Tutorial :How can I pass git SHA1 to compiler as definition using cmake?


In a Makefile this would be done with something like:

g++ -DGIT_SHA1="`git log -1 | head -n 1`" ...  

This is very useful, because the binary knows exact commit SHA1 so it can dump it in case of segfault.

How can I achieve the same with CMake?


I've made some CMake modules that peer into a git repo for versioning and similar purposes - they're all in my repository at https://github.com/rpavlik/cmake-modules

The good thing about these functions is, they will force a re-configure (a rerun of cmake) before a build every time the HEAD commit changes. Unlike doing something just once with execute_process, you don't need to remember to re-cmake to update the hash definition.

For this specific purpose, you'd need at least the GetGitRevisionDescription.cmake and GetGitRevisionDescription.cmake.in files. Then, in your main CMakeLists.txt file, you'd have something like this

list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/whereYouPutMyModules/")  include(GetGitRevisionDescription)  get_git_head_revision(GIT_REFSPEC GIT_SHA1)  

Then, you could either add it as a system-wide definition (which unfortunately would cause lots of rebuilding)


or, my suggested alternative: Make a generated source file. Create these two files in your source:


#define GIT_SHA1 "@GIT_SHA1@"  const char g_GIT_SHA1[] = GIT_SHA1;  


extern const char g_GIT_SHA1[];  

Add this to your CMakeLists.txt (assuming you have a list of source files in SOURCES):


Then, you have a global variable containing your SHA string - the header with the extern doesn't change when the SHA does, so you can just include that any place you want to refer to the string, and then only the generated CPP needs to be recompiled on every commit to give you access to the SHA everywhere.


I did this in such as way as to generate:

const std::string Version::GIT_SHA1 = "e7fb69fb8ee93ac66f006406781138562d0250fb";  const std::string Version::GIT_DATE = "Thu Jan 9 14:17:56 2014";  const std::string Version::GIT_COMMIT_SUBJECT = "Fix all the bugs";  

If the workspace that performed the build had pending, uncommitted changes, the above SHA1 string will be suffixed with -dirty.

In CMakeLists.txt:

# the commit's SHA1, and whether the building workspace was dirty or not  execute_process(COMMAND    "${GIT_EXECUTABLE}" describe --match=NeVeRmAtCh --always --abbrev=40 --dirty    WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"    OUTPUT_VARIABLE GIT_SHA1    ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)    # the date of the commit  execute_process(COMMAND    "${GIT_EXECUTABLE}" log -1 --format=%ad --date=local    WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"    OUTPUT_VARIABLE GIT_DATE    ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)    # the subject of the commit  execute_process(COMMAND    "${GIT_EXECUTABLE}" log -1 --format=%s    WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"    OUTPUT_VARIABLE GIT_COMMIT_SUBJECT    ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)    # generate version.cc  configure_file("${CMAKE_CURRENT_SOURCE_DIR}/version.cc.in" "${CMAKE_CURRENT_BINARY_DIR}/version.cc" @ONLY)    list(APPEND SOURCES "${CMAKE_CURRENT_BINARY_DIR}/version.cc" version.hh)  

This requires version.cc.in:

#include "version.hh"    using namespace my_app;    const std::string Version::GIT_SHA1 = "@GIT_SHA1@";  const std::string Version::GIT_DATE = "@GIT_DATE@";  const std::string Version::GIT_COMMIT_SUBJECT = "@GIT_COMMIT_SUBJECT@";  

And version.hh:

#pragma once    #include <string>    namespace my_app  {    struct Version    {      static const std::string GIT_SHA1;      static const std::string GIT_DATE;      static const std::string GIT_COMMIT_SUBJECT;    };  }  

Then in code I can write:

cout << "Build SHA1: " << Version::GIT_SHA1 << endl;  


I'd use sth. like this in my CMakeLists.txt:

exec_program(      "git"      ${CMAKE_CURRENT_SOURCE_DIR}      ARGS "describe"      OUTPUT_VARIABLE VERSION )    string( REGEX MATCH "-g.*$" VERSION_SHA1 ${VERSION} )  string( REGEX REPLACE "[-g]" "" VERSION_SHA1 ${VERSION_SHA1} )    add_definitions( -DGIT_SHA1="${VERSION_SHA1}" )  


It would be nice to have a solution that catches changes to the repository (from git describe --dirty), but only triggers recompilation if something about the git information has changed.

