xref: /openbmc/openbmc/poky/documentation/dev-manual/debugging.rst (revision 44b3caf2)

.. SPDX-License-Identifier: CC-BY-SA-2.0-UK

Debugging Tools and Techniques
******************************

The exact method for debugging build failures depends on the nature of
the problem and on the system's area from which the bug originates.
Standard debugging practices such as comparison against the last known
working version with examination of the changes and the re-application
of steps to identify the one causing the problem are valid for the Yocto
Project just as they are for any other system. Even though it is
impossible to detail every possible potential failure, this section
provides some general tips to aid in debugging given a variety of
situations.

.. note::

   A useful feature for debugging is the error reporting tool.
   Configuring the Yocto Project to use this tool causes the
   OpenEmbedded build system to produce error reporting commands as part
   of the console output. You can enter the commands after the build
   completes to log error information into a common database, that can
   help you figure out what might be going wrong. For information on how
   to enable and use this feature, see the
   ":ref:`dev-manual/error-reporting-tool:using the error reporting tool`"
   section.

The following list shows the debugging topics in the remainder of this
section:

-  ":ref:`dev-manual/debugging:viewing logs from failed tasks`" describes
   how to find and view logs from tasks that failed during the build
   process.

-  ":ref:`dev-manual/debugging:viewing variable values`" describes how to
   use the BitBake ``-e`` option to examine variable values after a
   recipe has been parsed.

-  ":ref:`dev-manual/debugging:viewing package information with \`\`oe-pkgdata-util\`\``"
   describes how to use the ``oe-pkgdata-util`` utility to query
   :term:`PKGDATA_DIR` and
   display package-related information for built packages.

-  ":ref:`dev-manual/debugging:viewing dependencies between recipes and tasks`"
   describes how to use the BitBake ``-g`` option to display recipe
   dependency information used during the build.

-  ":ref:`dev-manual/debugging:viewing task variable dependencies`" describes
   how to use the ``bitbake-dumpsig`` command in conjunction with key
   subdirectories in the :term:`Build Directory` to determine variable
   dependencies.

-  ":ref:`dev-manual/debugging:running specific tasks`" describes
   how to use several BitBake options (e.g. ``-c``, ``-C``, and ``-f``)
   to run specific tasks in the build chain. It can be useful to run
   tasks "out-of-order" when trying isolate build issues.

-  ":ref:`dev-manual/debugging:general BitBake problems`" describes how
   to use BitBake's ``-D`` debug output option to reveal more about what
   BitBake is doing during the build.

-  ":ref:`dev-manual/debugging:building with no dependencies`"
   describes how to use the BitBake ``-b`` option to build a recipe
   while ignoring dependencies.

-  ":ref:`dev-manual/debugging:recipe logging mechanisms`"
   describes how to use the many recipe logging functions to produce
   debugging output and report errors and warnings.

-  ":ref:`dev-manual/debugging:debugging parallel make races`"
   describes how to debug situations where the build consists of several
   parts that are run simultaneously and when the output or result of
   one part is not ready for use with a different part of the build that
   depends on that output.

-  ":ref:`dev-manual/debugging:debugging with the gnu project debugger (gdb) remotely`"
   describes how to use GDB to allow you to examine running programs, which can
   help you fix problems.

-  ":ref:`dev-manual/debugging:debugging with the gnu project debugger (gdb) on the target`"
   describes how to use GDB directly on target hardware for debugging.

-  ":ref:`dev-manual/debugging:other debugging tips`" describes
   miscellaneous debugging tips that can be useful.

Viewing Logs from Failed Tasks
==============================

You can find the log for a task in the file
``${``\ :term:`WORKDIR`\ ``}/temp/log.do_``\ `taskname`.
For example, the log for the
:ref:`ref-tasks-compile` task of the
QEMU minimal image for the x86 machine (``qemux86``) might be in
``tmp/work/qemux86-poky-linux/core-image-minimal/1.0-r0/temp/log.do_compile``.
To see the commands :term:`BitBake` ran
to generate a log, look at the corresponding ``run.do_``\ `taskname` file
in the same directory.

``log.do_``\ `taskname` and ``run.do_``\ `taskname` are actually symbolic
links to ``log.do_``\ `taskname`\ ``.``\ `pid` and
``log.run_``\ `taskname`\ ``.``\ `pid`, where `pid` is the PID the task had
when it ran. The symlinks always point to the files corresponding to the
most recent run.

Viewing Variable Values
=======================

Sometimes you need to know the value of a variable as a result of
BitBake's parsing step. This could be because some unexpected behavior
occurred in your project. Perhaps an attempt to :ref:`modify a variable
<bitbake-user-manual/bitbake-user-manual-metadata:modifying existing
variables>` did not work out as expected.

BitBake's ``-e`` option is used to display variable values after
parsing. The following command displays the variable values after the
configuration files (i.e. ``local.conf``, ``bblayers.conf``,
``bitbake.conf`` and so forth) have been parsed::

   $ bitbake -e

The following command displays variable values after a specific recipe has
been parsed. The variables include those from the configuration as well::

   $ bitbake -e recipename

.. note::

   Each recipe has its own private set of variables (datastore).
   Internally, after parsing the configuration, a copy of the resulting
   datastore is made prior to parsing each recipe. This copying implies
   that variables set in one recipe will not be visible to other
   recipes.

   Likewise, each task within a recipe gets a private datastore based on
   the recipe datastore, which means that variables set within one task
   will not be visible to other tasks.

In the output of ``bitbake -e``, each variable is preceded by a
description of how the variable got its value, including temporary
values that were later overridden. This description also includes
variable flags (varflags) set on the variable. The output can be very
helpful during debugging.

