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!

Solution

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.

Comments

[Admin]

From GCC documentation: -fopenmp flag also arranges for automatic linking of the OpenMP runtime library. See here: gcc.gnu.org/onlinedocs/libgomp/…

[Tim]

Thanks! I am considering the code with API and without directives. (1) 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) when using -fopenmp, why explicit linking to libgomp.so is not required?

[ejd]

(1) You can statically link OpenMP by using "-fopenmp -static". What you will find is that when this is specified it uses "-lgomp -lrt". (2) If the system supports the dynamic linker then it is used and when you specify -fopenmp it will link with libgomp.so.

[Royi]

It seems both the compiler and the linker requires the -fopenmp flag.

[ChrisZZ]

@ejd Hi, I would like to know how can people know "-fopenmp -static" links to openmp and rt libraries. I do man gcc and search -fopenmp -static but not found result, and search -fopenmp not get result of rt library.