Set up compilation environment

I choose Arch Linux as the Operating System. For the compiler, both gcc and clang are OK.

(1) Install gcc:

# pacman -S gcc

(2) Install clang:

# pacman -S clang
......
Optional dependencies for clang
    openmp: OpenMP support in clang with -fopenmp
    python2: for scan-view and git-clang-format
......

So to use OpenMP feature of clang, an additional openmp package needs to be installed:

# pacman -S openmp

Now that the environment is ready, let's write a simple program to get a first experience of OpenMP:

#include <omp.h>
#include <stdio.h>

int main(void) {
    #pragma omp parallel
    {
        printf("Thread id is %d\n", omp_get_thread_num());
    }
    return 0;
} 

The function of this program just spawns a herd of threads, and every thread prints its ID. My machine's CPU parameters are like following:

# lscpu
Architecture:        x86_64
CPU op-mode(s):      32-bit, 64-bit
Byte Order:          Little Endian
CPU(s):              2
On-line CPU(s) list: 0,1
Thread(s) per core:  1
Core(s) per socket:  2
Socket(s):           1
......

Use gcc to build and run it:

# gcc -fopenmp thread.c
# ./a.out
Thread id is 0
Thread id is 1

Switch to clang:

# clang -fopenmp thread.c
# ./a.out
Thread id is 0
Thread id is 1

The same output! The program creates 2 threads which is equal to my machine's logical CPU number.

BTW, use ldd to check the linked library of program built with gcc:

# ldd a.out
    linux-vdso.so.1 (0x00007ffcbbcf7000)
    libgomp.so.1 => /usr/lib/libgomp.so.1 (0x00007f15b75f7000)
    libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f15b73d9000)
    libc.so.6 => /usr/lib/libc.so.6 (0x00007f15b7021000)
    libdl.so.2 => /usr/lib/libdl.so.2 (0x00007f15b6e1d000)
    /lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007f15b7a27000)

built with clang:

# ldd a.out
    linux-vdso.so.1 (0x00007fffb5fb5000)
    libomp.so => /usr/lib/libomp.so (0x00007fcedc447000)
    libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007fcedc229000)
    libc.so.6 => /usr/lib/libc.so.6 (0x00007fcedbe71000)
    libdl.so.2 => /usr/lib/libdl.so.2 (0x00007fcedbc6d000)
    /lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007fcedc910000)

We can see gcc's OpenMP library is libgomp, while clang's is libomp.