Variables that are exported to the environment are preceded by
``export`` in the output of ``bitbake -e``. See the following example::

   export CC="i586-poky-linux-gcc -m32 -march=i586 --sysroot=/home/ulf/poky/build/tmp/sysroots/qemux86"

In addition to variable values, the output of the ``bitbake -e`` and
``bitbake -e`` recipe commands includes the following information:

-  The output starts with a tree listing all configuration files and
   classes included globally, recursively listing the files they include
   or inherit in turn. Much of the behavior of the OpenEmbedded build
   system (including the behavior of the :ref:`ref-manual/tasks:normal recipe build tasks`) is
   implemented in the :ref:`ref-classes-base` class and the
   classes it inherits, rather than being built into BitBake itself.

-  After the variable values, all functions appear in the output. For
   shell functions, variables referenced within the function body are
   expanded. If a function has been modified using overrides or using
   override-style operators like ``:append`` and ``:prepend``, then the
   final assembled function body appears in the output.

Viewing Package Information with ``oe-pkgdata-util``
====================================================

You can use the ``oe-pkgdata-util`` command-line utility to query
:term:`PKGDATA_DIR` and display
various package-related information. When you use the utility, you must
use it to view information on packages that have already been built.

Here are a few of the available ``oe-pkgdata-util`` subcommands.

.. note::

   You can use the standard \* and ? globbing wildcards as part of
   package names and paths.

-  ``oe-pkgdata-util list-pkgs [pattern]``: Lists all packages
   that have been built, optionally limiting the match to packages that
   match pattern.

-  ``oe-pkgdata-util list-pkg-files package ...``: Lists the
   files and directories contained in the given packages.

   .. note::

      A different way to view the contents of a package is to look at
      the
      ``${``\ :term:`WORKDIR`\ ``}/packages-split``
      directory of the recipe that generates the package. This directory
      is created by the
      :ref:`ref-tasks-package` task
      and has one subdirectory for each package the recipe generates,
      which contains the files stored in that package.

      If you want to inspect the ``${WORKDIR}/packages-split``
      directory, make sure that :ref:`ref-classes-rm-work` is not
      enabled when you build the recipe.

-  ``oe-pkgdata-util find-path path ...``: Lists the names of
   the packages that contain the given paths. For example, the following
   tells us that ``/usr/share/man/man1/make.1`` is contained in the
   ``make-doc`` package::

      $ oe-pkgdata-util find-path /usr/share/man/man1/make.1
      make-doc: /usr/share/man/man1/make.1

-  ``oe-pkgdata-util lookup-recipe package ...``: Lists the name
   of the recipes that produce the given packages.

For more information on the ``oe-pkgdata-util`` command, use the help
facility::

   $ oe-pkgdata-util --help
   $ oe-pkgdata-util subcommand --help

Viewing Dependencies Between Recipes and Tasks
==============================================

Sometimes it can be hard to see why BitBake wants to build other recipes
before the one you have specified. Dependency information can help you
understand why a recipe is built.

To generate dependency information for a recipe, run the following
command::

   $ bitbake -g recipename

This command writes the following files in the current directory:

-  ``pn-buildlist``: A list of recipes/targets involved in building
   `recipename`. "Involved" here means that at least one task from the
   recipe needs to run when building `recipename` from scratch. Targets
   that are in
   :term:`ASSUME_PROVIDED`
   are not listed.

-  ``task-depends.dot``: A graph showing dependencies between tasks.

The graphs are in :wikipedia:`DOT <DOT_%28graph_description_language%29>`
format and can be converted to images (e.g. using the ``dot`` tool from
`Graphviz <https://www.graphviz.org/>`__).

.. note::

   -  DOT files use a plain text format. The graphs generated using the
      ``bitbake -g`` command are often so large as to be difficult to
      read without special pruning (e.g. with BitBake's ``-I`` option)
      and processing. Despite the form and size of the graphs, the
      corresponding ``.dot`` files can still be possible to read and
      provide useful information.

      As an example, the ``task-depends.dot`` file contains lines such
      as the following::

         "libxslt.do_configure" -> "libxml2.do_populate_sysroot"

      The above example line reveals that the
      :ref:`ref-tasks-configure`
      task in ``libxslt`` depends on the
      :ref:`ref-tasks-populate_sysroot`
      task in ``libxml2``, which is a normal
      :term:`DEPENDS` dependency
      between the two recipes.

   -  For an example of how ``.dot`` files can be processed, see the
      ``scripts/contrib/graph-tool`` Python script, which finds and
      displays paths between graph nodes.

You can use a different method to view dependency information by using
either::

   $ bitbake -g -u taskexp recipename

or::

   $ bitbake -g -u taskexp_ncurses recipename

The ``-u taskdep`` option GUI window from which you can view build-time and
runtime dependencies for the recipes involved in building recipename. The
``-u taskexp_ncurses`` option uses ncurses instead of GTK to render the UI.

Viewing Task Variable Dependencies
==================================

As mentioned in the
":ref:`bitbake-user-manual/bitbake-user-manual-execution:checksums (signatures)`"
section of the BitBake User Manual, BitBake tries to automatically determine
what variables a task depends on so that it can rerun the task if any values of
the variables change. This determination is usually reliable. However, if you
do things like construct variable names at runtime, then you might have to
manually declare dependencies on those variables using ``vardeps`` as described
in the ":ref:`bitbake-user-manual/bitbake-user-manual-metadata:variable flags`"
section of the BitBake User Manual.

