How to use OpenMP in a C project built with meson - c

Question

I am quite new to meson and C, please forgive me if the answer to this question is trivial ...
I want to use OpenMP in a C project, and I am using meson as a build tool.
I want to compile the parallel for example from this tutorial.
My main.c looks very similar:
#include <omp.h>
#define N 1000
#define CHUNKSIZE 100
int main(int argc, char *argv[]) {
int i, chunk;
float a[N], b[N], c[N];
/* Some initializations */
for (i=0; i < N; i++)
a[i] = b[i] = i * 1.0;
chunk = CHUNKSIZE;
#pragma omp parallel for \
shared(a,b,c,chunk) private(i) \
schedule(static,chunk)
for (i=0; i < N; i++)
c[i] = a[i] + b[i];
return 0;
}
My short meson.build file contains this:
project('openmp_with_meson', 'c')
# add_project_arguments('-fopenmp', language: 'c')
exe = executable('some_exe', 'src/main.c') #, c_args: '-fopenmp')
I commented out the c_args keyword in the call to executable here.
Now I end up with the following scenarios:
without '-fopenmp' option, I get the warning, that the pragma is unknown and will be ignored (as I would expect): ../src/main.c:15:0: warning: ignoring pragma omp parallel [-Wunknown-pragmas] #pragma omp parallel for
with the option c_args: '-fopenmp' inserted, I do not get the above warning anymore, instead I get errors for undefined references to GOMP_parallel, omp_get_num_threads and omp_get_thread_num, and nothing gets built
when I use gcc manually with gcc -Wall -o manually_with_gcc ../src/main.c -fopenmp the program compiles and executes without any errors.
Can anyone tell me how to get the executable to compile with meson?

Answer 1

Meson 0.46 or later
Meson 0.46 (released Apr 23, 2018) added OpenMP support. So, if you have meson 0.46 or later,
project('openmp_with_meson', 'c')
omp = dependency('openmp')
exe = executable('some_exe', 'src/main.c',
dependencies : omp)
Should work with both GCC and Clang.
Meson 0.45 or earlier
If you happen to have older version, Debian Stretch, Ubuntu Bionic (18.04LTS), or Fedora 27, you can do the following:
You need another keyword arg link_args : '-fopenmp' for executable().
exe = executable('some_exe', 'src/main.c',
c_args: '-fopenmp',
link_args : '-fopenmp')
Meson builds C program in two phases, compiling and linking. You can pass extra arguments with c_args for compiling and link_args for linking.
The option -fopenmp enables OpenMP directives while compiling, and
the flag also arranges for automatic linking of the OpenMP runtime
library.
That is, -fopenmp is dual purpose option.
Now, the above is simple and good. Once you understand it, however, you can also compile your program with -fopenmp to activate the OpenMP directives and link the OpenMP libraries by yourself without -fopenmp to the link_args.
Here is a complete meson.build:
project('openmp_with_meson', 'c')
cc = meson.get_compiler('c')
libgomp = cc.find_library('gomp')
exe = executable('some_exe', 'src/main.c',
c_args: '-fopenmp',
dependencies : libgomp)

Answer 2

Meson >= 0.46 now has a builtin for this (docs):
openmp = dependency('openmp') # meson builtin