title>

            <para>
                To add a machine configuration you need to add a <filename>.conf</filename> file
                with details of the device being added to the <filename>conf/machine/</filename> file.
                The name of the file determines the name the OpenEmbedded build system
                uses to reference the new machine.
            </para>

            <para>
                The most important variables to set in this file are as follows:
                <itemizedlist>
                    <listitem><para><filename><ulink url='&YOCTO_DOCS_REF_URL;#var-TARGET_ARCH'>
                        TARGET_ARCH</ulink></filename> (e.g. "arm")</para></listitem>
                    <listitem><para><filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PREFERRED_PROVIDER'>
                        PREFERRED_PROVIDER</ulink></filename>_virtual/kernel (see below)</para></listitem>
                    <listitem><para><filename><ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_FEATURES'>
                        MACHINE_FEATURES</ulink></filename> (e.g. "apm screen wifi")</para></listitem>
                </itemizedlist>
            </para>

            <para>
                You might also need these variables:
                <itemizedlist>
                    <listitem><para><filename><ulink url='&YOCTO_DOCS_REF_URL;#var-SERIAL_CONSOLE'>
                        SERIAL_CONSOLE</ulink></filename> (e.g. "115200 ttyS0")</para></listitem>
                    <listitem><para><filename><ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_IMAGETYPE'>
                        KERNEL_IMAGETYPE</ulink></filename> (e.g. "zImage")</para></listitem>
                    <listitem><para><filename><ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_FSTYPES'>
                        IMAGE_FSTYPES</ulink></filename> (e.g. "tar.gz jffs2")</para></listitem>
                </itemizedlist>
            </para>

            <para>
                You can find full details on these variables in the reference section.
                You can leverage many existing machine <filename>.conf</filename> files from
                <filename>meta/conf/machine/</filename>.
            </para>
        </section>

        <section id="platdev-newmachine-kernel">
            <title>Adding a Kernel for the Machine</title>

            <para>
                The OpenEmbedded build system needs to be able to build a kernel for the machine.
                You need to either create a new kernel recipe for this machine, or extend an
                existing recipe.
                You can find several kernel examples in the
                Source Directory at <filename>meta/recipes-kernel/linux</filename>
                that you can use as references.
            </para>

            <para>
                If you are creating a new recipe, normal recipe-writing rules apply for setting
                up a
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'>SRC_URI</ulink></filename>.
                Thus, you need to specify any necessary patches and set
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-S'>S</ulink></filename> to point at the source code.
                You need to create a <filename>configure</filename> task that configures the
                unpacked kernel with a defconfig.
                You can do this by using a <filename>make defconfig</filename> command or,
                more commonly, by copying in a suitable <filename>defconfig</filename> file and and then running
                <filename>make oldconfig</filename>.
                By making use of <filename>inherit kernel</filename> and potentially some of the
                <filename>linux-*.inc</filename> files, most other functionality is
                centralized and the the defaults of the class normally work well.
            </para>

            <para>
                If you are extending an existing kernel, it is usually a matter of adding a
                suitable defconfig file.
                The file needs to be added into a location similar to defconfig files
                used for other machines in a given kernel.
                A possible way to do this is by listing the file in the
                <filename>SRC_URI</filename> and adding the machine to the expression in
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-COMPATIBLE_MACHINE'>COMPATIBLE_MACHINE</ulink></filename>:
                <literallayout class='monospaced'>
     COMPATIBLE_MACHINE = '(qemux86|qemumips)'
                </literallayout>
            </para>
        </section>

        <section id="platdev-newmachine-formfactor">
            <title>Adding a Formfactor Configuration File</title>

            <para>
                A formfactor configuration file provides information about the
                target hardware for which the image is being built and information that
                the build system cannot obtain from other sources such as the kernel.
                Some examples of information contained in a formfactor configuration file include
                framebuffer orientation, whether or not the system has a keyboard,
                the positioning of the keyboard in relation to the screen, and
                the screen resolution.
            </para>

            <para>
                The build system uses reasonable defaults in most cases, but if customization is
                necessary you need to create a <filename>machconfig</filename> file
                in the <filename>meta/recipes-bsp/formfactor/files</filename>
                directory.
                This directory contains directories for specific machines such as
                <filename>qemuarm</filename> and <filename>qemux86</filename>.
                For information about the settings available and the defaults, see the
                <filename>meta/recipes-bsp/formfactor/files/config</filename> file found in the
                same area.
                Following is an example for qemuarm:
                <literallayout class='monospaced'>
     HAVE_TOUCHSCREEN=1
     HAVE_KEYBOARD=1

     DISPLAY_CAN_ROTATE=0
     DISPLAY_ORIENTATION=0
     #DISPLAY_WIDTH_PIXELS=640
     #DISPLAY_HEIGHT_PIXELS=480
     #DISPLAY_BPP=16
     DISPLAY_DPI=150
     DISPLAY_SUBPIXEL_ORDER=vrgb
                </literallayout>
            </para>
        </section>
    </section>

    <section id="building-multiple-architecture-libraries-into-one-image">
        <title>Combining Multiple Versions of Library Files into One Image</title>

        <para>
            The build system offers the ability to build libraries with different
            target optimizations or architecture formats and combine these together
            into one system image.
            You can link different binaries in the image
            against the different libraries as needed for specific use cases.
            This feature is called "Multilib."
        </para>

        <para>
            An example would be where you have most of a system compiled in 32-bit
            mode using 32-bit libraries, but you have something large, like a database
            engine, that needs to be a 64-bit application and use 64-bit libraries.
            Multilib allows you to get the best of both 32-bit and 64-bit libraries.
        </para>

        <para>
            While the Multilib feature is most commonly used for 32 and 64-bit differences,
            the approach the build system uses facilitates different target optimizations.
            You could compile some binaries to use one set of libraries and other binaries
            to use other different sets of libraries.
            The libraries could differ in architecture, compiler options, or other
            optimizations.
        </para>

        <para>
            This section overviews the Multilib process only.
            For more details on how to implement Multilib, see the
            <ulink url='&YOCTO_WIKI_URL;/wiki/Multilib'>Multilib</ulink> wiki
            page.
        </para>

        <section id='preparing-to-use-multilib'>
            <title>Preparing to use Multilib</title>

            <para>
                User-specific requirements drive the Multilib feature,
                Consequently, there is no one "out-of-the-box" configuration that likely
                exists to meet your needs.
            </para>

            <para>
                In order to enable Multilib, you first need to ensure your recipe is
                extended to support multiple libraries.
                Many standard recipes are already extended and support multiple libraries.
                You can check in the <filename>meta/conf/multilib.conf</filename>
                configuration file in the Source Directory to see how this is
                done using the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-BBCLASSEXTEND'><filename>BBCLASSEXTEND</filename></ulink>
                variable.
                Eventually, all recipes will be covered and this list will be unneeded.
            </para>

            <para>
                For the most part, the Multilib class extension works automatically to
                extend the package name from <filename>${PN}</filename> to
                <filename>${MLPREFIX}${PN}</filename>, where <filename>MLPREFIX</filename>
                is the particular multilib (e.g. "lib32-" or "lib64-").
                Standard variables such as
                <ulink url='&YOCTO_DOCS_REF_URL;#var-DEPENDS'><filename>DEPENDS</filename></ulink>,
                <ulink url='&YOCTO_DOCS_REF_URL;#var-RDEPENDS'><filename>RDEPENDS</filename></ulink>,
                <filename>RPROVIDES</filename>,
                <ulink url='&YOCTO_DOCS_REF_URL;#var-RRECOMMENDS'><filename>RRECOMMENDS</filename></ulink>,
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES'><filename>PACKAGES</filename></ulink>,
                and <filename>PACKAGES_DYNAMIC</filename> are automatically extended by the system.
                If you are extending any manual code in the recipe, you can use the
                <filename>${MLPREFIX}</filename> variable to ensure those names are extended
                correctly.
                This automatic extension code resides in <filename>multilib.bbclass</filename>.
            </para>
        </section>

        <section id='using-multilib'>
            <title>Using Multilib</title>

            <para>
                After you have set up the recipes, you need to define the actual
                combination of multiple libraries you want to build.
                You accomplish this through your <filename>local.conf</filename>
                configuration file in the
                <link linkend='build-directory'>Build Directory</link>.
                An example configuration would be as follows:
                <literallayout class='monospaced'>
     MACHINE = "qemux86-64"
     require conf/multilib.conf
     MULTILIBS = "multilib:lib32"
     DEFAULTTUNE_virtclass-multilib-lib32 = "x86"
     IMAGE_INSTALL = "lib32-connman"
                </literallayout>
                This example enables an
                additional library named <filename>lib32</filename> alongside the
                normal target packages.
                When combining these "lib32" alternatives, the example uses "x86" for tuning.
                For information on this particular tuning, see
                <filename>meta/conf/machine/include/ia32/arch-ia32.inc</filename>.
            </para>

            <para>
                The example then includes <filename>lib32-connman</filename>
                in all the images, which illustrates one method of including a
                multiple library dependency.
                You can use a normal image build to include this dependency,
                for example:
                <literallayout class='monospaced'>
     $ bitbake core-image-sato
                </literallayout>
                You can also build Multilib packages specifically with a command like this:
                <literallayout class='monospaced'>
     $  bitbake lib32-connman
                </literallayout>
            </para>
        </section>

        <section id='additional-implementation-details'>
            <title>Additional Implementation Details</title>