If you are unsure whether a variable dependency is being picked up
automatically for a given task, you can list the variable dependencies
BitBake has determined by doing the following:

#. Build the recipe containing the task::

   $ bitbake recipename

#. Inside the :term:`STAMPS_DIR`
   directory, find the signature data (``sigdata``) file that
   corresponds to the task. The ``sigdata`` files contain a pickled
   Python database of all the metadata that went into creating the input
   checksum for the task. As an example, for the
   :ref:`ref-tasks-fetch` task of the
   ``db`` recipe, the ``sigdata`` file might be found in the following
   location::

      ${BUILDDIR}/tmp/stamps/i586-poky-linux/db/6.0.30-r1.do_fetch.sigdata.7c048c18222b16ff0bcee2000ef648b1

   For tasks that are accelerated through the shared state
   (:ref:`sstate <overview-manual/concepts:shared state cache>`) cache, an
   additional ``siginfo`` file is written into
   :term:`SSTATE_DIR` along with
   the cached task output. The ``siginfo`` files contain exactly the
   same information as ``sigdata`` files.

#. Run ``bitbake-dumpsig`` on the ``sigdata`` or ``siginfo`` file. Here
   is an example::

      $ bitbake-dumpsig ${BUILDDIR}/tmp/stamps/i586-poky-linux/db/6.0.30-r1.do_fetch.sigdata.7c048c18222b16ff0bcee2000ef648b1

   In the output of the above command, you will find a line like the
   following, which lists all the (inferred) variable dependencies for
   the task. This list also includes indirect dependencies from
   variables depending on other variables, recursively::

      Task dependencies: ['PV', 'SRCREV', 'SRC_URI', 'SRC_URI[sha256sum]', 'base_do_fetch']

   .. note::

      Functions (e.g. ``base_do_fetch``) also count as variable dependencies.
      These functions in turn depend on the variables they reference.

   The output of ``bitbake-dumpsig`` also includes the value each
   variable had, a list of dependencies for each variable, and
   :term:`BB_BASEHASH_IGNORE_VARS`
   information.

Debugging signature construction and unexpected task executions
===============================================================

There is a ``bitbake-diffsigs`` command for comparing two
``siginfo`` or ``sigdata`` files. This command can be helpful when
trying to figure out what changed between two versions of a task. If you
call ``bitbake-diffsigs`` with just one file, the command behaves like
``bitbake-dumpsig``.

You can also use BitBake to dump out the signature construction
information without executing tasks by using either of the following
BitBake command-line options::

   ‐‐dump-signatures=SIGNATURE_HANDLER
   -S SIGNATURE_HANDLER


.. note::

   Two common values for `SIGNATURE_HANDLER` are "none" and "printdiff", which
   dump only the signature or compare the dumped signature with the most recent one,
   respectively. "printdiff" will try to establish the most recent
   signature match (e.g. in the sstate cache) and then
   compare the matched signatures to determine the stamps and delta
   where these two stamp trees diverge. This can be used to determine why
   tasks need to be re-run in situations where that is not expected.

Using BitBake with either of these options causes BitBake to dump out
``sigdata`` files in the ``stamps`` directory for every task it would
have executed instead of building the specified target package.

Viewing Metadata Used to Create the Input Signature of a Shared State Task
==========================================================================

Seeing what metadata went into creating the input signature of a shared
state (sstate) task can be a useful debugging aid. This information is
available in signature information (``siginfo``) files in
:term:`SSTATE_DIR`. For
information on how to view and interpret information in ``siginfo``
files, see the
":ref:`dev-manual/debugging:viewing task variable dependencies`" section.

For conceptual information on shared state, see the
":ref:`overview-manual/concepts:shared state`"
section in the Yocto Project Overview and Concepts Manual.

Invalidating Shared State to Force a Task to Run
================================================

The OpenEmbedded build system uses
:ref:`checksums <overview-manual/concepts:checksums (signatures)>` and
:ref:`overview-manual/concepts:shared state` cache to avoid unnecessarily
rebuilding tasks. Collectively, this scheme is known as "shared state
code".

As with all schemes, this one has some drawbacks. It is possible that
you could make implicit changes to your code that the checksum
calculations do not take into account. These implicit changes affect a
task's output but do not trigger the shared state code into rebuilding a
recipe. Consider an example during which a tool changes its output.
Assume that the output of ``rpmdeps`` changes. The result of the change
should be that all the ``package`` and ``package_write_rpm`` shared
state cache items become invalid. However, because the change to the
output is external to the code and therefore implicit, the associated
shared state cache items do not become invalidated. In this case, the
build process uses the cached items rather than running the task again.
Obviously, these types of implicit changes can cause problems.

To avoid these problems during the build, you need to understand the
effects of any changes you make. Realize that changes you make directly
to a function are automatically factored into the checksum calculation.
Thus, these explicit changes invalidate the associated area of shared
state cache. However, you need to be aware of any implicit changes that
are not obvious changes to the code and could affect the output of a
given task.

When you identify an implicit change, you can easily take steps to
invalidate the cache and force the tasks to run. The steps you can take
are as simple as changing a function's comments in the source code. For
example, to invalidate package shared state files, change the comment
statements of
:ref:`ref-tasks-package` or the
comments of one of the functions it calls. Even though the change is
purely cosmetic, it causes the checksum to be recalculated and forces
the build system to run the task again.

.. note::

   For an example of a commit that makes a cosmetic change to invalidate
   shared state, see this
   :yocto_git:`commit </poky/commit/meta/classes/package.bbclass?id=737f8bbb4f27b4837047cb9b4fbfe01dfde36d54>`.

