The caveat of building OpenMP program

When building OpenMP program, you must be sure to use -fopenmp option in both compile and link stage (refer stackoverflow), else you may get a hit.

Take the following example:

#include <unistd.h>
#include <omp.h>

int main(void){

        #pragma omp parallel num_threads(4)
        for(;;)
        {
            sleep(1);
        }

        return 0;
}

Use gcc to build it (contains both compile and link):

gcc -fopenmp parallel.c

Execute the program and check the threads number:

$ ./a.out &
[1] 5684
$ ps -T 5684
  PID  SPID TTY      STAT   TIME COMMAND
 5684  5684 pts/16   Sl     0:00 ./a.out
 5684  5685 pts/16   Sl     0:00 ./a.out
 5684  5686 pts/16   Sl     0:00 ./a.out
 5684  5687 pts/16   Sl     0:00 ./a.out

There are 4 threads which as our expected.

Then we create a neat Makefile and split the compile and link stages separately:

all:
        gcc -fopenmp -c parallel.c -o parallel.o
        gcc parallel.o
clean:
        rm *.o a.out

Run the Makefie:

$ make
gcc -fopenmp -c parallel.c -o parallel.o
gcc parallel.o
parallel.o: In function `main':
parallel.c:(.text+0x19): undefined reference to `GOMP_parallel'
collect2: error: ld returned 1 exit status
make: *** [Makefile:3: all] Error 1

During the link phase, the gcc complained it can’t find GOMP_parallel. So we need to add -fopenmp in link command too:

all:
        gcc -fopenmp -c parallel.c -o parallel.o
        gcc -fopenmp parallel.o
clean:
        rm *.o a.out

This time all is OK:

$ make
gcc -fopenmp -c parallel.c -o parallel.o
gcc -fopenmp parallel.o
$ ./a.out &
[2] 6502
$ ps -T 6502
  PID  SPID TTY      STAT   TIME COMMAND
 6502  6502 pts/16   Sl     0:00 ./a.out
 6502  6503 pts/16   Sl     0:00 ./a.out
 6502  6504 pts/16   Sl     0:00 ./a.out
 6502  6505 pts/16   Sl     0:00 ./a.out

You can also use ldd tool to check a.out‘s dynamic libraries:

$ ldd /usr/lib/libgomp.so.1
    linux-vdso.so.1 (0x00007ffd9c0dd000)
    libgomp.so.1 => /usr/lib/libgomp.so.1 (0x00007fe5554ee000)
    libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007fe5552d0000)
    libc.so.6 => /usr/lib/libc.so.6 (0x00007fe554f2c000)
    libdl.so.2 => /usr/lib/libdl.so.2 (0x00007fe554d28000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fe55571c000)

The libgomp includes the GOMP_parallel definition.

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