            <para>
                Different packaging systems have different levels of native Multilib
                support.
                For the RPM Package Management System, the following implementation details
                exist:
                <itemizedlist>
                    <listitem><para>A unique architecture is defined for the Multilib packages,
                        along with creating a unique deploy folder under
                        <filename>tmp/deploy/rpm</filename> in the
                        <link linkend='build-directory'>Build Directory</link>.
                        For example, consider <filename>lib32</filename> in a
                        <filename>qemux86-64</filename> image.
                        The possible architectures in the system are "all", "qemux86_64",
                        "lib32_qemux86_64", and "lib32_x86".</para></listitem>
                    <listitem><para>The <filename>${MLPREFIX}</filename> variable is stripped from
                        <filename>${PN}</filename> during RPM packaging.
                        The naming for a normal RPM package and a Multilib RPM package in a
                        <filename>qemux86-64</filename> system resolves to something similar to
                        <filename>bash-4.1-r2.x86_64.rpm</filename> and
                        <filename>bash-4.1.r2.lib32_x86.rpm</filename>, respectively.
                        </para></listitem>
                    <listitem><para>When installing a Multilib image, the RPM backend first
                        installs the base image and then installs the Multilib libraries.
                        </para></listitem>
                    <listitem><para>The build system relies on RPM to resolve the identical files in the
                        two (or more) Multilib packages.</para></listitem>
                </itemizedlist>
            </para>

            <para>
                For the IPK Package Management System, the following implementation details exist:
                <itemizedlist>
                    <listitem><para>The <filename>${MLPREFIX}</filename> is not stripped from
                        <filename>${PN}</filename> during IPK packaging.
                        The naming for a normal RPM package and a Multilib IPK package in a
                        <filename>qemux86-64</filename> system resolves to something like
                        <filename>bash_4.1-r2.x86_64.ipk</filename> and
                        <filename>lib32-bash_4.1-rw_x86.ipk</filename>, respectively.
                        </para></listitem>
                    <listitem><para>The IPK deploy folder is not modified with
                        <filename>${MLPREFIX}</filename> because packages with and without
                        the Multilib feature can exist in the same folder due to the
                        <filename>${PN}</filename> differences.</para></listitem>
                    <listitem><para>IPK defines a sanity check for Multilib installation
                        using certain rules for file comparison, overridden, etc.
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>
    </section>

    <section id='configuring-the-kernel'>
        <title>Configuring the Kernel</title>

        <para>
            Configuring the Yocto Project kernel consists of making sure the <filename>.config</filename>
            file has all the right information in it for the image you are building.
            You can use the <filename>menuconfig</filename> tool and configuration fragments to
            make sure your <filename>.config</filename> file is just how you need it.
            This section describes how to use <filename>menuconfig</filename>, create and use
            configuration fragments, and how to interactively tweak your <filename>.config</filename>
            file to create the leanest kernel configuration file possible.
        </para>

        <para>
            For more information on kernel configuration, see the
            "<ulink url='&YOCTO_DOCS_KERNEL_DEV_URL;#changing-the-configuration'>Changing the Configuration</ulink>"
            section in the Yocto Project Linux Kernel Development Manual.
        </para>

        <section id='using-menuconfig'>
            <title>Using&nbsp;&nbsp;<filename>menuconfig</filename></title>

            <para>
                The easiest way to define kernel configurations is to set them through the
                <filename>menuconfig</filename> tool.
                This tool provides an interactive method with which
                to set kernel configurations.
                For general information on <filename>menuconfig</filename>, see
                <ulink url='http://en.wikipedia.org/wiki/Menuconfig'></ulink>.
            </para>

            <para>
                To use the <filename>menuconfig</filename> tool in the Yocto Project development
                environment, you must build the tool using BitBake.
                Thus, the environment must be set up using the <filename>&OE_INIT_FILE;</filename>
                script found in the
                <link linkend='build-directory'>Build Directory</link>.
                The following commands build and invoke <filename>menuconfig</filename> assuming the
                Source Directory top-level folder is <filename>~/poky</filename>:
                <literallayout class='monospaced'>
     $ cd ~/poky
     $ source oe-init-build-env
     $ bitbake linux-yocto -c menuconfig
                </literallayout>
                Once <filename>menuconfig</filename> comes up, its standard interface allows you to
                interactively examine and configure all the kernel configuration parameters.
                After making your changes, simply exit the tool and save your changes to
                create an updated version of the <filename>.config</filename> configuration file.
            </para>

            <para>
                Consider an example that configures the <filename>linux-yocto-3.4</filename>
                kernel.
                The OpenEmbedded build system recognizes this kernel as
                <filename>linux-yocto</filename>.
                Thus, the following commands from the shell in which you previously sourced the
                environment initialization script cleans the shared state cache and the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink>
                directory and then builds and launches <filename>menuconfig</filename>:
                <literallayout class='monospaced'>
     $ bitbake linux-yocto -c menuconfig
                </literallayout>
            </para>

            <para>
                Once <filename>menuconfig</filename> launches, you use the interface
                to navigate through the selections to find the configuration settings in
                which you are interested.
                For example, consider the <filename>CONFIG_SMP</filename> configuration setting.
                You can find it at <filename>Processor Type and Features</filename> under
                the configuration selection <filename>Symmetric Multi-processing Support</filename>.
                After highlighting the selection, you can use the arrow keys to select or deselect
                the setting.
                When you are finished with all your selections, exit out and save them.
            </para>

            <para>
                Saving the selections updates the <filename>.config</filename> configuration file.
                This is the file that the OpenEmbedded build system uses to configure the
                kernel during the build.
                You can find and examine this file in the Build Directory in
                <filename>tmp/work/</filename>.
                The actual <filename>.config</filename> is located in the area where the
                specific kernel is built.
                For example, if you were building a Linux Yocto kernel based on the
                Linux 3.4 kernel and you were building a QEMU image targeted for
                <filename>x86</filename> architecture, the
                <filename>.config</filename> file would be located here:
                <literallayout class='monospaced'>
     ~/poky/build/tmp/work/qemux86-poky-linux/linux-yocto-3.4.11+git1+84f...
        ...656ed30-r1/linux-qemux86-standard-build
                </literallayout>
                <note>
                    The previous example directory is artificially split and many of the characters
                    in the actual filename are omitted in order to make it more readable.
                    Also, depending on the kernel you are using, the exact pathname
                    for <filename>linux-yocto-3.4...</filename> might differ.
                </note>
            </para>

            <para>
                Within the <filename>.config</filename> file, you can see the kernel settings.
                For example, the following entry shows that symmetric multi-processor support
                is not set:
                <literallayout class='monospaced'>
     # CONFIG_SMP is not set
                </literallayout>
            </para>

            <para>
                A good method to isolate changed configurations is to use a combination of the
                <filename>menuconfig</filename> tool and simple shell commands.
                Before changing configurations with <filename>menuconfig</filename>, copy the
                existing <filename>.config</filename> and rename it to something else,
                use <filename>menuconfig</filename> to make
                as many changes an you want and save them, then compare the renamed configuration
                file against the newly created file.
                You can use the resulting differences as your base to create configuration fragments
                to permanently save in your kernel layer.
                <note>
                    Be sure to make a copy of the <filename>.config</filename> and don't just
                    rename it.
                    The build system needs an existing <filename>.config</filename>
                    from which to work.
                </note>
            </para>
        </section>

        <section id='creating-config-fragments'>
            <title>Creating Configuration Fragments</title>

            <para>
                Configuration fragments are simply kernel options that appear in a file
                placed where the OpenEmbedded build system can find and apply them.
                Syntactically, the configuration statement is identical to what would appear
                in the <filename>.config</filename> file, which is in the
                <link linkend='build-directory'>Build Directory</link> in
                <filename>tmp/work/&lt;arch&gt;-poky-linux/linux-yocto-&lt;release-specific-string&gt;/linux-&lt;arch&gt;-&lt;build-type&gt;</filename>.
            </para>

            <para>
                It is simple to create a configuration fragment.
                For example, issuing the following from the shell creates a configuration fragment
                file named <filename>my_smp.cfg</filename> that enables multi-processor support
                within the kernel:
                <literallayout class='monospaced'>
     $ echo "CONFIG_SMP=y" >> my_smp.cfg
                </literallayout>
                <note>
                    All configuration files must use the <filename>.cfg</filename> extension in order
                    for the OpenEmbedded build system to recognize them as a configuration fragment.
                </note>
            </para>

            <para>
                Where do you put your configuration files?
                You can place these configuration files in the same area pointed to by
                <filename>SRC_URI</filename>.
                The OpenEmbedded build system will pick up the configuration and add it to the
                kernel's configuration.
                For example, suppose you had a set of configuration options in a file called
                <filename>myconfig.cfg</filename>.
                If you put that file inside a directory named <filename>/linux-yocto</filename>
                that resides in the same directory as the kernel's append file and then add
                a <filename>SRC_URI</filename> statement such as the following to the kernel's append file,
                those configuration options will be picked up and applied when the kernel is built.
                <literallayout class='monospaced'>
     SRC_URI += "file://myconfig.cfg"
                </literallayout>
            </para>

            <para>
                As mentioned earlier, you can group related configurations into multiple files and
                name them all in the <filename>SRC_URI</filename> statement as well.
                For example, you could group separate configurations specifically for Ethernet and graphics
                into their own files and add those by using a <filename>SRC_URI</filename> statement like the
                following in your append file:
                <literallayout class='monospaced'>
     SRC_URI += "file://myconfig.cfg \
            file://eth.cfg \
            file://gfx.cfg"
                </literallayout>
            </para>
        </section>

        <section id='fine-tuning-the-kernel-configuration-file'>
            <title>Fine-tuning the Kernel Configuration File</title>

            <para>
                You can make sure the <filename>.config</filename> is as lean or efficient as
                possible by reading the output of the kernel configuration fragment audit,
                noting any issues, making changes to correct the issues, and then repeating.
            </para>