Running Specific Tasks
======================

Any given recipe consists of a set of tasks. The standard BitBake
behavior in most cases is: :ref:`ref-tasks-fetch`, :ref:`ref-tasks-unpack`, :ref:`ref-tasks-patch`,
:ref:`ref-tasks-configure`, :ref:`ref-tasks-compile`, :ref:`ref-tasks-install`, :ref:`ref-tasks-package`,
:ref:`do_package_write_* <ref-tasks-package_write_deb>`, and :ref:`ref-tasks-build`. The default task is
:ref:`ref-tasks-build` and any tasks on which it depends build first. Some tasks,
such as :ref:`ref-tasks-devshell`, are not part of the default build chain. If you
wish to run a task that is not part of the default build chain, you can
use the ``-c`` option in BitBake. Here is an example::

   $ bitbake matchbox-desktop -c devshell

The ``-c`` option respects task dependencies, which means that all other
tasks (including tasks from other recipes) that the specified task
depends on will be run before the task. Even when you manually specify a
task to run with ``-c``, BitBake will only run the task if it considers
it "out of date". See the
":ref:`overview-manual/concepts:stamp files and the rerunning of tasks`"
section in the Yocto Project Overview and Concepts Manual for how
BitBake determines whether a task is "out of date".

If you want to force an up-to-date task to be rerun (e.g. because you
made manual modifications to the recipe's
:term:`WORKDIR` that you want to try
out), then you can use the ``-f`` option.

.. note::

   The reason ``-f`` is never required when running the
   :ref:`ref-tasks-devshell` task is because the
   [\ :ref:`nostamp <bitbake-user-manual/bitbake-user-manual-metadata:variable flags>`\ ]
   variable flag is already set for the task.

The following example shows one way you can use the ``-f`` option::

   $ bitbake matchbox-desktop
             .
             .
   make some changes to the source code in the work directory
             .
             .
   $ bitbake matchbox-desktop -c compile -f
   $ bitbake matchbox-desktop

This sequence first builds and then recompiles ``matchbox-desktop``. The
last command reruns all tasks (basically the packaging tasks) after the
compile. BitBake recognizes that the :ref:`ref-tasks-compile` task was rerun and
therefore understands that the other tasks also need to be run again.

Another, shorter way to rerun a task and all
:ref:`ref-manual/tasks:normal recipe build tasks`
that depend on it is to use the ``-C`` option.

.. note::

   This option is upper-cased and is separate from the ``-c``
   option, which is lower-cased.

Using this option invalidates the given task and then runs the
:ref:`ref-tasks-build` task, which is
the default task if no task is given, and the tasks on which it depends.
You could replace the final two commands in the previous example with
the following single command::

   $ bitbake matchbox-desktop -C compile

Internally, the ``-f`` and ``-C`` options work by tainting (modifying)
the input checksum of the specified task. This tainting indirectly
causes the task and its dependent tasks to be rerun through the normal
task dependency mechanisms.

.. note::

   BitBake explicitly keeps track of which tasks have been tainted in
   this fashion, and will print warnings such as the following for
   builds involving such tasks:

   .. code-block:: none

      WARNING: /home/ulf/poky/meta/recipes-sato/matchbox-desktop/matchbox-desktop_2.1.bb.do_compile is tainted from a forced run


   The purpose of the warning is to let you know that the work directory
   and build output might not be in the clean state they would be in for
   a "normal" build, depending on what actions you took. To get rid of
   such warnings, you can remove the work directory and rebuild the
   recipe, as follows::

      $ bitbake matchbox-desktop -c clean
      $ bitbake matchbox-desktop


You can view a list of tasks in a given package by running the
:ref:`ref-tasks-listtasks` task as follows::

   $ bitbake matchbox-desktop -c listtasks

The results appear as output to the console and are also in
the file ``${WORKDIR}/temp/log.do_listtasks``.

General BitBake Problems
========================

You can see debug output from BitBake by using the ``-D`` option. The
debug output gives more information about what BitBake is doing and the
reason behind it. Each ``-D`` option you use increases the logging
level. The most common usage is ``-DDD``.

The output from ``bitbake -DDD -v targetname`` can reveal why BitBake
chose a certain version of a package or why BitBake picked a certain
provider. This command could also help you in a situation where you
think BitBake did something unexpected.

Building with No Dependencies
=============================

To build a specific recipe (``.bb`` file), you can use the following
command form::

   $ bitbake -b somepath/somerecipe.bb

This command form does
not check for dependencies. Consequently, you should use it only when
you know existing dependencies have been met.

.. note::

   You can also specify fragments of the filename. In this case, BitBake
   checks for a unique match.

Recipe Logging Mechanisms
=========================

The Yocto Project provides several logging functions for producing
debugging output and reporting errors and warnings. For Python
functions, the following logging functions are available. All of these functions
log to ``${T}/log.do_``\ `task`, and can also log to standard output
(stdout) with the right settings:

-  ``bb.plain(msg)``: Writes msg as is to the log while also
   logging to stdout.

-  ``bb.note(msg)``: Writes "NOTE: msg" to the log. Also logs to
   stdout if BitBake is called with "-v".

-  ``bb.debug(level, msg)``: Writes "DEBUG: msg" to the log. Also logs to
   stdout if the log level is greater than or equal to level. See the
   ":ref:`bitbake-user-manual/bitbake-user-manual-intro:usage and syntax`"
   option in the BitBake User Manual for more information.

-  ``bb.warn(msg)``: Writes "WARNING: msg" to the log while also
   logging to stdout.

-  ``bb.error(msg)``: Writes "ERROR: msg" to the log while also
   logging to standard out (stdout).

   .. note::

      Calling this function does not cause the task to fail.

-  ``bb.fatal(msg)``: This logging function is similar to
   ``bb.error(msg)`` but also causes the calling task to fail.

