Patching a package

While integrating a new package or updating an existing one, it may be necessary to patch the source of the software to get it cross-built within Buildroot.

Buildroot offers an infrastructure to automatically handle this during the builds. It supports three ways of applying patch sets: downloaded patches, patches supplied within buildroot and patches located in a user-defined global patch directory.

Providing patches

Downloaded

If it is necessary to apply a patch that is available for download, then add it to the <packagename>_PATCH variable. If an entry contains ://, then Buildroot will assume it is a full URL and download the patch from this location. Otherwise, Buildroot will assume that the patch should be downloaded from <packagename>_SITE. It can be a single patch, or a tarball containing a patch series.

Like for all downloads, a hash should be added to the <packagename>.hash file.

This method is typically used for packages from Debian.

Within Buildroot

Most patches are provided within Buildroot, in the package directory; these typically aim to fix cross-compilation, libc support, or other such issues.

These patch files should be named <number>-<description>.patch.

Notes

  • The patch files coming with Buildroot should not contain any package version reference in their filename.

  • The field <number> in the patch file name refers to the 'apply order', and shall start at 1; It is preferred to pad the number with zeros up to 4 digits, like 'git-format-patch' does. E.g.: 0001-foobar-the-buz.patch

  • Previously, it was mandatory for patches to be prefixed with the name of the package, like <package>-<number>-<description>.patch, but that is no longer the case. Existing packages will be fixed as time passes. 'Do not prefix patches with the package name.'

  • Previously, a series file, as used by quilt, could also be added in the package directory. In that case, the series file defines the patch application order. This is deprecated, and will be removed in the future. 'Do not use a series file.'

Global patch directory

The BR2_GLOBAL_PATCH_DIR configuration file option can be used to specify a space separated list of one or more directories containing global package patches. See [customize-patches] for details.

How patches are applied

  1. Run the <packagename>_PRE_PATCH_HOOKS commands if defined;

  2. Cleanup the build directory, removing any existing *.rej files;

  3. If <packagename>_PATCH is defined, then patches from these tarballs are applied;

  4. If there are some *.patch files in the package’s Buildroot directory or in a package subdirectory named <packageversion>, then:

    • If a series file exists in the package directory, then patches are applied according to the series file;

    • Otherwise, patch files matching *.patch are applied in alphabetical order. So, to ensure they are applied in the right order, it is highly recommended to name the patch files like this: <number>-<description>.patch, where <number> refers to the 'apply order'.

  5. If BR2_GLOBAL_PATCH_DIR is defined, the directories will be enumerated in the order they are specified. The patches are applied as described in the previous step.

  6. Run the <packagename>_POST_PATCH_HOOKS commands if defined.

If something goes wrong in the steps 3 or 4, then the build fails.

Format and licensing of the package patches

Patches are released under the same license as the software they apply to (see [legal-info-buildroot]).

A message explaining what the patch does, and why it is needed, should be added in the header commentary of the patch.

You should add a Signed-off-by statement in the header of the each patch to help with keeping track of the changes and to certify that the patch is released under the same license as the software that is modified.

If the software is under version control, it is recommended to use the upstream SCM software to generate the patch set.

Otherwise, concatenate the header with the output of the diff -purN package-version.orig/ package-version/ command.

If you update an existing patch (e.g. when bumping the package version), make sure the existing From header and Signed-off-by tags are not removed, but do update the rest of the patch comment when appropriate.

At the end, the patch should look like:

configure.ac: add C`` support test

Signed-off-by: John Doe <john.doe@noname.org>

--- configure.ac.orig
``+ configure.ac
@@ -40,2 +40,12 @@

AC_PROG_MAKE_SET
+
`AC_CACHE_CHECK([whether the C`+ compiler works],
+               [rw_cv_prog_cxx_works],
`               [AC_LANG_PUSH([C`+])
+                AC_LINK_IFELSE([AC_LANG_PROGRAM([], [])],
+                               [rw_cv_prog_cxx_works=yes],
+                               [rw_cv_prog_cxx_works=no])
`                AC_LANG_POP([C`+])])
+
+AM_CONDITIONAL([CXX_WORKS], [test "x$rw_cv_prog_cxx_works" = "xyes"])

Integrating patches found on the Web

When integrating a patch of which you are not the author, you have to add a few things in the header of the patch itself.

Depending on whether the patch has been obtained from the project repository itself, or from somewhere on the web, add one of the following tags:

Backported from: <some commit id>

or

Fetch from: <some url>

It is also sensible to add a few words about any changes to the patch that may have been necessary.