infix/doc/override-package.md

Package override

This guide demonstrates how the local.mk file is utilized to override a Linux Buildroot package. The example of tcpdump serves to illustrate this process.

In an instance such as Infix using tcpdump, the local.mk file is modified as shown below:

TCPDUMP_OVERRIDE_SRCDIR = /path/to/tcpdump/repo

and stored in the output/ folder, alongside the .config file:

user@PC:~/infix$ll output/ | grep -e 'local.mk' -e '.config'
-rw-r--r--   1 group user 119936 Nov 10 18:04 .config
-rw-r--r--   1 group user     43 Nov 10 18:25 local.mk

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.

user@PC:~/infix$ make tcpdump-rebuild all

user@PC:~/infix$ 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/

Buildroot follows a process of downloading and processing tarballs (extraction, configuration, compilation, and installation). The source code is stored in a temporary directory: output/build/<package>-<version> (i.e. tcpdump-4.99.4/), which is removed and recreated with each make command. That is why the direct modifications in the output/build directory are generally not recommended.

To manage the development changes more effectively, where the package source code remains untouched, Buildroot incorporates the <pkg>_OVERRIDE_SRCDIR feature.

For a comprehensive understanding of utilizing Buildroot during development, including detailed elaboration on the <pkg>_OVERRIDE_SRCDIR feature, refer to section 8.13.6 in Using Buildroot during development.