            <para>
                As part of the kernel build process, the
                <filename>kernel_configcheck</filename> task runs.
                This task validates the kernel configuration by checking the final
                <filename>.config</filename> file against the input files.
                During the check, the task produces warning messages for the following
                issues:
                <itemizedlist>
                    <listitem><para>Requested options that did not make the final
                        <filename>.config</filename> file.</para></listitem>
                    <listitem><para>Configuration items that appear twice in the same
                        configuration fragment.</para></listitem>
                    <listitem><para>Configuration items tagged as 'required' were overridden.
                        </para></listitem>
                    <listitem><para>A board overrides a non-board specific option.</para></listitem>
                    <listitem><para>Listed options not valid for the kernel being processed.
                        In other words, the option does not appear anywhere.</para></listitem>
                </itemizedlist>
                <note>
                    The <filename>kernel_configcheck</filename> task can also optionally report
                    if an option is overridden during processing.
                </note>
            </para>

            <para>
                For each output warning, a message points to the file
                that contains a list of the options and a pointer to the config
                fragment that defines them.
                Collectively, the files are the key to streamlining the configuration.
            </para>

            <para>
                To streamline the configuration, do the following:
                <orderedlist>
                    <listitem><para>Start with a full configuration that you know
                        works - it builds and boots successfully.
                        This configuration file will be your baseline.</para></listitem>
                    <listitem><para>Separately run the <filename>configme</filename> and
                        <filename>kernel_configcheck</filename> tasks.</para></listitem>
                    <listitem><para>Take the resulting list of files from the
                        <filename>kernel_configcheck</filename> task warnings and do the following:
                        <itemizedlist>
                            <listitem><para>Drop values that are redefined in the fragment but do not
                                change the final <filename>.config</filename> file.</para></listitem>
                            <listitem><para>Analyze and potentially drop values from the
                                <filename>.config</filename> file that override required
                                configurations.</para></listitem>
                            <listitem><para>Analyze and potentially remove non-board specific options.
                                </para></listitem>
                            <listitem><para>Remove repeated and invalid options.</para></listitem>
                        </itemizedlist></para></listitem>
                    <listitem><para>After you have worked through the output of the kernel configuration
                        audit, you can re-run the <filename>configme</filename>
                        and <filename>kernel_configcheck</filename> tasks to see the results of your
                        changes.
                        If you have more issues, you can deal with them as described in the
                        previous step.</para></listitem>
                </orderedlist>
            </para>

            <para>
                Iteratively working through steps two through four eventually yields
                a minimal, streamlined configuration file.
                Once you have the best <filename>.config</filename>, you can build the Linux
                Yocto kernel.
            </para>
        </section>
    </section>

    <section id="patching-the-kernel">
        <title>Patching the Kernel</title>

        <para>
            Patching the kernel involves changing or adding configurations to an existing kernel,
            changing or adding recipes to the kernel that are needed to support specific hardware features,
            or even altering the source code itself.
            <note>
                You can use the <filename>yocto-kernel</filename> script
                found in the <link linkend='source-directory'>Source Directory</link>
                under <filename>scripts</filename> to manage kernel patches and configuration.
                See the "<ulink url='&YOCTO_DOCS_BSP_URL;#managing-kernel-patches-and-config-items-with-yocto-kernel'>Managing kernel Patches and Config Items with yocto-kernel</ulink>"
                section in the Yocto Project Board Support Packages (BSP) Developer's Guide for
                more information.</note>
        </para>

        <para>
            This example creates a simple patch by adding some QEMU emulator console
            output at boot time through <filename>printk</filename> statements in the kernel's
            <filename>calibrate.c</filename> source code file.
            Applying the patch and booting the modified image causes the added
            messages to appear on the emulator's console.
        </para>

        <para>
            The example assumes a clean build exists for the <filename>qemux86</filename>
            machine in a Source Directory named <filename>poky</filename>.
            Furthermore, the <link linkend='build-directory'>Build Directory</link> is
            <filename>build</filename> and is located in <filename>poky</filename> and
            the kernel is based on the Linux 3.4 kernel.
            For general information on how to configure the most efficient build, see the
            "<ulink url='&YOCTO_DOCS_QS_URL;#building-image'>Building an Image</ulink>" section
            in the Yocto Project Quick Start.
        </para>

        <para>
            Also, for more information on patching the kernel, see the
            "<ulink url='&YOCTO_DOCS_KERNEL_DEV_URL;#applying-patches'>Applying Patches</ulink>"
            section in the Yocto Project Linux Kernel Development Manual.
        </para>

        <section id='create-a-layer-for-your-changes'>
            <title>Create a Layer for your Changes</title>

            <para>
                The first step is to create a layer so you can isolate your changes:
                <literallayout class='monospaced'>
     $cd ~/poky
     $mkdir meta-mylayer
                </literallayout>
                Creating a directory that follows the Yocto Project layer naming
                conventions sets up the layer for your changes.
                The layer is where you place your configuration files, append
                files, and patch files.
                To learn more about creating a layer and filling it with the
                files you need, see the "<link linkend='understanding-and-creating-layers'>Understanding
                and Creating Layers</link>" section.
            </para>
        </section>

        <section id='finding-the-kernel-source-code'>
            <title>Finding the Kernel Source Code</title>

            <para>
                Each time you build a kernel image, the kernel source code is fetched
                and unpacked into the following directory:
                <literallayout class='monospaced'>
     ${S}/linux
                </literallayout>
                See the "<link linkend='finding-the-temporary-source-code'>Finding the Temporary Source Code</link>"
                section and the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-S'><filename>S</filename></ulink> variable
                for more information about where source is kept during a build.
            </para>

            <para>
                For this example, we are going to patch the
                <filename>init/calibrate.c</filename> file
                by adding some simple console <filename>printk</filename> statements that we can
                see when we boot the image using QEMU.
            </para>
        </section>

        <section id='creating-the-patch'>
            <title>Creating the Patch</title>

            <para>
                Two methods exist by which you can create the patch:
                <link linkend='using-a-git-workflow'>Git workflow</link> and
                <link linkend='using-a-quilt-workflow'>Quilt workflow</link>.
                For kernel patches, the Git workflow is more appropriate.
                This section assumes the Git workflow and shows the steps specific to
                this example.
                <orderedlist>
                    <listitem><para><emphasis>Change the working directory</emphasis>:
                        Change to where the kernel source code is before making
                        your edits to the <filename>calibrate.c</filename> file:
                        <literallayout class='monospaced'>
     $ cd ~/poky/build/tmp/work/qemux86-poky-linux/linux-yocto-${PV}-${PR}/linux
                        </literallayout>
                        Because you are working in an established Git repository,
                        you must be in this directory in order to commit your changes
                        and create the patch file.
                        <note>The <ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink> and
                            <ulink url='&YOCTO_DOCS_REF_URL;#var-PR'><filename>PR</filename></ulink> variables
                            represent the version and revision for the
                            <filename>linux-yocto</filename> recipe.
                            The <filename>PV</filename> variable includes the Git meta and machine
                            hashes, which make the directory name longer than you might
                            expect.
                        </note></para></listitem>
                    <listitem><para><emphasis>Edit the source file</emphasis>:
                        Edit the <filename>init/calibrate.c</filename> file to have the
                        following changes:
                        <literallayout class='monospaced'>
     void __cpuinit calibrate_delay(void)
     {
         unsigned long lpj;
         static bool printed;
         int this_cpu = smp_processor_id();

         printk("*************************************\n");
         printk("*                                   *\n");
         printk("*        HELLO YOCTO KERNEL         *\n");
         printk("*                                   *\n");
         printk("*************************************\n");

     	if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
               .
               .
               .
                        </literallayout></para></listitem>
                    <listitem><para><emphasis>Stage and commit your changes</emphasis>:
                        These Git commands list out the changed file, stage it, and then
                        commit the file:
                        <literallayout class='monospaced'>
     $ git status
     $ git add init/calibrate.c
     $ git commit -m "calibrate: Add printk example"
                        </literallayout></para></listitem>
                    <listitem><para><emphasis>Generate the patch file</emphasis>:
                        This Git command creates the a patch file named
                        <filename>0001-calibrate-Add-printk-example.patch</filename>
                        in the current directory.
                        <literallayout class='monospaced'>
     $ git format-patch -1
                        </literallayout>
                        </para></listitem>
                </orderedlist>
            </para>
        </section>

        <section id='get-your-layer-setup-for-the-build'>
            <title>Get Your Layer Setup for the Build</title>

            <para>These steps get your layer set up for the build:
                <orderedlist>
                    <listitem><para><emphasis>Create additional structure</emphasis>:
                        Create the additional layer structure:
                        <literallayout class='monospaced'>
     $ cd ~/poky/meta-mylayer
     $ mkdir conf
     $ mkdir recipes-kernel
     $ mkdir recipes-kernel/linux
     $ mkdir recipes-kernel/linux/linux-yocto
                         </literallayout>
                         The <filename>conf</filename> directory holds your configuration files, while the
                         <filename>recipes-kernel</filename> directory holds your append file and
                         your patch file.</para></listitem>
                    <listitem><para><emphasis>Create the layer configuration file</emphasis>:
                        Move to the <filename>meta-mylayer/conf</filename> directory and create
                        the <filename>layer.conf</filename> file as follows:
                        <literallayout class='monospaced'>
     # We have a conf and classes directory, add to BBPATH
     BBPATH := "${LAYERDIR}:${BBPATH}"

     # We have a packages directory, add to BBFILES
     BBFILES := "${BBFILES} ${LAYERDIR}/recipes-*/*/*.bb \
                 ${LAYERDIR}/recipes-*/*/*.bbappend"

