Can't use openmp in c++ project [closed]

Question

I've included openmp to my project. I've got -fopenmp in compliler flags.

g++ -std=c++0x -O3 -Wall -c -fmessage-length=0 -march=core2 -fopenmp -ffast-math -fPIC

#pragma omp parallel for for (int i = 0; i < rows; i++) { MatrixXd frame = frames.row(i); output.row(i) = dem(frame); } return output;

And I have this output when compiling.

hello.cpp:(.text+0x2d88): undefined reference to `omp_get_num_threads' hello.cpp:(.text+0x2d8f): undefined reference to `omp_get_thread_num' ./hello.o: In function `demodulateMatrix': hello.cpp:(.text+0x315f): undefined reference to `GOMP_parallel_start' hello.cpp:(.text+0x316c): undefined reference to `GOMP_parallel_end'

I've tried to add -fopenmp flag to the linker and I have this output with it

g++: error: unrecognized command line option ‘-fopenmp,’ make: *** [libhello.so] Error 1

GCC version is 4.8.2 right now.

Are you using GCC version < 4.2? If so, possible duplicate of help with openmp compilation problems. Try gcc --version. — Tom Fenech
– Tom Fenech, Commented May 21, 2014 at 10:11
I've got the latest stable release of gcc. 4.7 or something. Definitely not 4.2 — user1685095
– user1685095, Commented May 21, 2014 at 10:13
OK, well it would still be useful to find out exactly what version you have and add it to the question. — Tom Fenech
– Tom Fenech, Commented May 21, 2014 at 10:14
What linker are you using? Make sure it’s the right version, and it’s the C++ linker (g++). Clearly compilation does work for you, so GCC correctly recognises the -fopenmp flag. Provide a complete minimal example, along with all command line calls you issue. — Konrad Rudolph
– Konrad Rudolph, Commented May 21, 2014 at 10:26

Fred Foo · Accepted Answer · 2014-05-21 10:31:37Z

You need to pass -fopenmp to the linker as well. It looks like you tried to do that, but you have a syntax error that causes the linker to see -fopenmp, (with a stray comma at the end). Check your makefile.

(Also you seem to pass hello.cpp to the linker instead of hello.o, so your code is compiled twice.)

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