Some of the existing solutions:

  1. Use 'execute_process'. This only gets the git information at configure time, and can miss changes to the repository.
  2. Depend on .git/logs/HEAD. This only triggers recompilation when something in the repo changes, but misses the changes to get the '-dirty' state.
  3. Use a custom command to rebuild the version information every time a build is run. This catches changes resulting in the -dirty state, but triggers a recompilation all the time (based on the updated timestamp of the version information file)

One fix to the third solution is to use the CMake 'copy_if_different' command, so the timestamp on the version information file only changes if the contents change.

The steps in the custom command are:

  1. Collect the git information to a temporary file
  2. Use 'copy_if_different' to copy the temporary file to the real file
  3. Delete the temporary file, to trigger the custom command to run again on the next 'make'

The code (borrowing heavily from kralyk's solution):

# The 'real' git information file  SET(GITREV_BARE_FILE git-rev.h)  # The temporary git information file  SET(GITREV_BARE_TMP git-rev-tmp.h)  SET(GITREV_FILE ${CMAKE_BINARY_DIR}/${GITREV_BARE_FILE})  SET(GITREV_TMP ${CMAKE_BINARY_DIR}/${GITREV_BARE_TMP})    ADD_CUSTOM_COMMAND(    OUTPUT ${GITREV_TMP}    COMMAND ${CMAKE_COMMAND} -E echo_append "#define GIT_BRANCH_RAW " > ${GITREV_TMP}    COMMAND ${GIT_EXECUTABLE} rev-parse --abbrev-ref HEAD >> ${GITREV_TMP}    COMMAND ${CMAKE_COMMAND} -E echo_append "#define GIT_HASH_RAW " >> ${GITREV_TMP}    COMMAND ${GIT_EXECUTABLE} describe --always --dirty --abbrev=40 --match="NoTagWithThisName" >> ${GITREV_TMP}    COMMAND ${CMAKE_COMMAND} -E copy_if_different ${GITREV_TMP} ${GITREV_FILE}    COMMAND ${CMAKE_COMMAND} -E remove ${GITREV_TMP}    WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}    VERBATIM  )  # Finally, the temporary file should be added as a dependency to the target    ADD_EXECUTABLE(test source.cpp ${GITREV_TMP})  


The following solution is based on the observation that Git updates the HEAD log whenever you pull or commit something. Note that e.g. Drew's suggestion above will update the Git information only if you rebuild the CMake cache manually after every commit.

I use a CMake "custom command" that generates a one-line header file ${SRCDIR}/gitrevision.hh where ${SRCDIR} is the root of your source tree. It will be re-made only when a new commit is made. Here is the necessary CMake magic with some comments:

# Generate gitrevision.hh if Git is available  # and the .git directory is present  # this is the case when the software is checked out from a Git repo  find_program(GIT_SCM git DOC "Git version control")  mark_as_advanced(GIT_SCM)  find_file(GITDIR NAMES .git PATHS ${CMAKE_SOURCE_DIR} NO_DEFAULT_PATH)  if (GIT_SCM AND GITDIR)      # Create gitrevision.hh      # that depends on the Git HEAD log      add_custom_command(OUTPUT ${SRCDIR}/gitrevision.hh          COMMAND ${CMAKE_COMMAND} -E echo_append "#define GITREVISION " > ${SRCDIR}/gitrevision.hh          COMMAND ${GIT_SCM} log -1 "--pretty=format:%h %ai" >> ${SRCDIR}/gitrevision.hh          DEPENDS ${GITDIR}/logs/HEAD          VERBATIM      )  else()      # No version control      # e.g. when the software is built from a source tarball      # and gitrevision.hh is packaged with it but no Git is available      message(STATUS "Will not remake ${SRCDIR}/gitrevision.hh")  endif()  

The contents of gitrevision.hh will look like this:

#define GITREVISION cb93d53 2014-03-13 11:08:15 +0100  

If you want to change this then edit the --pretty=format: specification accordingly. E.g. using %H instead of %h will print the full SHA1 digest. See the Git manual for details.