     BBFILE_COLLECTIONS += "mylayer"
     BBFILE_PATTERN_mylayer := "^${LAYERDIR}/"
     BBFILE_PRIORITY_mylayer = "5"
                         </literallayout>
                         Notice <filename>mylayer</filename> as part of the last three
                         statements.</para></listitem>
                    <listitem><para><emphasis>Create the kernel recipe append file</emphasis>:
                        Move to the <filename>meta-mylayer/recipes-kernel/linux</filename> directory and create
                        the <filename>linux-yocto_3.4.bbappend</filename> file as follows:
                        <literallayout class='monospaced'>
     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"

     SRC_URI += "file://0001-calibrate-Add-printk-example.patch"

     PRINC := "${@int(PRINC) + 1}"
                        </literallayout>
                        The <filename>FILESEXTRAPATHS</filename> and <filename>SRC_URI</filename>
                        statements enable the OpenEmbedded build system to find the patch file.
                        </para></listitem>
                    <listitem><para><emphasis>Put the patch file in your layer</emphasis>:
                        Move the <filename>0001-calibrate-Add-printk-example.patch</filename> file to
                        the <filename>meta-mylayer/recipes-kernel/linux/linux-yocto</filename>
                        directory.</para></listitem>
                </orderedlist>
            </para>
        </section>

        <section id='set-up-for-the-build'>
            <title>Set Up for the Build</title>

            <para>
                Do the following to make sure the build parameters are set up for the example.
                Once you set up these build parameters, they do not have to change unless you
                change the target architecture of the machine you are building:
                <itemizedlist>
                    <listitem><para><emphasis>Build for the Correct Target Architecture:</emphasis> Your
                        selected <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>
                        definition within the <filename>local.conf</filename> file in the Build Directory
                        specifies the target architecture used when building the Linux kernel.
                        By default, <filename>MACHINE</filename> is set to
                        <filename>qemux86</filename>, which specifies a 32-bit
                        <trademark class='registered'>Intel</trademark> Architecture
                        target machine suitable for the QEMU emulator.</para></listitem>
                    <listitem><para><emphasis>Identify Your <filename>meta-mylayer</filename>
                        Layer:</emphasis> The <filename>BBLAYERS</filename> variable in the
                        <filename>bblayers.conf</filename> file found in the
                        <filename>poky/build/conf</filename> directory needs to have the path to your local
                        <filename>meta-mylayer</filename> layer.
                        By default, the <filename>BBLAYERS</filename> variable contains paths to
                        <filename>meta</filename>, <filename>meta-yocto</filename>, and
                        <filename>meta-yocto-bsp</filename> in the
                        <filename>poky</filename> Git repository.
                        Add the path to your <filename>meta-mylayer</filename> location:
                        <literallayout class='monospaced'>
     BBLAYERS ?= " \
       $HOME/poky/meta \
       $HOME/poky/meta-yocto \
       $HOME/poky/meta-yocto-bsp \
       $HOME/poky/meta-mylayer \
       "

     BBLAYERS_NON_REMOVABLE ?= " \
       $HOME/poky/meta \
       $HOME/poky/meta-yocto \
       "
                        </literallayout></para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='build-and-booting-the-modified-qemu-kernel-image'>
            <title>Build and Booting the Modified QEMU Kernel Image</title>

            <para>
                The following steps build and boot your modified kernel image:
                <orderedlist>
                    <listitem><para><emphasis>Be sure your build environment is initialized</emphasis>:
                        Your environment should be set up since you previously sourced
                        the <filename>&OE_INIT_FILE;</filename> script.
                        If it is not, source the script again from <filename>poky</filename>.
                        <literallayout class='monospaced'>
     $ cd ~/poky
     $ source &OE_INIT_FILE;
                        </literallayout>
                        </para></listitem>
                    <listitem><para><emphasis>Clean up</emphasis>:
                        Be sure to clean the shared state out by running the
                        <filename>cleansstate</filename> BitBake task as follows from your Build Directory:
                        <literallayout class='monospaced'>
     $ bitbake -c cleansstate linux-yocto
                        </literallayout></para>
                        <para><note>Never remove any files by hand from the <filename>tmp/deploy</filename>
                        directory inside the Build Directory.
                        Always use the various BitBake clean tasks to clear out previous
                        build artifacts.
                        </note></para></listitem>
                    <listitem><para><emphasis>Build the image</emphasis>:
                        Next, build the kernel image using this command:
                        <literallayout class='monospaced'>
     $ bitbake -k linux-yocto
                        </literallayout></para></listitem>
                </orderedlist>
            </para>
        </section>

        <section id='verify-your-changes'>
            <title>Verify Your Changes</title>

            <para>
                These steps boot the image and allow you to see the changes
                <orderedlist>
                    <listitem><para><emphasis>Boot the image</emphasis>:
                        Boot the modified image in the QEMU emulator
                        using this command:
                        <literallayout class='monospaced'>
     $ runqemu qemux86
                        </literallayout></para></listitem>
                    <listitem><para><emphasis>Verify the changes</emphasis>:
                        Log into the machine using <filename>root</filename> with no password and then
                        use the following shell command to scroll through the console's boot output.
                        <literallayout class='monospaced'>
     # dmesg | less
                        </literallayout>
                        You should see the results of your <filename>printk</filename> statements
                        as part of the output.</para></listitem>
                </orderedlist>
            </para>
        </section>
    </section>

    <section id="usingpoky-changes-updatingimages">
        <title>Updating Existing Images</title>

        <para>
            Often, rather than re-flashing a new image, you might wish to install updated
            packages into an existing running system.
            You can do this by first sharing the <filename>tmp/deploy/ipk/</filename> directory
            through a web server and then by changing <filename>/etc/opkg/base-feeds.conf</filename>
            to point at the shared server.
            Following is an example:
            <literallayout class='monospaced'>
     $ src/gz all http://www.mysite.com/somedir/deploy/ipk/all
     $ src/gz armv7a http://www.mysite.com/somedir/deploy/ipk/armv7a
     $ src/gz beagleboard http://www.mysite.com/somedir/deploy/ipk/beagleboard
            </literallayout>
        </para>
    </section>

    <section id='working-with-packages'>
        <title>Working with Packages</title>

        <para>
            This section describes a few tasks that involve packages:
            <itemizedlist>
                <listitem><para>Incrementing a package revision number
                    </para></listitem>
                <listitem><para>Handling a package name alias
                    </para></listitem>
                <listitem><para>Handling option module packaging
                    </para></listitem>
            </itemizedlist>
        </para>

        <section id="usingpoky-changes-prbump">
            <title>Incrementing a Package Revision Number</title>

            <para>
                If a committed change results in changing the package output,
                then the value of the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PR'>PR</ulink></filename>
                variable needs to be increased
                (or "bumped") as part of that commit.
                For new recipes you should add the <filename>PR</filename>
                variable and set its initial value equal to "r0", which is the default.
                Even though the default value is "r0", the practice of adding it to a new recipe makes
                it harder to forget to bump the variable when you make changes
                to the recipe in future.
            </para>

            <para>
                If you are sharing a common <filename>.inc</filename> file with multiple recipes,
                you can also use the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-INC_PR'>INC_PR</ulink></filename>
                variable to ensure that
                the recipes sharing the <filename>.inc</filename> file are rebuilt when the
                <filename>.inc</filename> file itself is changed.
                The <filename>.inc</filename> file must set <filename>INC_PR</filename>
                (initially to "r0"), and all recipes referring to it should set <filename>PR</filename>
                to "$(INC_PR).0" initially, incrementing the last number when the recipe is changed.
                If the <filename>.inc</filename> file is changed then its
                <filename>INC_PR</filename> should be incremented.
            </para>

            <para>
                When upgrading the version of a package, assuming the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PV'>PV</ulink></filename>
                changes, the <filename>PR</filename> variable should be reset to "r0"
                (or "$(INC_PR).0" if you are using <filename>INC_PR</filename>).
            </para>

            <para>
                Usually, version increases occur only to packages.
                However, if for some reason <filename>PV</filename> changes but does not
                increase, you can increase the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PE'>PE</ulink></filename>
                variable (Package Epoch).
                The <filename>PE</filename> variable defaults to "0".
            </para>

            <para>
                Version numbering strives to follow the
                <ulink url='http://www.debian.org/doc/debian-policy/ch-controlfields.html'>
                Debian Version Field Policy Guidelines</ulink>.
                These guidelines define how versions are compared and what "increasing" a version means.
            </para>

            <para>
                There are two reasons for following the previously mentioned guidelines.
                First, to ensure that when a developer updates and rebuilds, they get all the changes to
                the repository and do not have to remember to rebuild any sections.
                Second, to ensure that target users are able to upgrade their
                devices using package manager commands such as <filename>opkg upgrade</filename>
                (or similar commands for dpkg/apt or rpm-based systems).
            </para>

            <para>
                The goal is to ensure the Yocto Project has packages that can be upgraded in all cases.
            </para>
        </section>

        <section id="usingpoky-configuring-DISTRO_PN_ALIAS">
            <title>Handling a Package Name Alias</title>
            <para>
                Sometimes a package name you are using might exist under an alias or as a similarly named
                package in a different distribution.
                The OpenEmbedded build system implements a <filename>distro_check</filename>
                task that automatically connects to major distributions
                and checks for these situations.
                If the package exists under a different name in a different distribution, you get a
                <filename>distro_check</filename> mismatch.
                You can resolve this problem by defining a per-distro recipe name alias using the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-DISTRO_PN_ALIAS'>DISTRO_PN_ALIAS</ulink></filename>
                variable.
            </para>

