How to save the build time of a C++ DLL - c++

Question

My project is a C++ DLL (cmake project), that expose methods from a c interface to another C# project,
I want to add a methode getBuildTime() in the c interface that return the buildtime (the exact date and time) so from the C# project we can know when the DLL was built.
Is there a way to do this stuff ?
I don't know if this useful or no but I'm using git as a version control system

Answer 1

Build time in C/C++ code can be obtained by built-in macros __DATE__ and __TIME__, e.g. do something like string build_time = __DATE__ " " __TIME__;.
Also file which contains these macros usage needs to be touched before each build to change file's modify time. touch is a unix command, which can be avaialble in Windows too through Cygwin or other means. This is needed to force it to recompile to use new date. Do file touching by adding touch build_time.cpp command to Pre-Build Events, they should be located in your project config somewhere. Alternatively you may add touching to your make file. Altertnatively some compile environments and possibly MSVC too can configure which files to rebuild always which is tweaked inside project's settings.
Also if you use git version control system then you might want to use last git commit hash and time together or instead of build time above. Git commit hash and time is sometimes better than to use build time like suggested above, because git hash and time remain same on each re-compile before next commit, this ensures that your DLL build time changes only when code changes/committed, this may help to have reproducible DLL compiles. To support git hash and time do next things:
If you have unix echo and bash and git commands in your system e.g. by installing Cygwin or new native Windows SubSystem for Linux then you may do next things in command script before_build.cmd that you run before build, also this shell script can be run on Unix systems almost without modifications:
linux_echo -n "" > cfg.h
linux_echo -n "#define GIT_COMMIT """ >> cfg.h
linux_bash -c "echo -n $(git rev-parse --short=8 HEAD)" >> cfg.h
linux_echo """" >> cfg.h
linux_echo -n "#define GIT_COMMIT_TIME """ >> cfg.h
linux_bash -c "echo -n $(git show -s --format=%%ci $(git rev-parse --short=8 HEAD) | tr -d '\n')" >> cfg.h
linux_echo """" >> cfg.h
linux_echo -n "#define COMPILE_TIME """ >> cfg.h
linux_bash -c 'echo -n $(date +"%%Y-%%m-%%d %%H:%%M:%%S %%z")' >> cfg.h
linux_echo """" >> cfg.h
and you will get cfg.h like:
#define GIT_COMMIT "fe0a7891"
#define GIT_COMMIT_TIME "2020-05-13 17:42:55 +0100"
#define COMPILE_TIME "2020-05-13 18:20:28 +0100"
these config file and macros you can use in your code.

Answer 2

The C++ preprocessor predefines several macros (backward compatible with C) including __DATE__ and __TIME__. These expand to string literals containing the date and time of the build (in particular, the date and time that the preprocessor runs on that particular file).
You may need to force the compilation unit containing them to be rebuilt on every change. With make that can be done with .PHONY, with cmake it looks like add_custom_target is the ticket.
Add a target with no output so it will always be built.
Besides using __DATE__ and __TIME__ to capture when the compiler ran, the linker will store a timestamp in the PE header of the DLL it creates. There's no configuration necessary to make this happen, but then you have to write code to read the PE header and convert the timestamp to your favorite date and time format.

Answer 3

The preprocessor solution is way simpler, but the following approach may be convenient, if you need to provide other properties of the build system as well.
Create a cmake script file that generates a source file. Use this script to generate the source file providing the info:
generate_timestamp.cmake
#parameter: OUTPUT_FILE
string(TIMESTAMP _TIMESTAMP "%Y-%m-%dT%H:%M:%S")
file(WRITE ${OUTPUT_FILE} "#include \"buildtime.h\"
const char* getBuildTime() {
return \"${_TIMESTAMP}\";
}")
CMakeLists.txt
set(_BUILD_TIME_FILE buildtime.cpp)
add_library(mylib ${_BUILD_TIME_FILE} ...)
set_source_files_properties(${_BUILD_TIME_FILE} PROPERTIES GENERATED 1)
get_filename_component(_BUILD_TIME_FILE_ABSOLUTE ${_BUILD_TIME_FILE} ABSOLUTE)
# generate source using cmake script file
add_custom_target(buildtime_info COMMAND "${CMAKE_BUILD_TOOL}" -D "OUTPUT_FILE=${_BUILD_TIME_FILE_ABSOLUTE}" -P generate_timestamp.cmake)
# generate file before building mylib
add_dependencies(mylib buildtime_info)