   .. note::

      ``bb.fatal()`` raises an exception, which means you do not need to put a
      "return" statement after the function.

The same logging functions are also available in shell functions, under
the names ``bbplain``, ``bbnote``, ``bbdebug``, ``bbwarn``, ``bberror``,
and ``bbfatal``. The :ref:`ref-classes-logging` class
implements these functions. See that class in the ``meta/classes``
folder of the :term:`Source Directory` for information.

Logging With Python
-------------------

When creating recipes using Python and inserting code that handles build
logs, keep in mind the goal is to have informative logs while keeping
the console as "silent" as possible. Also, if you want status messages
in the log, use the "debug" loglevel.

Here is an example written in Python. The code handles logging for
a function that determines the number of tasks needed to be run. See the
":ref:`ref-tasks-listtasks`"
section for additional information::

   python do_listtasks() {
       bb.debug(2, "Starting to figure out the task list")
       if noteworthy_condition:
           bb.note("There are 47 tasks to run")
       bb.debug(2, "Got to point xyz")
       if warning_trigger:
           bb.warn("Detected warning_trigger, this might be a problem later.")
       if recoverable_error:
           bb.error("Hit recoverable_error, you really need to fix this!")
       if fatal_error:
           bb.fatal("fatal_error detected, unable to print the task list")
       bb.plain("The tasks present are abc")
       bb.debug(2, "Finished figuring out the tasklist")
   }

Logging With Bash
-----------------

When creating recipes using Bash and inserting code that handles build
logs, you have the same goals --- informative with minimal console output.
The syntax you use for recipes written in Bash is similar to that of
recipes written in Python described in the previous section.

Here is an example written in Bash. The code logs the progress of
the ``do_my_function`` function::

   do_my_function() {
       bbdebug 2 "Running do_my_function"
       if [ exceptional_condition ]; then
           bbnote "Hit exceptional_condition"
       fi
       bbdebug 2  "Got to point xyz"
       if [ warning_trigger ]; then
           bbwarn "Detected warning_trigger, this might cause a problem later."
       fi
       if [ recoverable_error ]; then
           bberror "Hit recoverable_error, correcting"
       fi
       if [ fatal_error ]; then
           bbfatal "fatal_error detected"
       fi
       bbdebug 2 "Completed do_my_function"
   }


Debugging Parallel Make Races
=============================

A parallel ``make`` race occurs when the build consists of several parts
that are run simultaneously and a situation occurs when the output or
result of one part is not ready for use with a different part of the
build that depends on that output. Parallel make races are annoying and
can sometimes be difficult to reproduce and fix. However, there are some simple
tips and tricks that can help you debug and fix them. This section
presents a real-world example of an error encountered on the Yocto
Project autobuilder and the process used to fix it.

.. note::

   If you cannot properly fix a ``make`` race condition, you can work around it
   by clearing either the :term:`PARALLEL_MAKE` or :term:`PARALLEL_MAKEINST`
   variables.

The Failure
-----------

For this example, assume that you are building an image that depends on
the "neard" package. And, during the build, BitBake runs into problems
and creates the following output.

.. note::

   This example log file has longer lines artificially broken to make
   the listing easier to read.

If you examine the output or the log file, you see the failure during
``make``:

.. code-block:: none

   | DEBUG: SITE files ['endian-little', 'bit-32', 'ix86-common', 'common-linux', 'common-glibc', 'i586-linux', 'common']
   | DEBUG: Executing shell function do_compile
   | NOTE: make -j 16
   | make --no-print-directory all-am
   | /bin/mkdir -p include/near
   | /bin/mkdir -p include/near
   | /bin/mkdir -p include/near
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/types.h include/near/types.h
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/log.h include/near/log.h
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/plugin.h include/near/plugin.h
   | /bin/mkdir -p include/near
   | /bin/mkdir -p include/near
   | /bin/mkdir -p include/near
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/tag.h include/near/tag.h
   | /bin/mkdir -p include/near
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/adapter.h include/near/adapter.h
   | /bin/mkdir -p include/near
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/ndef.h include/near/ndef.h
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/tlv.h include/near/tlv.h
   | /bin/mkdir -p include/near
   | /bin/mkdir -p include/near
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/setting.h include/near/setting.h
   | /bin/mkdir -p include/near
   | /bin/mkdir -p include/near
   | /bin/mkdir -p include/near
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/device.h include/near/device.h
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/nfc_copy.h include/near/nfc_copy.h
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/snep.h include/near/snep.h
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/version.h include/near/version.h
   | ln -s /home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/work/i586-poky-linux/neard/
     0.14-r0/neard-0.14/include/dbus.h include/near/dbus.h
   | ./src/genbuiltin nfctype1 nfctype2 nfctype3 nfctype4 p2p > src/builtin.h
   | i586-poky-linux-gcc  -m32 -march=i586 --sysroot=/home/pokybuild/yocto-autobuilder/nightly-x86/
     build/build/tmp/sysroots/qemux86 -DHAVE_CONFIG_H -I. -I./include -I./src -I./gdbus  -I/home/pokybuild/
     yocto-autobuilder/nightly-x86/build/build/tmp/sysroots/qemux86/usr/include/glib-2.0
     -I/home/pokybuild/yocto-autobuilder/nightly-x86/build/build/tmp/sysroots/qemux86/usr/
     lib/glib-2.0/include  -I/home/pokybuild/yocto-autobuilder/nightly-x86/build/build/
     tmp/sysroots/qemux86/usr/include/dbus-1.0 -I/home/pokybuild/yocto-autobuilder/
     nightly-x86/build/build/tmp/sysroots/qemux86/usr/lib/dbus-1.0/include  -I/home/pokybuild/yocto-autobuilder/
     nightly-x86/build/build/tmp/sysroots/qemux86/usr/include/libnl3
     -DNEAR_PLUGIN_BUILTIN -DPLUGINDIR=\""/usr/lib/near/plugins"\"
     -DCONFIGDIR=\""/etc/neard\"" -O2 -pipe -g -feliminate-unused-debug-types -c
     -o tools/snep-send.o tools/snep-send.c
   | In file included from tools/snep-send.c:16:0:
   | tools/../src/near.h:41:23: fatal error: near/dbus.h: No such file or directory
   |  #include <near/dbus.h>
   |                        ^
   | compilation terminated.
   | make[1]: *** [tools/snep-send.o] Error 1
   | make[1]: *** Waiting for unfinished jobs....
   | make: *** [all] Error 2
   | ERROR: oe_runmake failed