            <para>
                Following is an example that shows how you specify the <filename>DISTRO_PN_ALIAS</filename>
                variable:
                <literallayout class='monospaced'>
     DISTRO_PN_ALIAS_pn-PACKAGENAME = "distro1=package_name_alias1 \
                                       distro2=package_name_alias2 \
                                       distro3=package_name_alias3 \
                                       ..."
                </literallayout>
            </para>

            <para>
                If you have more than one distribution alias, separate them with a space.
                Note that the build system currently automatically checks the
                Fedora, OpenSuSE, Debian, Ubuntu,
                and Mandriva distributions for source package recipes without having to specify them
                using the <filename>DISTRO_PN_ALIAS</filename> variable.
                For example, the following command generates a report that lists the Linux distributions
                that include the sources for each of the recipes.
                <literallayout class='monospaced'>
     $ bitbake world -f -c distro_check
                </literallayout>
                The results are stored in the <filename>build/tmp/log/distro_check-${DATETIME}.results</filename>
                file found in the Source Directory.
            </para>
        </section>

        <section id='handling-optional-module-packaging'>
            <title>Handling Optional Module Packaging</title>

            <para>
                Many pieces of software split functionality into optional
                modules (or plugins) and the plugins that are built
                might depend on configuration options.
                To avoid having to duplicate the logic that determines what
                modules are available in your recipe or to avoid having
                to package each module by hand, the OpenEmbedded build system
                provides functionality to handle module packaging dynamically.
            </para>

            <para>
                To handle optional modual packaging, you need to do two things:
                <itemizedlist>
                    <listitem><para>Ensure the module packaging is actually
                        done</para></listitem>
                    <listitem><para>Ensure that any dependencies on optional
                        modules from other recipes are satisfied by your recipe
                        </para></listitem>
                </itemizedlist>
            </para>

            <section id='making-sure-the-packaging-is-done'>
                <title>Making Sure the Packaging is Done</title>

                <para>
                    To ensure the module packaging actually gets done, you use
                    the <filename>do_split_packages</filename> function within
                    the <filename>populate_packages</filename> python function
                    in your recipe.
                    The <filename>do_split_packages</filename> function
                    searches for a pattern of files or directories under a
                    specified path and creates a package for each one it finds
                    by appending to the <filename>PACKAGES</filename> variable
                    and setting the appropriate values for
                    <filename>FILES_packagename</filename>,
                    <filename>RDEPENDS_packagename</filename>,
                    <filename>DESCRIPTION_packagename</filename>, and so forth.
                    Here is an example from the <filename>lighttpd</filename>
                    recipe:
                    <literallayout class='monospaced'>
     python populate_packages_prepend () {
         lighttpd_libdir = d.expand('${libdir}')
         do_split_packages(d, lighttpd_libdir, '^mod_(.*)\.so$',
                          'lighttpd-module-%s', 'Lighttpd module for %s',
                           extra_depends='')
     }
                    </literallayout>
                    The previous example specifies a number of things in the
                    call to <filename>do_split_packages</filename>.
                    <itemizedlist>
                        <listitem><para>A directory within the files installed
                            by your recipe through <filename>do_install</filename>
                            in which to search.</para></listitem>
                        <listitem><para>A regular expression to match module
                            files in that directory.
                            In the example, note the parentheses () that mark
                            the part of the expression from which the module
                            name should be derived.</para></listitem>
                        <listitem><para>A pattern to use for the package names.
                            </para></listitem>
                        <listitem><para>A description for each package.
                            </para></listitem>
                        <listitem><para>An empty string for
                            <filename>extra_depends</filename>, which disables
                            the default dependency on the main
                            <filename>lighttpd</filename> package.
                            Thus, if a file in <filename>${libdir}</filename>
                            called <filename>mod_alias.so</filename> is found,
                            a package called <filename>lighttpd-module-alias</filename>
                            is created for it and the <filename>DESCRIPTION</filename>
                            is set to "Lighttpd module for alias".</para></listitem>
                    </itemizedlist>
                </para>

                <para>
                    Often, packaging modules is as simple as the previous
                    example.
                    However, more advanced options exist that you can employ
                    to <filename>do_split_packages</filename> to modify its
                    behavior.
                    And, if you need to, you can add more logic by specifying
                    a hook function that is called for each package.
                    It is also perfectly acceptable to call
                    <filename>do_split_packages</filename> multiple times if
                    you have more than one set of modules to package.
                </para>

                <para>
                    For more examples that show how to use
                    <filename>do_split_packages</filename>, see the
                    <filename>connman.inc</filename> file in the
                    <filename>meta/recipes-connectivity/connman/</filename>
                    directory of the <filename>poky</filename> source repository.
                    You can also find examples in
                    <filename>meta/classes/kernel.bbclass</filename>.
                 </para>

                 <para>
                     Following is a reference that shows
                     <filename>do_split_packages</filename> mandatory and
                     optional arguments:
                     <literallayout class='monospaced'>
     Mandatory arguments

     root
        The path in which to search
     file_regex
        Regular expression to match searched files.
        Use parentheses () to mark the part of this
        expression that should be used to derive the
        module name (to be substituted where %s is
        used in other function arguments as noted below)
     output_pattern
        Pattern to use for the package names. Must
        include %s.
     description
        Description to set for each package. Must
        include %s.

     Optional arguments

     postinst
        Postinstall script to use for all packages
        (as a string)
     recursive
        True to perform a recursive search - default
        False
     hook
        A hook function to be called for every match.
        The function will be called with the following
        arguments (in the order listed):

        f
           Full path to the file/directory match
        pkg
           The package name
        file_regex
           As above
        output_pattern
           As above
        modulename
           The module name derived using file_regex

     extra_depends
        Extra runtime dependencies (RDEPENDS) to be
        set for all packages. The default value of None
        causes a dependency on the main package
        (${PN}) - if you do not want this, pass empty
        string '' for this parameter.
     aux_files_pattern
        Extra item(s) to be added to FILES for each
        package. Can be a single string item or a list
        of strings for multiple items. Must include %s.
     postrm
        postrm script to use for all packages (as a
        string)
     allow_dirs
        True to allow directories to be matched -
        default False
     prepend
        If True, prepend created packages to PACKAGES
        instead of the default False which appends them
     match_path
        match file_regex on the whole relative path to
        the root rather than just the file name
     aux_files_pattern_verbatim
        Extra item(s) to be added to FILES for each
        package, using the actual derived module name
        rather than converting it to something legal
        for a package name. Can be a single string item
        or a list of strings for multiple items. Must
        include %s.
     allow_links
        True to allow symlinks to be matched - default
        False
                     </literallayout>
                 </para>
            </section>

            <section id='satisfying-dependencies'>
                <title>Satisfying Dependencies</title>

                <para>
                    The second part for handling optional module packaging
                    is to ensure that any dependencies on optional modules
                    from other recipes are satisfied by your recipe.
                    You can be sure these dependencies are satisfied by
                    using the
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES_DYNAMIC'><filename>PACKAGES_DYNAMIC</filename></ulink> variable.
                    Here is an example that continues with the
                    <filename>lighttpd</filename> recipe shown earlier:
                    <literallayout class='monospaced'>
     PACKAGES_DYNAMIC = "lighttpd-module-.*"
                    </literallayout>
                    The name specified in the regular expression can of
                    course be anything.
                    In this example, it is <filename>lighttpd-module-</filename>
                    and is specified as the prefix to ensure that any
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-RDEPENDS'><filename>RDEPENDS</filename></ulink>
                    and <ulink url='&YOCTO_DOCS_REF_URL;#var-RRECOMMENDS'><filename>RRECOMMENDS</filename></ulink>
                    on a package name starting with the prefix are satisfied
                    during build time.
                    If you are using <filename>do_split_packages</filename>
                    as described in the previous section, the value you put in
                    <filename>PACKAGES_DYNAMIC</filename> should correspond to
                    the name pattern specified in the call to
                    <filename>do_split_packages</filename>.
                </para>
            </section>
        </section>
    </section>

    <section id="building-software-from-an-external-source">
        <title>Building Software from an External Source</title>

        <para>
            By default, the OpenEmbedded build system does its work from within the
            <link linkend='build-directory'>Build Directory</link>.
            The build process involves fetching the source files, unpacking them, and then patching them
            if necessary before the build takes place.
        </para>

        <para>
            Situations exist where you might want to build software from source files that are external to
            and thus outside of the <link linkend='source-directory'>Source Directory</link>.
            For example, suppose you have a project that includes a new BSP with a heavily customized
            kernel, a very minimal image, and some new user-space recipes.
            And, you want to minimize exposing the build system to the
            development team so that they can focus on their project and maintain everyone's workflow
            as much as possible.
            In this case, you want a kernel source directory on the development machine where the
            development occurs.
            You want the recipe's
            <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
            variable to point to the external directory and use it as is, not copy it.
        </para>

        <para>
            To build from software that comes from an external source, all you need to do is
            change your recipe so that it inherits the
            <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-externalsrc'><filename>externalsrc.bbclass</filename></ulink>
            class and then sets the
            <ulink url='&YOCTO_DOCS_REF_URL;#var-S'><filename>S</filename></ulink>
            variable to point to your external source code.
            Here are the statements to put in your recipe:
            <literallayout class='monospaced'>
     inherit externalsrc
     S = "/some/path/to/your/package/source"
            </literallayout>
        </para>

