How to use Linux-specific APIs and libraries only on Linux builds with CMake?

Question

I have a project that I run on Linux (primarily), but sometimes on Darwin/Mac OS X. I use CMake to generate Makefiles on Linux and an Xcode project on Mac OS X. So far, this has worked well.

Now I want to use some Linux-specific functions (clock_gettime() and related functions). I get linker errors on Mac OS X when I try to use clock_gettime(), so I assume it is only available on Linux. I am prepared to introduce conditionally-compiled code in the .c files to use clock_gettime() on Linux and plain old clock() on Mac OS. (BTW I was planning to use #include <unistd.h> and #if _POSIX_TIMERS > 0 as the preprocessor expression, unless someone has a better alternative.)

Things get tricky when it comes to the CMakeLists.txt file. What is the preferred way of introducing linkage to Linux-specific APIs only under the Linux build in a cross-platform CMake project?

Note: An earlier revision of this question contained references to glibc, which was overly specific and confusing. The question is really about the right way to use Linux-specific APIs and libraries in a cross-platform CMake project.

Answer 1

Abstracting away from your examples, and answering only this question:

How to use Linux-specific APIs and libraries only on Linux builds with CMake?

CMake provides numerous useful constants that you can check in order to determine which system you are running:

if (${UNIX})
  # *nix-specific includes or actions
elsif (${WIN32})
  # Windows-specific includes or actions
elsif (${APPLE})
  # ...
endif (${UNIX})

Answer 2

(I know you're asking about glibc, but you really want to know whether clock_gettime is present, right? But nothing in your question is Linux-specific...)

If you want to check for clock_gettime, you can use the preprocessor. If clock_gettime is present, then _POSIX_TIMERS will be defined. The clock_gettime function is part of an optional POSIX extension (see spec), so it is not Linux-specific but not universal either. Mac OS X does not have clock_gettime: it is not declared in any header nor defined in any library.

#include <time.h>
#include <unistd.h> /* for _POSIX_TIMERS definition, if present */

#if _POSIX_TIMERS
...use clock_gettime()...
#else
...use something else...
#endif

This doesn't solve the problem that you still have to link with -lrt on Linux. This is typically solved with something like AC_CHECK_LIB in Autoconf, I'm sure there's an equivalent in CMake.

From man 2 clock_gettime:

On POSIX systems on which these functions are available, the symbol _POSIX_TIMERS is defined in <unistd.h> to a value greater than 0. The symbols _POSIX_MONOTONIC_CLOCK, _POSIX_CPUTIME, _POSIX_THREAD_CPUTIME indicate that CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID, CLOCK_THREAD_CPUTIME_ID are available. (See also sysconf(3).)

On Darwin you can use the mach_absolute_time function if you need a high-resolution monotonic clock. If you don't need the resolution or monotonicity, you should probably be using gettimeofday on both platforms.