Question

I originally asked this question on CMake mailing list: How to configure target or command to preprocess C file?

I’m porting build configuration based on GNU Autotools to CMake and I have to deal with C preprocessing to generate a file.

The input for preprocessor is SQL file with C preprocessor directives used, like #include "another.sql", etc.

Currently, Makefile uses the following rule to generate plain SQL file as output:

myfile.sql: myfile.sql.in.c
    cpp -I../common $< | grep -v '^#' > $@

So, the myfile.sql is meant to be one of products of the build process, similar to share libraries or executables.

What CMake tools should I use to achieve the same effect?

It’s unclear to me if I should use add_custom_command, add_custom_target or combine both.

Obviously, I’m looking for a portable solution that would work at least with GNU GCC and Visual Studio toolsets. I presume I will have to define platform-specific custom commands, one for cpp preprocessor, one for cl.exe /P.

Or, does CMake provide any kind of abstraction for C preprocessor?

I scanned the archives, but I only found preprocessing of fortran files or solutions based on make capabilities: make myfile.i
So, it’s not quite what I’m looking for.

UPDATE: Added answer based on solution received from Petr Kmoch on CMake mailing list.

Answer 1

To take advantage of CMake’ make myfile.i feature, you can do this:

add_library(sql_cpp_target EXCLUDE_FROM_ALL myfile.sql.in.c)

Now running make myfile.sql.in.c.i will produce preprocessed source for you, using defined CMAKE_C_FLAGS. It might be possible to change output name and dir for preprocessed file.

At any rate, you’d need to wrap these make invocations into add_custom_target(ALL ...) to make CMake run them during build.

Use CMAKE_MAKE_PROGRAM variable in targets definitions.

If you want to abstract from build tool, you can call cmake itself to build a target for you. Use ${CMAKE_COMMAND} --build ${CMAKE_BINARY_DIR} --target targetname when defining custom target.

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Alternatively, you can just add_custom_command() which runs specified compiler to preprocess files and put them at the appropriate place. But this seems to be less flexible, IMO.