        <para>
            It is important to know that the <filename>externalsrc.bbclass</filename> assumes that the
            source directory <filename>S</filename> and the Build Directory
            <ulink url='&YOCTO_DOCS_REF_URL;#var-B'><filename>B</filename></ulink>
            are different even though by default these directories are the same.
            This assumption is important because it supports building different variants of the recipe
            by using the
            <ulink url='&YOCTO_DOCS_REF_URL;#var-BBCLASSEXTEND'><filename>BBCLASSEXTEND</filename></ulink>
            variable.
            You could allow the Build Directory to be the same as the source directory but you would
            not be able to build more than one variant of the recipe.
            Consequently, if you are building multiple variants of the recipe, you need to establish a
            Build Directory that is different than the source directory.
        </para>
    </section>

    <section id='excluding-recipes-from-the-build'>
        <title>Excluding Recipes From the Build</title>

        <para>
            You might find that there are groups of recipes you want to filter
            out of the build process.
            For example, recipes you know you will never use or want should not
            be part of the build.
            Removing these recipes from parsing speeds up parts of the build.
        </para>

        <para>
            It is possible to filter or mask out <filename>.bb</filename> and
            <filename>.bbappend</filename> files.
            You can do this by providing an expression with the
            <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-BBMASK'>BBMASK</ulink></filename>
            variable.
            Here is an example:
            <literallayout class='monospaced'>
     BBMASK = ".*/meta-mymachine/recipes-maybe/"
            </literallayout>
            Here, all <filename>.bb</filename> and <filename>.bbappend</filename> files
            in the directory that match the expression are ignored during the build
            process.
        </para>
    </section>

    <section id="platdev-appdev-srcrev">
        <title>Using an External SCM</title>

        <para>
            If you're working on a recipe that pulls from an external Source Code Manager (SCM), it
            is possible to have the OpenEmbedded build system notice new recipe changes added to the
            SCM and then build the resulting package that depends on the new recipes by using the latest
            versions.
            This only works for SCMs from which it is possible to get a sensible revision number for changes.
            Currently, you can do this with Apache Subversion (SVN), Git, and Bazaar (BZR) repositories.
        </para>

        <para>
            To enable this behavior, simply add the following to the <filename>local.conf</filename>
            configuration file found in the
            <ulink url='&YOCTO_DOCS_DEV_URL;#build-directory'>Build Directory</ulink>:
            <literallayout class='monospaced'>
     SRCREV_pn-&lt;PN&gt; = "${AUTOREV}"
            </literallayout>
            where <ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink>
            is the name of the recipe for which you want to enable automatic source
            revision updating.
        </para>
    </section>

    <section id="platdev-gdb-remotedebug">
        <title>Debugging With the GNU Project Debugger (GDB) Remotely</title>

        <para>
            GDB allows you to examine running programs, which in turn helps you to understand and fix problems.
            It also allows you to perform post-mortem style analysis of program crashes.
            GDB is available as a package within the Yocto Project and by default is
            installed in SDK images.
            See the "<ulink url='&YOCTO_DOCS_REF_URL;#ref-images'>Images</ulink>" chapter
            in the Yocto Project Reference Manual for a description of these images.
            You can find information on GDB at <ulink url="http://sourceware.org/gdb/"/>.
        </para>

        <tip>
            For best results, install <filename>-dbg</filename> packages for the applications
            you are going to debug.
            Doing so makes available extra debug symbols that give you more meaningful output.
        </tip>

        <para>
            Sometimes, due to memory or disk space constraints, it is not possible
            to use GDB directly on the remote target to debug applications.
            These constraints arise because GDB needs to load the debugging information and the
            binaries of the process being debugged.
            Additionally, GDB needs to perform many computations to locate information such as function
            names, variable names and values, stack traces and so forth - even before starting the
            debugging process.
            These extra computations place more load on the target system and can alter the
            characteristics of the program being debugged.
        </para>

        <para>
            To help get past the previously mentioned constraints, you can use Gdbserver.
            Gdbserver runs on the remote target and does not load any debugging information
            from the debugged process.
            Instead, a GDB instance processes the debugging information that is run on a
            remote computer - the host GDB.
            The host GDB then sends control commands to Gdbserver to make it stop or start the debugged
            program, as well as read or write memory regions of that debugged program.
            All the debugging information loaded and processed as well
            as all the heavy debugging is done by the host GDB.
            Offloading these processes gives the Gdbserver running on the target a chance to remain
            small and fast.
        </para>

        <para>
            Because the host GDB is responsible for loading the debugging information and
            for doing the necessary processing to make actual debugging happen, the
            user has to make sure the host can access the unstripped binaries complete
            with their debugging information and also be sure the target is compiled with no optimizations.
            The host GDB must also have local access to all the libraries used by the
            debugged program.
            Because Gdbserver does not need any local debugging information, the binaries on
            the remote target can remain stripped.
            However, the binaries must also be compiled without optimization
            so they match the host's binaries.
        </para>

        <para>
            To remain consistent with GDB documentation and terminology, the binary being debugged
            on the remote target machine is referred to as the "inferior" binary.
            For documentation on GDB see the
            <ulink url="http://sourceware.org/gdb/documentation/">GDB site</ulink>.
        </para>

        <section id="platdev-gdb-remotedebug-launch-gdbserver">
            <title>Launching Gdbserver on the Target</title>

            <para>
                First, make sure Gdbserver is installed on the target.
                If it is not, install the package <filename>gdbserver</filename>, which needs the
                <filename>libthread-db1</filename> package.
            </para>

            <para>
                As an example, to launch Gdbserver on the target and make it ready to "debug" a
                program located at <filename>/path/to/inferior</filename>, connect
                to the target and launch:
                <literallayout class='monospaced'>
     $ gdbserver localhost:2345 /path/to/inferior
                </literallayout>
                Gdbserver should now be listening on port 2345 for debugging
                commands coming from a remote GDB process that is running on the host computer.
                Communication between Gdbserver and the host GDB are done using TCP.
                To use other communication protocols, please refer to the
                <ulink url='http://www.gnu.org/software/gdb/'>Gdbserver documentation</ulink>.
            </para>
        </section>

        <section id="platdev-gdb-remotedebug-launch-gdb">
            <title>Launching GDB on the Host Computer</title>

            <para>
                Running GDB on the host computer takes a number of stages.
                This section describes those stages.
            </para>

            <section id="platdev-gdb-remotedebug-launch-gdb-buildcross">
                <title>Building the Cross-GDB Package</title>
                <para>
                    A suitable GDB cross-binary is required that runs on your host computer but
                    also knows about the the ABI of the remote target.
                    You can get this binary from the meta-toolchain.
                    Here is an example:
                    <literallayout class='monospaced'>
     /usr/local/poky/eabi-glibc/arm/bin/arm-poky-linux-gnueabi-gdb
                    </literallayout>
                    where <filename>arm</filename> is the target architecture and
                    <filename>linux-gnueabi</filename> the target ABI.
                </para>

                <para>
                    Alternatively, you can use BitBake to build the <filename>gdb-cross</filename> binary.
                    Here is an example:
                    <literallayout class='monospaced'>
     $ bitbake gdb-cross
                    </literallayout>
                    Once the binary is built, you can find it here:
                    <literallayout class='monospaced'>
     tmp/sysroots/&lt;host-arch&gt;/usr/bin/&lt;target-abi&gt;-gdb
                    </literallayout>
                </para>
            </section>

            <section id="platdev-gdb-remotedebug-launch-gdb-inferiorbins">
                <title>Making the Inferior Binaries Available</title>

                <para>
                    The inferior binary (complete with all debugging symbols), as well as any
                    libraries (and their debugging symbols) on which the inferior binary depends,
                    needs to be available.
                    There are a number of ways you can make these items available.
                </para>

                <para>
                    Perhaps the easiest way is to have an SDK image that corresponds to the plain
                    image installed on the device.
                    In the case of <filename>core-image-sato</filename>,
                    <filename>core-image-sato-sdk</filename> would contain suitable symbols.
                    Because the SDK images already have the debugging symbols installed, it is just a
                    question of expanding the archive to some location and then informing GDB.
                </para>

                <para>
                    Alternatively, the OpenEmbedded build system can build a custom directory of files
                    for a specific
                    debugging purpose by reusing its <filename>tmp/rootfs</filename> directory.
                    This directory contains the contents of the last built image.
                    This process assumes two things:
                    <itemizedlist>
                        <listitem><para>The image running on the target was the last image to
                        be built.</para></listitem>
                        <listitem><para>The package (<filename>foo</filename> in the following
                        example) that contains the inferior binary to be debugged has been built
                        without optimization and has debugging information available.</para></listitem>
                    </itemizedlist>
                </para>

                <para>
                    The following steps show how to build the custom directory of files:
                    <orderedlist>
                        <listitem><para>Install the package (<filename>foo</filename> in this case) to
                            <filename>tmp/rootfs</filename>:
                            <literallayout class='monospaced'>
     $ tmp/sysroots/i686-linux/usr/bin/opkg-cl -f \
     tmp/work/&lt;target-abi&gt;/core-image-sato-1.0-r0/temp/opkg.conf -o \
     tmp/rootfs/ update
                            </literallayout></para></listitem>
                        <listitem><para>Install the debugging information:
                            <literallayout class='monospaced'>
     $ tmp/sysroots/i686-linux/usr/bin/opkg-cl -f \
     tmp/work/&lt;target-abi&gt;/core-image-sato-1.0-r0/temp/opkg.conf \
     -o tmp/rootfs install foo

     $ tmp/sysroots/i686-linux/usr/bin/opkg-cl -f \
     tmp/work/&lt;target-abi&gt;/core-image-sato-1.0-r0/temp/opkg.conf \
     -o tmp/rootfs install foo-dbg
                            </literallayout></para></listitem>
                    </orderedlist>
                </para>
            </section>

