Compile-time version strings in CMake

At compile-time, I would like to embed a few strings into my code that reflects the current state of the Git repository:

The short commit hash, with a trailing + if there are uncommitted changes.
The tag (if any)
The current branch

This is the kind of thing that should be easy, but is surprisingly non-obvious.

First, I write a standalone CMake script named version.cmake:

execute_process(COMMAND git log --pretty=format:'%h' -n 1
                OUTPUT_VARIABLE GIT_REV
                ERROR_QUIET)

# Check whether we got any revision (which isn't
# always the case, e.g. when someone downloaded a zip
# file from Github instead of a checkout)
if ("${GIT_REV}" STREQUAL "")
    set(GIT_REV "N/A")
    set(GIT_DIFF "")
    set(GIT_TAG "N/A")
    set(GIT_BRANCH "N/A")
else()
    execute_process(
        COMMAND bash -c "git diff --quiet --exit-code || echo +"
        OUTPUT_VARIABLE GIT_DIFF)
    execute_process(
        COMMAND git describe --exact-match --tags
        OUTPUT_VARIABLE GIT_TAG ERROR_QUIET)
    execute_process(
        COMMAND git rev-parse --abbrev-ref HEAD
        OUTPUT_VARIABLE GIT_BRANCH)

    string(STRIP "${GIT_REV}" GIT_REV)
    string(SUBSTRING "${GIT_REV}" 1 7 GIT_REV)
    string(STRIP "${GIT_DIFF}" GIT_DIFF)
    string(STRIP "${GIT_TAG}" GIT_TAG)
    string(STRIP "${GIT_BRANCH}" GIT_BRANCH)
endif()

set(VERSION "const char* GIT_REV=\"${GIT_REV}${GIT_DIFF}\";
const char* GIT_TAG=\"${GIT_TAG}\";
const char* GIT_BRANCH=\"${GIT_BRANCH}\";")

if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/version.cpp)
    file(READ ${CMAKE_CURRENT_SOURCE_DIR}/version.cpp VERSION_)
else()
    set(VERSION_ "")
endif()

if (NOT "${VERSION}" STREQUAL "${VERSION_}")
    file(WRITE ${CMAKE_CURRENT_SOURCE_DIR}/version.cpp "${VERSION}")
endif()

This runs git to get relevant info, inserts version and revision strings into a preformatted chunk of C code, then writes it to a local file if it has changed.

The resulting file looks like this:

const char* GIT_REV="c629110+";
const char* GIT_TAG="";
const char* GIT_BRANCH="master";

Next, I add the following lines to my main CMakeLists.txt:

# Add a custom command that produces version.cpp, plus
# a dummy output that's not actually produced, in order
# to force version.cmake to always be re-run before the build
ADD_CUSTOM_COMMAND(
    OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/version.cpp
           ${CMAKE_CURRENT_BINARY_DIR}/_version.cpp
    COMMAND ${CMAKE_COMMAND} -P
            ${CMAKE_CURRENT_SOURCE_DIR}/version.cmake)

Finally, I add the generated file

${CMAKE_CURRENT_BINARY_DIR}/version.cpp

to the list of files in my build.

This works beautifully: whenever I run a build, it checks the state of the repo and writes a new copy of version.cpp if things have changed. In turn, if that file is modified, then the usual compilation process rebuilds it and re-links the resulting libraries.

This is also a fast solution. If nothing has changed, a build runs in about 0.09 seconds (using ninja); if we need to regenerate version.cpp, rebuild it, and re-link the libraries, it takes 0.28 sec.

To actually use these variables, I add a few small accessor functions (to ensure that they're accessible outside of the shared library):

// These variables are autogenerated and compiled
// into the library by the version.cmake script
extern "C"
{
    extern const char* GIT_TAG;
    extern const char* GIT_REV;
    extern const char* GIT_BRANCH;
}

const char* libfive_git_version(void)
{
    return GIT_TAG;
}

const char* libfive_git_revision(void)
{
    return GIT_REV;
}

const char* libfive_git_branch(void)
{
    return GIT_BRANCH;
}

Matt Keeter // blog

Compile-time version strings in CMake