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Package Override

This guide demonstrates how the local.mk file is utilized to override a Linux Buildroot package. As an example we use tcpdump to illustrate this process.

For a comprehensive guide to utilizing Buildroot during development, including the <pkg>_OVERRIDE_SRCDIR mechanism, shown below, please see Using Buildroot during development in the official docs.

Setup

Since the output/ directory is often wiped for rebuilds, make sure you keep the file local.mk in the top directory and only symlink it to your build directory:

~$ cd infix/
~/infix(main)$ touch local.mk
~/infix/output(main)$ ln -s ../local.mk .
~/infix/output(main)$ cd ..

Override

Now edit the file:

~/infix(main)$ editor local.mk

Add an override for tcpdump. The file is a Makefile snippet so you can add a lot of things. Comment out lines with the UNIX comment # character if needed:

TCPDUMP_OVERRIDE_SRCDIR = /path/to/tcpdump/repo

Building

The execution of make tcpdump-rebuild all triggers a process where Buildroot synchronizes the tcpdump source code from the specified override directory to output/build/tcpdump-custom, followed by the rebuilding of the entire project.

~/infix$(main)$ make tcpdump-rebuild all

Buildroot follows a process of downloading and processing tarballs: extraction, configuration, compilation, and installation. The source for each package is extracted to a temporary build directory:

output/build/<package>-<version>    # e.g., tcpdump-4.99.4

Let's have a look at what we got:

~/infix$(main)$ ll /output/build/ | grep tcpdump
drwxr-xr-x   7 group user 20480 Nov 10 18:26 tcpdump-4.99.4/
drwxr-xr-x   7 group user 12288 Nov 10 18:28 tcpdump-custom/

As long as your local override is in place, Buildroot will use your custom version.

Remember: the build directory is ephemeral, so be careful to change any of the files therein. It can be useful though during debugging, but just make sure to learn the difference between the various Buildroot commands to build, clean, reconfigure, etc.