Making gitrevision.hh a fully-fledged C++ header file with include guards etc. is left as an exercise to the reader :-)


I can't help you with the CMake side, but with respect to Git side I would recommend taking a look how Linux kernel and Git project itself does it, via GIT-VERSION-GEN script, or how tig does it in its Makefile, by using git describe if there is git repository present, falling back to "version" / "VERSION" / "GIT-VERSION-FILE" generated and present in tarballs, finally falling back to default value hardcoded in script (or Makefile).

The first part (using git describe) requires that you tag releases using annotated (and possibly GPG signed) tags. Or use git describe --tags to use also lightweight tags.


Here's my solution, which I think is reasonably short yet effective ;-)

First, a file is needed in the source tree (I name it git-rev.h.in), it should looks something like this:

#define STR_EXPAND(x) #x  #define STR(x) STR_EXPAND(x)  #define GIT_REV STR(GIT_REV_)  #define GIT_REV_ \      

(Please never mind those macros, that's a little bit insane trick to make a string out of a raw value.) It is essential that this file has exactly one empty newline at the end so that value can be appended.

And now this code goes in respective CMakeLists.txt file:

# --- Git revision ---  add_dependencies(your_awesome_target gitrev)      #put name of your target here  include_directories(${CMAKE_CURRENT_BINARY_DIR})  #so that the include file is found  set(gitrev_in git-rev.h.in)                       #just filenames, feel free to change them...  set(gitrev git-rev.h)  add_custom_target(gitrev    ${CMAKE_COMMAND} -E remove -f ${CMAKE_CURRENT_BINARY_DIR}/${gitrev}    COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/${gitrev_in} ${CMAKE_CURRENT_BINARY_DIR}/${gitrev}    COMMAND git rev-parse HEAD >> ${CMAKE_CURRENT_BINARY_DIR}/${gitrev}    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}         #very important, otherwise git repo might not be found in shadow build    VERBATIM                                              #portability wanted  )  

This command ensuers that the git-rev.h.in is copied in the build tree as git-rev.h and git revision is appended at its end.

So all you need to do next is include git-rev.h in one of your files and do whatever you want with the GIT_REV macro, which yields current git revision hash as a string value.

The nice thing about this solution is that the git-rev.h is recreated each time you build the associated target, so you don't have to run cmake over and over again.

It also should be pretty portable - no non-portable external tools were used and even the bloody stupid windows cmd supports the > and >> operators ;-)


If CMake doesn't have a built-in capability to do this substitution, then you could write a wrapper shell script that reads a template file, substitutes the SHA1 hash as above in the correct location (using sed, for example), creates the real CMake build file, and then calls CMake to build your project.

A slightly different approach might be to make the SHA1 substitution optional. You would create the CMake file with a dummy hash value such as "NO_OFFICIAL_SHA1_HASH". When developers build their own builds from their working directories, the built code would not include a SHA1 hash value (only the dummy value) because the code from the working directory doesn't even have a corresponding SHA1 hash value yet.

On the other hand, when an official build is made by your build server, from sources pulled from a central repository, then you know the SHA1 hash value for the source code. At that point, you can substitute the hash value in the CMake file and then run CMake.



Simply adding some code to only 2 files: CMakeList.txt and main.cpp.

1. CMakeList.txt

# git commit hash macro  execute_process(    COMMAND git log -1 --format=%h    WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}    OUTPUT_VARIABLE GIT_COMMIT_HASH    OUTPUT_STRIP_TRAILING_WHITESPACE  )  add_definitions("-DGIT_COMMIT_HASH=\"${GIT_COMMIT_HASH}\"")  

2. main.cpp

inline void LogGitCommitHash() {  #ifndef GIT_COMMIT_HASH  #define GIT_COMMIT_HASH "0000000" // 0000000 means uninitialized  

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