            <section id="platdev-gdb-remotedebug-launch-gdb-launchhost">
                <title>Launch the Host GDB</title>

                <para>
                    To launch the host GDB, you run the <filename>cross-gdb</filename> binary and provide
                    the inferior binary as part of the command line.
                    For example, the following command form continues with the example used in
                    the previous section.
                    This command form loads the <filename>foo</filename> binary
                    as well as the debugging information:
                    <literallayout class='monospaced'>
     $ &lt;target-abi&gt;-gdb rootfs/usr/bin/foo
                    </literallayout>
                    Once the GDB prompt appears, you must instruct GDB to load all the libraries
                    of the inferior binary from <filename>tmp/rootfs</filename> as follows:
                    <literallayout class='monospaced'>
     $ set solib-absolute-prefix /path/to/tmp/rootfs
                    </literallayout>
                    The pathname <filename>/path/to/tmp/rootfs</filename> must either be
                    the absolute path to <filename>tmp/rootfs</filename> or the location at which
                    binaries with debugging information reside.
                </para>

                <para>
                    At this point you can have GDB connect to the Gdbserver that is running
                    on the remote target by using the following command form:
                    <literallayout class='monospaced'>
     $ target remote remote-target-ip-address:2345
                    </literallayout>
                    The <filename>remote-target-ip-address</filename> is the IP address of the
                    remote target where the Gdbserver is running.
                    Port 2345 is the port on which the GDBSERVER is running.
                </para>
            </section>

            <section id="platdev-gdb-remotedebug-launch-gdb-using">
                <title>Using the Debugger</title>

                <para>
                    You can now proceed with debugging as normal - as if you were debugging
                    on the local machine.
                    For example, to instruct GDB to break in the "main" function and then
                    continue with execution of the inferior binary use the following commands
                    from within GDB:
                    <literallayout class='monospaced'>
     (gdb) break main
     (gdb) continue
                    </literallayout>
                </para>

                <para>
                    For more information about using GDB, see the project's online documentation at
                    <ulink url="http://sourceware.org/gdb/download/onlinedocs/"/>.
                </para>
            </section>
        </section>
    </section>

    <section id="platdev-oprofile">
        <title>Profiling with OProfile</title>

        <para>
            <ulink url="http://oprofile.sourceforge.net/">OProfile</ulink> is a
            statistical profiler well suited for finding performance
            bottlenecks in both userspace software and in the kernel.
            This profiler provides answers to questions like "Which functions does my application spend
            the most time in when doing X?"
            Because the OpenEmbedded build system is well integrated with OProfile, it makes profiling
            applications on target hardware straightforward.
            <note>
                For more information on how to set up and run OProfile, see the
                "<ulink url='&YOCTO_DOCS_PROF_URL;#profile-manual-oprofile'>OProfile</ulink>"
                section in the Yocto Project Profiling and Tracing Manual.
            </note>
        </para>

        <para>
            To use OProfile, you need an image that has OProfile installed.
            The easiest way to do this is with <filename>tools-profile</filename> in the
            <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_FEATURES'>IMAGE_FEATURES</ulink></filename> variable.
            You also need debugging symbols to be available on the system where the analysis
            takes place.
            You can gain access to the symbols by using <filename>dbg-pkgs</filename> in the
            <filename>IMAGE_FEATURES</filename> variable or by
            installing the appropriate <filename>-dbg</filename> packages.
        </para>

        <para>
            For successful call graph analysis, the binaries must preserve the frame
            pointer register and should also be compiled with the
            <filename>-fno-omit-framepointer</filename> flag.
            You can achieve this by setting the
            <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-SELECTED_OPTIMIZATION'>SELECTED_OPTIMIZATION</ulink></filename>
            variable with the following options:
            <literallayout class='monospaced'>
     -fexpensive-optimizations
     -fno-omit-framepointer
     -frename-registers
     -O2
            </literallayout>
            You can also achieve it by setting the
            <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-DEBUG_BUILD'>DEBUG_BUILD</ulink></filename>
            variable to "1" in the <filename>local.conf</filename> configuration file.
            If you use the <filename>DEBUG_BUILD</filename> variable,
            you will also add extra debug information that can make the debug packages large.
        </para>

        <section id="platdev-oprofile-target">
            <title>Profiling on the Target</title>

            <para>
                Using OProfile you can perform all the profiling work on the target device.
                A simple OProfile session might look like the following:
            </para>

            <para>
                <literallayout class='monospaced'>
     # opcontrol --reset
     # opcontrol --start --separate=lib --no-vmlinux -c 5
              .
              .
        [do whatever is being profiled]
              .
              .
     # opcontrol --stop
     $ opreport -cl
                </literallayout>
            </para>

            <para>
                In this example, the <filename>reset</filename> command clears any previously profiled data.
                The next command starts OProfile.
                The options used when starting the profiler separate dynamic library data
                within applications, disable kernel profiling, and enable callgraphing up to
                five levels deep.
                <note>
                    To profile the kernel, you would specify the
                    <filename>--vmlinux=/path/to/vmlinux</filename> option.
                    The <filename>vmlinux</filename> file is usually in the source directory in the
                    <filename>/boot/</filename> directory and must match the running kernel.
                </note>
            </para>

            <para>
                After you perform your profiling tasks, the next command stops the profiler.
                After that, you can view results with the <filename>opreport</filename> command with options
                to see the separate library symbols and callgraph information.
            </para>

            <para>
                Callgraphing logs information about time spent in functions and about a function's
                calling function (parent) and called functions (children).
                The higher the callgraphing depth, the more accurate the results.
                However, higher depths also increase the logging overhead.
                Consequently, you should take care when setting the callgraphing depth.
                <note>
                    On ARM, binaries need to have the frame pointer enabled for callgraphing to work.
                    To accomplish this use the <filename>-fno-omit-framepointer</filename> option
                    with <filename>gcc</filename>.
                </note>
            </para>

            <para>
                For more information on using OProfile, see the OProfile
                online documentation at
                <ulink url="http://oprofile.sourceforge.net/docs/"/>.
            </para>
        </section>

        <section id="platdev-oprofile-oprofileui">
            <title>Using OProfileUI</title>

            <para>
                A graphical user interface for OProfile is also available.
                You can download and build this interface from the Yocto Project at
                <ulink url="&YOCTO_GIT_URL;/cgit.cgi/oprofileui/"></ulink>.
                If the "tools-profile" image feature is selected, all necessary binaries
                are installed onto the target device for OProfileUI interaction.
            </para>

            <para>
                Even though the source directory usually includes all needed patches on the target device, you
                might find you need other OProfile patches for recent OProfileUI features.
                If so, see the <ulink url='&YOCTO_GIT_URL;/cgit.cgi/oprofileui/tree/README'>
                OProfileUI README</ulink> for the most recent information.
            </para>

            <section id="platdev-oprofile-oprofileui-online">
                <title>Online Mode</title>

                <para>
                    Using OProfile in online mode assumes a working network connection with the target
                    hardware.
                    With this connection, you just need to run "oprofile-server" on the device.
                    By default, OProfile listens on port 4224.
                    <note>
                        You can change the port using the <filename>--port</filename> command-line
                        option.
                    </note>
                </para>

                <para>
                    The client program is called <filename>oprofile-viewer</filename> and its UI is relatively
                    straightforward.
                    You access key functionality through the buttons on the toolbar, which
                    are duplicated in the menus.
                    Here are the buttons:
                    <itemizedlist>
                        <listitem><para><emphasis>Connect:</emphasis> Connects to the remote host.
                            You can also supply the IP address or hostname.</para></listitem>
                        <listitem><para><emphasis>Disconnect:</emphasis> Disconnects from the target.
                            </para></listitem>
                        <listitem><para><emphasis>Start:</emphasis> Starts profiling on the device.
                            </para></listitem>
                        <listitem><para><emphasis>Stop:</emphasis> Stops profiling on the device and
                            downloads the data to the local host.
                            Stopping the profiler generates the profile and displays it in the viewer.
                            </para></listitem>
                        <listitem><para><emphasis>Download:</emphasis> Downloads the data from the
                            target and generates the profile, which appears in the viewer.</para></listitem>
                        <listitem><para><emphasis>Reset:</emphasis> Resets the sample data on the device.
                            Resetting the data removes sample information collected from previous
                            sampling runs.
                            Be sure you reset the data if you do not want to include old sample information.
                            </para></listitem>
                        <listitem><para><emphasis>Save:</emphasis> Saves the data downloaded from the
                            target to another directory for later examination.</para></listitem>
                        <listitem><para><emphasis>Open:</emphasis> Loads previously saved data.
                            </para></listitem>
                    </itemizedlist>
                </para>

                <para>
                    The client downloads the complete 'profile archive' from
                    the target to the host for processing.
                    This archive is a directory that contains the sample data, the object files,
                    and the debug information for the object files.
                    The archive is then converted using the <filename>oparchconv</filename> script, which is
                    included in this distribution.
                    The script uses <filename>opimport</filename> to convert the archive from
                    the target to something that can be processed on the host.
                </para>

                <para>
                    Downloaded archives reside in the Build Directory in
                    <filename>/tmp</filename> and are cleared up when they are no longer in use.
                </para>

                <para>
                    If you wish to perform kernel profiling, you need to be sure
                    a <filename>vmlinux</filename> file that matches the running kernel is available.
                    In the source directory, that file is usually located in
                    <filename>/boot/vmlinux-KERNELVERSION</filename>, where
                    <filename>KERNEL-version</filename> is the version of the kernel.
                    The OpenEmbedded build system generates separate <filename>vmlinux</filename>
                    packages for each kernel it builds.
                    Thus, it should just be a question of making sure a matching package is
                    installed (e.g. <filename>opkg install kernel-vmlinux</filename>.
                    The files are automatically installed into development and profiling images
                    alongside OProfile.
                    A configuration option exists within the OProfileUI settings page that you can use to
                    enter the location of the <filename>vmlinux</filename> file.
                </para>

