Why doesn't CHECK_FUNCTION_EXISTS find clock_gettime in CMake?

c, cmake

Solution

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()

Problem

How come `CHECK_FUNCTION_EXISTS` doesn't find `clock_gettime`? I use the following code in my `CMakeLists.txt`: ``` include(CheckFunctionExists) set(CMAKE_EXTRA_INCLUDE_FILES time.h) CHECK_FUNCTION_EXISTS(clock_gettime HAVE_CLOCK_GETTIME) ``` This is on a POSIX system I know has `clock_gettime`. Yet I simply get: ``` -- Looking for clock_gettime - not found ```

Original source