configure_file breaks multiline comments

Development
1

CMake version: 3.20.5 (and older)

I’m not sure if my observation is intended behavior or a bug. I noticed that configure_file()

  • does not check for C (multiline) comments
  • removes all text in a line before #cmakedefine…
  • expands #cmakedefine… inside C/C++ comments

Example: ‘#cmakedefine HAVE_JPEG 1’ inside a multiline C comment triggers a compilation error if HAVE_JPEG is false. After expansion the terminating ‘*/’ ends the existing multiline comment which leads to a compilation error in the following line(s).

This was a real case; I was trying to document CMake syntax in the input file.

All examples in the docs show ‘#’ at the beginning of the line. There’s no clue what will happen if it is inside a comment block and whether it is expanded if ‘#’ is not the first character or maybe after optional whitespace.

Please see more details in the attached complete project in text form:
configure_file_breaks_comments.txt (1.7 KB)

Questions:
(1) Is this behavior intended?
(2) If yes (or no) could (should) this be documented?

Thanks for reading and for any clues.

PS: I mitigated this in my project by replacing ‘#’ with ‘[#]’, hence there’s no problem for me anymore. Please consider this as an attempt to report a bug or trigger improvement of the documentation.

2

BTW: This code:
// NOT USED: #cmakedefine HAVE_ZLIB 1
gets expanded to:
// NOT USED: #define HAVE_ZLIB 1
(if HAVE_ZLIB is true, obviously)

which I think is at least inconsistent because the leading ‘// comment’ is NOT removed - and removing it would in all cases lead to unexpected behavior.

My suggestion would be to not expand #cmakedefine at all if it’s not the first token (after optional whitespace) in a source line.

3

Please file an issue. I don’t know that fixing it is all that doable since it would (probably) need to be a policy that probably isn’t all that easy to implement. I think mentioning this behavior as a note in the documentation is certainly the place to start. Any new policy which fixes it can then update that documentation to mention the policy.

4

Thanks for reading and for your reply.

I’ll file an issue as requested.

Done: https://gitlab.kitware.com/cmake/cmake/-/issues/22417