Reproducing the Error
---------------------

Because race conditions are intermittent, they do not manifest
themselves every time you do the build. In fact, most times the build
will complete without problems even though the potential race condition
exists. Thus, once the error surfaces, you need a way to reproduce it.

In this example, compiling the "neard" package is causing the problem.
So the first thing to do is build "neard" locally. Before you start the
build, set the
:term:`PARALLEL_MAKE` variable
in your ``local.conf`` file to a high number (e.g. "-j 20"). Using a
high value for :term:`PARALLEL_MAKE` increases the chances of the race
condition showing up::

   $ bitbake neard

Once the local build for "neard" completes, start a ``devshell`` build::

   $ bitbake neard -c devshell

For information on how to use a ``devshell``, see the
":ref:`dev-manual/development-shell:using a development shell`" section.

In the ``devshell``, do the following::

   $ make clean
   $ make tools/snep-send.o

The ``devshell`` commands cause the failure to clearly
be visible. In this case, there is a missing dependency for the ``neard``
Makefile target. Here is some abbreviated, sample output with the
missing dependency clearly visible at the end::

   i586-poky-linux-gcc  -m32 -march=i586 --sysroot=/home/scott-lenovo/......
      .
      .
      .
   tools/snep-send.c
   In file included from tools/snep-send.c:16:0:
   tools/../src/near.h:41:23: fatal error: near/dbus.h: No such file or directory
    #include <near/dbus.h>
                     ^
   compilation terminated.
   make: *** [tools/snep-send.o] Error 1
   $


Creating a Patch for the Fix
----------------------------

Because there is a missing dependency for the Makefile target, you need
to patch the ``Makefile.am`` file, which is generated from
``Makefile.in``. You can use Quilt to create the patch::

   $ quilt new parallelmake.patch
   Patch patches/parallelmake.patch is now on top
   $ quilt add Makefile.am
   File Makefile.am added to patch patches/parallelmake.patch

For more information on using Quilt, see the
":ref:`dev-manual/quilt:using quilt in your workflow`" section.

At this point you need to make the edits to ``Makefile.am`` to add the
missing dependency. For our example, you have to add the following line
to the file::

   tools/snep-send.$(OBJEXT): include/near/dbus.h

Once you have edited the file, use the ``refresh`` command to create the
patch::

   $ quilt refresh
   Refreshed patch patches/parallelmake.patch

Once the patch file is created, you need to add it back to the originating
recipe folder. Here is an example assuming a top-level
:term:`Source Directory` named ``poky``::

   $ cp patches/parallelmake.patch poky/meta/recipes-connectivity/neard/neard

The final thing you need to do to implement the fix in the build is to
update the "neard" recipe (i.e. ``neard-0.14.bb``) so that the
:term:`SRC_URI` statement includes
the patch file. The recipe file is in the folder above the patch. Here
is what the edited :term:`SRC_URI` statement would look like::

   SRC_URI = "${KERNELORG_MIRROR}/linux/network/nfc/${BPN}-${PV}.tar.xz \
              file://neard.in \
              file://neard.service.in \
              file://parallelmake.patch \
             "

With the patch complete and moved to the correct folder and the
:term:`SRC_URI` statement updated, you can exit the ``devshell``::

   $ exit

Testing the Build
-----------------

With everything in place, you can get back to trying the build again
locally::

   $ bitbake neard

This build should succeed.

Now you can open up a ``devshell`` again and repeat the clean and make
operations as follows::

   $ bitbake neard -c devshell
   $ make clean
   $ make tools/snep-send.o

The build should work without issue.

As with all solved problems, if they originated upstream, you need to
submit the fix for the recipe in OE-Core and upstream so that the
problem is taken care of at its source. See the
":doc:`../contributor-guide/submit-changes`" section for more information.

Debugging With the GNU Project Debugger (GDB) Remotely
======================================================

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 is installed in SDK images by default. See the
":ref:`ref-manual/images:Images`" chapter in the Yocto
Project Reference Manual for a description of these images. You can find
information on GDB at https://sourceware.org/gdb/.

.. note::

   For best results, install debug (``-dbg``) packages for the applications you
   are going to debug. Doing so makes extra debug symbols available that give
   you more meaningful output.

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.

To help get past the previously mentioned constraints, there are two
methods you can use: running a debuginfod server and using gdbserver.

Using the debuginfod server method
----------------------------------

``debuginfod`` from ``elfutils`` is a way to distribute ``debuginfo`` files.
Running a ``debuginfod`` server makes debug symbols readily available,
which means you don't need to download debugging information
and the binaries of the process being debugged. You can just fetch
debug symbols from the server.

