Specify OpenMP to GCC

Question

For OpenMP, when my code is using the functions in its API (for example, omp_get_thread_num()) without using its directives (such as those #pragma omp ...),

1. why directly specifying libgomp.a to gcc instead of using -fopenmp doesn't work, such as

   gcc hello.c /usr/lib/gcc/i686-linux-gnu/4.4/libgomp.a  -o hello 

   Update: I just found that linking to libgomp.a does not work, but linking to libgomp.so works. Does it mean OpenMP can not be static linked?

2. Why -fopenmp only works without specifying the library files

   gcc hello.c -fopenmp -o hello 

   Update: In other words, when using -fopenmp, why explicit linking to libgomp.so is not required?

3. Why does this also compile:

   gcc hello.c -L/usr/lib/gcc/i686-linux-gnu/4.4/ -lgomp -o hello 

   Will this ignore OpenMP directives in the code if there is any?

Thanks and regards!

Answer 1

In general, keep in mind that the directives and the functions are different things; the former are controlled by -fopenmp and the latter are controlled by linking to the OpenMP library.

1. (Updated to incorporate comments) Try using the -fopenmp and -static options to statically link OpenMP. Because this implies -lgomp -lrt, the following command won't compile correctly unless you also specify the location of librt.a.

   gcc hello.c /usr/lib/gcc/i686-linux-gnu/4.4/libgomp.a  -o hello 

2. (Updated to incorporate comments) I imagine that the following command is compiling correctly because the OpenMP library is already in your library path and your system's dynamic linker is automatically linking with libgomp.so.

   gcc hello.c -fopenmp -o hello 

3. The following command is probably compiling properly because it is linking to the shared object for OpenMP (libgomp.so). Note that the -static option is not used.

   gcc hello.c -L/usr/lib/gcc/i686-linux-gnu/4.4/ -lgomp -o hello 

   If you don't specify the -fopenmp option, OpenMP directives should be ignored.