How replace string in a file with value of current directory using CMake

Question

I'm trying to make another guy's research code reproducible, so that others don't have the same trouble I'm having right now. But, I'm lacking the experience with cmake for that. Here is what I could do after reading the docs:
In the same folder as my CMakeLists.txt I have a file called io_utils.h with a ROOT_PATH variable whose value is VALUE_ROOT_PATH. I want to replace that string with the value of the file's current directory. So I tried to add the following to CMakeLists.txt:

# Set ROOT_PATH in io_utils.h
FIND_PATH(BUILD_PATH CMakeLists.txt . )
FILE(READ ${BUILD_PATH}io_utils.h IO_UTILS)
STRING(REGEX REPLACE "VALUE_ROOT_PATH" "${BUILD_PATH}" MOD_IO_UTILS "${IO_UTILS}" )
FILE(WRITE ${BUILD_PATH}io_utils.h "${MOD_IO_UTILS}")

I tried to make and install that but it is not working, the file isn't changed. What's wrong with what I did?

Solution 1

I suggest a different approach using the configure_file cmake macro. First, you create a template file that references any variables you plan to set in cmake, then the macro substitutes the actual value in place of the variable. For example, let's say we have a template file io_utils.h.in that looks something like this:

const char* ROOT_PATH = "${BUILD_PATH}";

In your CMakeLists.txt, you can do something like this:

configure_file( io_utils.h.in io_utils.h )

When you run cmake, it will use the first file as a template (io_utils.h.in here) to generate the second file (called io_utils.h here) with the value substituted in:

const char* ROOT_PATH = "/path/to/CMakeLists.txt";

By the way, CMake has a built-in variable that references the directory with the top-level CMakeLists.txt called CMAKE_SOURCE_DIR. Try replacing BUILD_PATH above with this variable.

Solution 2

In this case it seems to be a lot easier to define a macro and let the preprocessor do the job:

add_definition(ROOT_PATH ${VALUE_ROOT_PATH})

Comments

[ideasman42]

Since not all files are headers which you may want to search/replace, it would still be interesting to have an answer to the original question, as well as this suggested alternative.

[ComicSansMS]

This has the disadvantage that the file becomes dependent on the define from the build environment. For instance, a header in a library that you want to ship to clients probably should be self-contained and not have such a dependency. But if that is of no concern, this is a valid solution.