How can I run a custom command at build time in meson? - build

Question

in my projects I adopt a semantic versioning scheme following the standard described by semver. I obtain something like this: product_v1.2.3-alpha-dirty.elf .
I work with embedded system and with make I usually generate a version_autogen.h file at compile time that contains both information of the version number , e.g 1.4.3.1, and current git repository, e.g --dirty, --clean and so on, using shell commands.
I'm starting to using meson and it is very easy and flexible but the custom commands like
run_command('command', 'arg1', 'arg2', 'arg3')
are available only at configure time while I need them at compile time to retrieve information like git status and similar.
How can I do that?

Answer 1

After a deeper research I found that custom_target() (as suggested by nielsdg) can do my job. I did something like this:
# versioning
version_autogen_h = custom_target(
'version_autogen.h',
output : 'version_autogen.h',
input : 'version_creator.sh',
command : ['#INPUT#', '0', '0', '1', 'alpha.1', '#OUTPUT#' ],
)
where version_creator.sh is my bash script that retrieves git info and creates the file version_autogen.h given the version numbers passed as the command arguments. The custom target is created at compile time, so my script is executed at compile time as well, exactly when I want it to be.
I also discovered that in meson there is the possibility to use generators to do something similar to this but in that case they transform an input file in one or more output file, so they didn't fit my case where I didn't need to have a file as input but just versioning numbers.

Answer 2

meson has specialized command for this job - vcs_tag
This command detects revision control commit information at build time
and places it in the specified output file. This file is guaranteed to
be up to date on every build. Keywords are similar to custom_target.
, so it'd look a bit shorter with possibility to avoid generation script and have just
git_version_h = vcs_tag(input : 'version.h.in',
output : 'version.h')
where version.h.in file that you should provide with #VCS_TAG# string that will be replaced, e.g.
#define MYPROJ_VERSION "#VCS_TAG#"
of course, you can have file header and naming according to your project style and possibly to add other definitions also. It is also possible to use another replace string and own command line to generate version, e.g.
vcs_tag(command: [
'git', '--git-dir', meson.build_root(),
'describe', '--tags', '--long',
'--match', '?.*.*', '--always'
],
...
)
which I found and adapted from here