What is the #cmakedefine preprocessor directive?

Question

I am looking through the source-code for ZeroMQ, which I want to build from source. Inside I found platform.hpp.in, which contains:

...
#cmakedefine ZMQ_HAVE_SO_PEERCRED
#cmakedefine ZMQ_HAVE_LOCAL_PEERCRED

#cmakedefine ZMQ_HAVE_SOCK_CLOEXEC
#cmakedefine ZMQ_HAVE_SO_KEEPALIVE
#cmakedefine ZMQ_HAVE_TCP_KEEPCNT
...

I assume these cmakedefine macros are used as templates to generate a header file, but how exactly do they work in CMake? How can I determine what are valid values? How are values set by the user when building the project?

Answer 1

It's part of a file that's processed by CMake's configure_file command. When configure_file is called for the file, #cmakedefine FOO is replaced by:

• #define FOO - if the CMake variable FOO is set to ON or TRUE.
• /* #undef FOO */ - otherwise.

And that is one way to pass values from CMake into C or C++ source code: The result of configure_file() is a C/C++ header file, which is included by the code you want to respect the CMake variable values.