How can I get my C code to automatically print out its Git version hash?

Is there an easy way to write C code that can access its Git version hash?

I wrote software in C to collect scientific data in a laboratory setting. My code records the data it collects in a .yaml file for later analysis. My experiments change from day-to-day and I often have to modify the code. To keep track of revisions, I use a git repository.

I would like to be able to include the Git revision hash as a comment in my .yaml data files. That way, I could look at the .yaml file and know exactly what code was used to generate the data shown in that file. Is there an easy way to do this automatically?

Solution 1:

If you are using a make-based build, you can put this in the Makefile:

GIT_VERSION := "$(shell git describe --abbrev=4 --dirty --always --tags)"

(See man git describe for what the switches do)

then add this to your CFLAGS:

-DVERSION=\"$(GIT_VERSION)\"

Then you can just reference the version directly in the program as though it was a #define:

printf("Version: %s\n", VERSION);

By default this just prints an abbreviated git commit id, but optionally you can tag particular releases with something like:

git tag -a v1.1 -m "Release v1.1"

then it will print out:

Version: v1.1-2-g766d

which means, 2 commits past v1.1, with a git commit id beginning with "766d".

If there are uncommitted changes in your tree, it will append "-dirty".

There is no dependency scanning so you have to do an explicit make clean to force the version to be updated. This can be solved however.

The advantages are that it is simple and doesn't require any extra build dependencies like perl or awk. I have used this approach with GNU automake and with Android NDK builds.

Solution 2:

In my program, I hold the git version number and the date of the build in a separate file, called version.c, which looks like this:

#include "version.h"
const char * build_date = "2009-11-10 11:09";
const char * build_git_sha = "6b54ea36e92d4907aba8b3fade7f2d58a921b6cd";

There is also a header file, which looks like this:

#ifndef VERSION_H
#define VERSION_H
extern const char * build_date; /* 2009-11-10 11:09 */
extern const char * build_git_sha; /* 6b54ea36e92d4907aba8b3fade7f2d58a921b6cd */
#endif /* VERSION_H */

Both the header file and the C file are generated by a Perl script which looks like this:

my $git_sha = `git rev-parse HEAD`;
$git_sha =~ s/\s+//g;
# This contains all the build variables.
my %build;
$build{date} = make_date_time ();
$build{git_sha} = $git_sha;

hash_to_c_file ("version.c", \%build, "build_");

Here hash_to_c_file does all the work of creating version.c and version.h and make_date_time makes a string as shown.

In the main program, I have a routine

#include "version.h"

// The name of this program.
const char * program_name = "magikruiser";
// The version of this program.
const char * version = "0.010";

/* Print an ID stamp for the program. */

static void _program_id_stamp (FILE * output)
{
    fprintf (output, "%s / %s / %s / %s\n",
             program_name, version,
             build_date, build_git_sha);
}

I'm not that knowledgeable about git, so I'd welcome comments if there is a better way to do this.

Solution 3:

I ended up using something very similar to @Kinopiko's answer, but I used awk instead of perl. This is useful if your stuck on windows machines which have awk installed by nature of mingw, but not perl. Here's how it works.

My makefile has a line in it that invokes git, date, and awk to create a c file:

$(MyLibs)/version.c: FORCE 
    $(GIT) rev-parse HEAD | awk ' BEGIN {print "#include \"version.h\""} {print "const char * build_git_sha = \"" $$0"\";"} END {}' > $(MyLibs)/version.c
    date | awk 'BEGIN {} {print "const char * build_git_time = \""$$0"\";"} END {} ' >> $(MyLibs)/version.c 

Everytime I compile my code, the awk command generates a version.c file that looks like this:

/* version.c */
#include "version.h"
const char * build_git_sha = "ac5bffc90f0034df9e091a7b3aa12d150df26a0e";
const char * build_git_time = "Thu Dec  3 18:03:58 EST 2009";

I have a static version.h file that looks like this:

/*version.h*/
#ifndef VERSION_H_
#define VERSION_H_

extern const char * build_git_time;
extern const char * build_git_sha;


#endif /* VERSION_H_ */

The rest of my code can now access the build-time and the git hash by simply including the version.h header. To wrap it all up, I tell git to ignore version.c by adding a line to my .gitignore file. This way git isn't constantly giving me merge conflicts. Hope this helps!