To run a ``debuginfod`` server, you need to do the following:

-  Ensure that ``debuginfod`` is present in :term:`DISTRO_FEATURES`
   (it already is in ``OpenEmbedded-core`` defaults and ``poky`` reference distribution).
   If not, set in your distro config file or in ``local.conf``::

      DISTRO_FEATURES:append = " debuginfod"

   This distro feature enables the server and client library in ``elfutils``,
   and enables ``debuginfod`` support in clients (at the moment, ``gdb`` and ``binutils``).

-  Run the following commands to launch the ``debuginfod`` server on the host::

      $ oe-debuginfod

-  To use ``debuginfod`` on the target, you need to know the ip:port where
   ``debuginfod`` is listening on the host (port defaults to 8002), and export
   that into the shell environment, for example in ``qemu``::

      root@qemux86-64:~# export DEBUGINFOD_URLS="http://192.168.7.1:8002/"

-  Then debug info fetching should simply work when running the target ``gdb``,
   ``readelf`` or ``objdump``, for example::

      root@qemux86-64:~# gdb /bin/cat
      ...
      Reading symbols from /bin/cat...
      Downloading separate debug info for /bin/cat...
      Reading symbols from /home/root/.cache/debuginfod_client/923dc4780cfbc545850c616bffa884b6b5eaf322/debuginfo...

-  It's also possible to use ``debuginfod-find`` to just query the server::

      root@qemux86-64:~# debuginfod-find debuginfo /bin/ls
      /home/root/.cache/debuginfod_client/356edc585f7f82d46f94fcb87a86a3fe2d2e60bd/debuginfo


Using the gdbserver method
--------------------------

gdbserver, which 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.

Because the host GDB is responsible for loading the debugging
information and for doing the necessary processing to make actual
debugging happen, you have 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.

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 `GDB
site <https://sourceware.org/gdb/documentation/>`__.

The following steps show you how to debug using the GNU project
debugger.

#. *Configure your build system to construct the companion debug
   filesystem:*

   In your ``local.conf`` file, set the following::

      IMAGE_GEN_DEBUGFS = "1"
      IMAGE_FSTYPES_DEBUGFS = "tar.bz2"

   These options cause the
   OpenEmbedded build system to generate a special companion filesystem
   fragment, which contains the matching source and debug symbols to
   your deployable filesystem. The build system does this by looking at
   what is in the deployed filesystem, and pulling the corresponding
   ``-dbg`` packages.

   The companion debug filesystem is not a complete filesystem, but only
   contains the debug fragments. This filesystem must be combined with
   the full filesystem for debugging. Subsequent steps in this procedure
   show how to combine the partial filesystem with the full filesystem.

#. *Configure the system to include gdbserver in the target filesystem:*

   Make the following addition in your ``local.conf`` file::

      EXTRA_IMAGE_FEATURES:append = " tools-debug"

   The change makes
   sure the ``gdbserver`` package is included.

#. *Build the environment:*

   Use the following command to construct the image and the companion
   Debug Filesystem::

      $ bitbake image

   Build the cross GDB component and
   make it available for debugging. Build the SDK that matches the
   image. Building the SDK is best for a production build that can be
   used later for debugging, especially during long term maintenance::

      $ bitbake -c populate_sdk image

   Alternatively, you can build the minimal toolchain components that
   match the target. Doing so creates a smaller than typical SDK and
   only contains a minimal set of components with which to build simple
   test applications, as well as run the debugger::

      $ bitbake meta-toolchain

   A final method is to build Gdb itself within the build system::

      $ bitbake gdb-cross-<architecture>

   Doing so produces a temporary copy of
   ``cross-gdb`` you can use for debugging during development. While
   this is the quickest approach, the two previous methods in this step
   are better when considering long-term maintenance strategies.

   .. note::

      If you run ``bitbake gdb-cross``, the OpenEmbedded build system suggests
      the actual image (e.g. ``gdb-cross-i586``). The suggestion is usually the
      actual name you want to use.

#. *Set up the* ``debugfs``\ *:*

   Run the following commands to set up the ``debugfs``::

      $ mkdir debugfs
      $ cd debugfs
      $ tar xvfj build-dir/tmp/deploy/images/machine/image.rootfs.tar.bz2
      $ tar xvfj build-dir/tmp/deploy/images/machine/image-dbg.rootfs.tar.bz2

#. *Set up GDB:*

   Install the SDK (if you built one) and then source the correct
   environment file. Sourcing the environment file puts the SDK in your
   ``PATH`` environment variable and sets ``$GDB`` to the SDK's debugger.

   If you are using the build system, Gdb is located in
   `build-dir`\ ``/tmp/sysroots/``\ `host`\ ``/usr/bin/``\ `architecture`\ ``/``\ `architecture`\ ``-gdb``

#. *Boot the target:*

   For information on how to run QEMU, see the `QEMU
   Documentation <https://wiki.qemu.org/Documentation/GettingStartedDevelopers>`__.

   .. note::

      Be sure to verify that your host can access the target via TCP.

#. *Debug a program:*

   Debugging a program involves running gdbserver on the target and then
   running Gdb on the host. The example in this step debugs ``gzip``:

   .. code-block:: shell

      root@qemux86:~# gdbserver localhost:1234 /bin/gzip —help

   For
   additional gdbserver options, see the `GDB Server
   Documentation <https://www.gnu.org/software/gdb/documentation/>`__.

   After running gdbserver on the target, you need to run Gdb on the
   host and configure it and connect to the target. Use these commands::

      $ cd directory-holding-the-debugfs-directory
      $ arch-gdb
      (gdb) set sysroot debugfs
      (gdb) set substitute-path /usr/src/debug debugfs/usr/src/debug
      (gdb) target remote IP-of-target:1234

   At this
   point, everything should automatically load (i.e. matching binaries,
   symbols and headers).