                <para>
                    Waiting for debug symbols to transfer from the device can be slow, and it
                    is not always necessary to actually have them on the device for OProfile use.
                    All that is needed is a copy of the filesystem with the debug symbols present
                    on the viewer system.
                    The "<link linkend='platdev-gdb-remotedebug-launch-gdb'>Launching GDB on the Host Computer</link>"
                    section covers how to create such a directory with
                    the source directory and how to use the OProfileUI Settings dialog to specify the location.
                    If you specify the directory, it will be used when the file checksums
                    match those on the system you are profiling.
                </para>
            </section>

            <section id="platdev-oprofile-oprofileui-offline">
                <title>Offline Mode</title>

                <para>
                    If network access to the target is unavailable, you can generate
                    an archive for processing in <filename>oprofile-viewer</filename> as follows:
                    <literallayout class='monospaced'>
     # opcontrol --reset
     # opcontrol --start --separate=lib --no-vmlinux -c 5
            .
            .
     [do whatever is being profiled]
            .
            .
     # opcontrol --stop
     # oparchive -o my_archive
                    </literallayout>
                </para>

                <para>
                    In the above example, <filename>my_archive</filename> is the name of the
                    archive directory where you would like the profile archive to be kept.
                    After the directory is created, you can copy it to another host and load it
                    using <filename>oprofile-viewer</filename> open functionality.
                    If necessary, the archive is converted.
                </para>
            </section>
        </section>
    </section>

    <section id='maintaining-open-source-license-compliance-during-your-products-lifecycle'>
        <title>Maintaining Open Source License Compliance During Your Product's Lifecycle</title>

        <para>
            One of the concerns for a development organization using open source
            software is how to maintain compliance with various open source
            licensing during the lifecycle of the product.
            While this section does not provide legal advice or
            comprehensively cover all scenarios, it does
            present methods that you can use to
            assist you in meeting the compliance requirements during a software
            release.
        </para>

        <para>
            With hundreds of different open source licenses that the Yocto
            Project tracks, it is difficult to know the requirements of each
            and every license.
            However, we can begin to cover the requirements of the major FLOSS licenses, by
            assuming that there are three main areas of concern:
            <itemizedlist>
                <listitem><para>Source code must be provided.</para></listitem>
                <listitem><para>License text for the software must be
                    provided.</para></listitem>
                <listitem><para>Compilation scripts and modifications to the
                    source code must be provided.
                    </para></listitem>
            </itemizedlist>
            There are other requirements beyond the scope of these
            three and the methods described in this section
            (e.g. the mechanism through which source code is distributed).
            As different organizations have different methods of complying with
            open source licensing, this section is not meant to imply that
            there is only one single way to meet your compliance obligations,
            but rather to describe one method of achieving compliance.
        </para>

        <para>
            The remainder of this section describes methods supported to meet the
            previously mentioned three requirements.
            Once you take steps to meet these requirements,
            and prior to releasing images, sources, and the build system,
            you should audit all artifacts to ensure completeness.
            The Yocto Project generates a license manifest during
            image creation that is located
            in <filename>${DEPLOY_DIR}/licenses/&lt;image_name-datestamp&gt;</filename>
            to assist with any audits.
        </para>

        <section id='providing-the-source-code'>
            <title>Providing the Source Code</title>

            <para>
                Compliance activities should begin before you generate the
                final image.
                The first thing you should look at is the requirement that
                tops the list for most compliance groups - providing
                the source.
                The Yocto Project has a few ways of meeting this
                requirement.
            </para>

            <para>
                One of the easiest ways to meet this requirement is
                to provide the entire
                <ulink url='&YOCTO_DOCS_REF_URL;#var-DL_DIR'><filename>DL_DIR</filename></ulink>
                used by the build.
                This method, however, has a few issues.
                The most obvious is the size of the directory since it includes
                all sources used in the build and not just the source used in
                the released image.
                It will include toolchain source, and other artifacts which
                you would not generally release.
                But, the more serious issue for most companies is accidental
                release of proprietary software.
                The Yocto Project provides an archiver class to help avoid
                some of these concerns.
            </para>

            <para>
                Before you employ <filename>DL_DIR</filename> or the
                archiver class, you need to decide how you choose to
                provide source.
                The source archiver class can generate tarballs and SRPMs
                and can create them with various levels of compliance in mind.
                One way of doing this (but certainly not the only way) is to
                release just the original source as a tarball.
                You can do this by adding the following to the
                <filename>local.conf</filename> file found in the
                <link linkend='build-directory'>Build Directory</link>:
                <literallayout class='monospaced'>
     ARCHIVER_MODE ?= "original"
     ARCHIVER_CLASS = "${@'archive-${ARCHIVER_MODE}-source' if
     ARCHIVER_MODE != 'none' else ''}"
     INHERIT += "${ARCHIVER_CLASS}"
     SOURCE_ARCHIVE_PACKAGE_TYPE = "tar"
                </literallayout>
                During the creation of your image, the source from all
                recipes that deploy packages to the image is placed within
                subdirectories of
                <filename>DEPLOY_DIR/sources</filename> based on the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-LICENSE'><filename>LICENSE</filename></ulink>
                for each recipe.
                Releasing the entire directory enables you to comply with
                requirements concerning providing the unmodified source.
                It is important to note that the size of the directory can
                get large.
            </para>

            <para>
                A way to help mitigate the size issue is to only release
                tarballs for licenses that require the release of
                source.
                Let's assume you are only concerned with GPL code as
                identified with the following:
                <literallayout class='monospaced'>
     $ cd poky/build/tmp/deploy/sources
     $ mkdir ~/gpl_source_release
     $ for dir in */*GPL*; do cp -r $dir ~/gpl_source_release; done
                </literallayout>
                At this point, you could create a tarball from the
                <filename>gpl_source_release</filename> directory and
                provide that to the end user.
                This method would be a step toward achieving compliance
                with section 3a of GPLv2 and with section 6 of GPLv3.
            </para>
        </section>

        <section id='providing-license-text'>
            <title>Providing License Text</title>

            <para>
                One requirement that is often overlooked is inclusion
                of license text.
                This requirement also needs to be dealt with prior to
                generating the final image.
                Some licenses require the license text to accompany
                the binary.
                You can achieve this by adding the following to your
                <filename>local.conf</filename> file:
                <literallayout class='monospaced'>
     COPY_LIC_MANIFEST = "1"
     COPY_LIC_DIRS = "1"
                </literallayout>
                Adding these statements to the configuration file ensures
                that the licenses collected during package generation
                are included on your image.
                As the source archiver has already archived the original
                unmodified source which would contain the license files,
                you would have already met the requirements for inclusion
                of the license information with source as defined by the GPL
                and other open source licenses.
            </para>
        </section>

        <section id='providing-compilation-scripts-and-source-code-modifications'>
            <title>Providing Compilation Scripts and Source Code Modifications</title>

            <para>
                At this point, we have addressed all we need to address
                prior to generating the image.
                The next two requirements are addressed during the final
                packaging of the release.
            </para>

            <para>
                By releasing the version of the OpenEmbedded build system
                and the layers used during the build, you will be providing both
                compilation scripts and the source code modifications in one
                step.
            </para>

            <para>
                If the deployment team has a
                <ulink url='&YOCTO_DOCS_BSP_URL;#bsp-layers'>BSP layer</ulink>
                and a distro layer, and those those layers are used to patch,
                compile, package, or modify (in any way) any open source
                software included in your released images, you
                may be required to to release those layers under section 3 of
                GPLv2 or section 1 of GPLv3.
                One way of doing that is with a clean
                checkout of the version of the Yocto Project and layers used
                during your build.
                Here is an example:
                <literallayout class='monospaced'>
     # We built using the &DISTRO_NAME; branch of the poky repo
     $ git clone -b &DISTRO_NAME; git://git.yoctoproject.org/poky
     $ cd poky
     # We built using the release_branch for our layers
     $ git clone -b release_branch git://git.mycompany.com/meta-my-bsp-layer
     $ git clone -b release_branch git://git.mycompany.com/meta-my-software-layer
     # clean up the .git repos
     $ find . -name ".git" -type d -exec rm -rf {} \;
                </literallayout>
                One thing a development organization might want to consider
                for end-user convenience is to modify
                <filename>meta-yocto/conf/bblayers.conf.sample</filename> to
                ensure that when the end user utilizes the released build
                system to build an image, the development organization's
                layers are included in the <filename>bblayers.conf</filename>
                file automatically:
                <literallayout class='monospaced'>
     # LAYER_CONF_VERSION is increased each time build/conf/bblayers.conf
     # changes incompatibly
     LCONF_VERSION = "6"

     BBPATH = "${TOPDIR}"
     BBFILES ?= ""

     BBLAYERS ?= " \
       ##COREBASE##/meta \
       ##COREBASE##/meta-yocto \
       ##COREBASE##/meta-yocto-bsp \
       ##COREBASE##/meta-mylayer \
       "

     BBLAYERS_NON_REMOVABLE ?= " \
       ##COREBASE##/meta \
       ##COREBASE##/meta-yocto \
       "
                </literallayout>
                Creating and providing an archive of the metadata layers
                (recipes, configuration files, and so forth)
                enables you to meet your
                requirements to include the scripts to control compilation
                as well as any modifications to the original source.
            </para>
        </section>
    </section>
</chapter>

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