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

Question

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?

Answer 1

Your program can shell out to git describe, either at runtime or as part of the build process.

Answer 2

What you need to do is to generate a header file (eg using echo from cmd line) something like this:

#define GIT_HASH \
"098709a0b098c098d0e"

To generate it use something like this:

echo #define GIT_HASH \ > file.h
echo " > file.h
echo git status <whatever cmd to get the hash> > file.h
echo " > file.h

Might need to play with the quotes and backslashes a bit to get it to compile, but you get the idea.

Answer 3

When I need to do this, I use a tag, like RELEASE_1_23. I can decide what the tag can be without knowing the SHA-1. I commit then tag. You can store that tag in your program anyway that you like.

Answer 4

You can see how I did it for memcached in the original commit.

Basically, tag occasionally, and make sure the thing you deliver comes from make dist or similar.

Answer 5

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.

Answer 6

There are two things that you can do:

• You can make Git to embed some version information in the file for you.

  The simpler way is to use ident attribute, which means putting (for example)

  *.yaml    ident
  

  in .gitattributes file, and $Id$ in the appropriate place. It would be automatically expanded to SHA-1 identifier of the contents of the file (blob id): this is NOT file version, or the last commit.

  Git does support $Id$ keyword in this way to avoid touching files which were not changed during branch switching, rewinding branch etc. If you really want Git to put commit (version) identifier or description in the file, you can (ab)use filter attribute, using clean/ smudge filter to expand some keyword (e.g. $Revision$) on checkout, and clean it up for commit.

• You can make build process to do that for you, like Linux kernel or Git itself does.

  Take a look at GIT-VERSION-GEN script and its use in Git Makefile, or for example how this Makefile embeds version information during generation / configuration of gitweb/gitweb.cgi file.

  GIT-VERSION-GEN uses git describe to generate version description. It needs to work better that you tag (using signed / annotated tags) releases / milestones of your project.

Answer 7

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!