How to run a command at compile time within Makefile generated by CMake?

Question

I would like to pass some options to a compiler. The option would have to be calculated at compile time - everytime when \'make\' is invoked, not when \'cmake\', so execute_

Answer 1

The solution above (using a separate CMake script file to generate a header file) seems very flexible but a bit complicated for what is being done in the example.

An alternative is to set a COMPILE_DEFINITIONS property on either an individual source file and or target, in which case the defined pre-processor variables will only be set for the source file or files in the target are compiled.

The COMPILE_DEFINITIONS properties have a different format from that used in the add_definitions command, and have the advantage that you don't need to worry about "-D" or "\D" syntax and they work cross-platform.

Example code

-- CMakeLists.txt --

execute_process(COMMAND svnversion
    WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
    OUTPUT_VARIABLE SVN_REV)
string(STRIP ${SVN_REV} SVN_REV)

execute_process(COMMAND date "+%Y-%m-%d-%H:%M"
    OUTPUT_VARIABLE BUILD_TIME)
string(STRIP ${BUILD_TIME} BUILD_TIME)

set_source_files_properties(./VersionInfo.cpp
    PROPERTIES COMPILE_DEFINITIONS SVN_REV=\"${SVN_REV}\";BUILD_TIME=\"${BUILD_TIME}\"")

The first line runs a shell command svnversion and puts the result in the variable SVN_REV. The string(STRIP ...) command is needed to remove trailing newline characters from the output.

Note this is assuming that the command being run is cross-platform. If not you may need to have alternatives for different platforms. For example I use the cygwin implementation of the Unix date command, and have:

if(WIN32)
 execute_process(COMMAND cmd /C win_date.bat
    OUTPUT_VARIABLE BUILD_TIME)
else(WIN32)
  execute_process(COMMAND date "+%Y-%m-%d-%H:%M"
    OUTPUT_VARIABLE BUILD_TIME)
endif(WIN32)
string(STRIP ${BUILD_TIME} BUILD_TIME)

for the date commands, where win_date.bat is a bat file that outputs the date in the desired format.

The two pre-processor variables are not available in the file ./VersionInfo.cpp but not set in any other files. You could then have

-- VersionInfo.cpp --

std::string version_build_time=BUILD_TIME;
std::string version_svn_rev=SVN_REV;

This seems to work nicely across platforms and minimizes the amount of platform-specific code.