Why doesn't CHECK_FUNCTION_EXISTS find clock_gettime in CMake?

Because clock_gettime is found in librt we need to link to that when doing the check (otherwise CMake will simply fail to compile the test program it generates to test if the function exists).

This is not possible with CHECK_FUNCTION_EXISTS. Instead CHECK_LIBRARY_EXISTS must be used:

include(CheckLibraryExists)
CHECK_LIBRARY_EXISTS(rt clock_gettime "" HAVE_CLOCK_GETTIME)

This will now work and output:

-- Looking for clock_gettime in rt - found

Update: In newer glibc 2.17+ clock_gettime has been moved from librt to libc.

So to be sure to find clock_gettime on all systems you would need to do two checks:

include(CheckLibraryExists)
CHECK_LIBRARY_EXISTS(rt clock_gettime "" HAVE_CLOCK_GETTIME)

if (NOT HAVE_CLOCK_GETTIME)
   set(CMAKE_EXTRA_INCLUDE_FILES time.h)
   CHECK_FUNCTION_EXISTS(clock_gettime HAVE_CLOCK_GETTIME)
   SET(CMAKE_EXTRA_INCLUDE_FILES)
endif()