   .. note::

      The Gdb ``set`` commands in the previous example can be placed into the
      users ``~/.gdbinit`` file. Upon starting, Gdb automatically runs whatever
      commands are in that file.

#. *Deploying without a full image rebuild:*

   In many cases, during development you want a quick method to deploy a
   new binary to the target and debug it, without waiting for a full
   image build.

   One approach to solving this situation is to just build the component
   you want to debug. Once you have built the component, copy the
   executable directly to both the target and the host ``debugfs``.

   If the binary is processed through the debug splitting in
   OpenEmbedded, you should also copy the debug items (i.e. ``.debug``
   contents and corresponding ``/usr/src/debug`` files) from the work
   directory. Here is an example::

      $ bitbake bash
      $ bitbake -c devshell bash
      $ cd ..
      $ scp packages-split/bash/bin/bash target:/bin/bash
      $ cp -a packages-split/bash-dbg/\* path/debugfs

Debugging with the GNU Project Debugger (GDB) on the Target
===========================================================

The previous section addressed using GDB remotely for debugging
purposes, which is the most usual case due to the inherent hardware
limitations on many embedded devices. However, debugging in the target
hardware itself is also possible with more powerful devices. This
section describes what you need to do in order to support using GDB to
debug on the target hardware.

To support this kind of debugging, you need do the following:

-  Ensure that GDB is on the target. You can do this by making
   the following addition to your ``local.conf`` file::

      EXTRA_IMAGE_FEATURES:append = " tools-debug"

-  Ensure that debug symbols are present. You can do so by adding the
   corresponding ``-dbg`` package to :term:`IMAGE_INSTALL`::

      IMAGE_INSTALL:append = " packagename-dbg"

   Alternatively, you can add the following to ``local.conf`` to include
   all the debug symbols::

      EXTRA_IMAGE_FEATURES:append = " dbg-pkgs"

.. note::

   To improve the debug information accuracy, you can reduce the level
   of optimization used by the compiler. For example, when adding the
   following line to your ``local.conf`` file, you will reduce optimization
   from :term:`FULL_OPTIMIZATION` of "-O2" to :term:`DEBUG_OPTIMIZATION`
   of "-O -fno-omit-frame-pointer"::

           DEBUG_BUILD = "1"

   Consider that this will reduce the application's performance and is
   recommended only for debugging purposes.

Enabling Minidebuginfo
======================

Enabling the :term:`DISTRO_FEATURES` minidebuginfo adds a compressed ELF section ``.gnu_debugdata``
to all binary files, containing only function names, and thus increasing the size of the
binaries only by 5 to 10%. For comparison, full debug symbols can be 10 times as big as
a stripped binary, and it is thus not always possible to deploy full debug symbols.
Minidebuginfo data allows, on the one side, to retrieve a call-stack using
GDB (command backtrace) without deploying full debug symbols to the target. It also
allows to retrieve a symbolicated call-stack when using ``systemd-coredump`` to manage
coredumps (commands ``coredumpctl list`` and ``coredumpctl info``).

This feature was created by Fedora, see https://fedoraproject.org/wiki/Features/MiniDebugInfo for
more details.

Other Debugging Tips
====================

Here are some other tips that you might find useful:

-  When adding new packages, it is worth watching for undesirable items
   making their way into compiler command lines. For example, you do not
   want references to local system files like ``/usr/lib/`` or
   ``/usr/include/``.

-  If you want to remove the ``psplash`` boot splashscreen, add
   ``psplash=false`` to the kernel command line. Doing so prevents
   ``psplash`` from loading and thus allows you to see the console. It
   is also possible to switch out of the splashscreen by switching the
   virtual console (e.g. Fn+Left or Fn+Right on a Zaurus).

-  Removing :term:`TMPDIR` (usually ``tmp/``, within the
   :term:`Build Directory`) can often fix temporary build issues. Removing
   :term:`TMPDIR` is usually a relatively cheap operation, because task output
   will be cached in :term:`SSTATE_DIR` (usually ``sstate-cache/``, which is
   also in the :term:`Build Directory`).

   .. note::

      Removing :term:`TMPDIR` might be a workaround rather than a fix.
      Consequently, trying to determine the underlying cause of an issue before
      removing the directory is a good idea.

-  Understanding how a feature is used in practice within existing
   recipes can be very helpful. It is recommended that you configure
   some method that allows you to quickly search through files.

   Using GNU Grep, you can use the following shell function to
   recursively search through common recipe-related files, skipping
   binary files, ``.git`` directories, and the :term:`Build Directory`
   (assuming its name starts with "build")::

      g() {
          grep -Ir \
               --exclude-dir=.git \
               --exclude-dir='build*' \
               --include='*.bb*' \
               --include='*.inc*' \
               --include='*.conf*' \
               --include='*.py*' \
               "$@"
      }

   Here are some usage examples::

      $ g FOO # Search recursively for "FOO"
      $ g -i foo # Search recursively for "foo", ignoring case
      $ g -w FOO # Search recursively for "FOO" as a word, ignoring e.g. "FOOBAR"

   If figuring
   out how some feature works requires a lot of searching, it might
   indicate that the documentation should be extended or improved. In
   such cases, consider filing a documentation bug using the Yocto
   Project implementation of
   :yocto_bugs:`Bugzilla <>`. For information on
   how to submit a bug against the Yocto Project, see the Yocto Project
   Bugzilla :yocto_wiki:`wiki page </Bugzilla_Configuration_and_Bug_Tracking>`
   and the ":doc:`../contributor-guide/report-defect`" section.

   .. note::

      The manuals might not be the right place to document variables
      that are purely internal and have a limited scope (e.g. internal
      variables used to implement a single ``.bbclass`` file).