Linking error OpenMP on Linux

[vebjoro]

Hi,

I am trying to link the code for problem 8: g++ prob_8.o state.o -larmadillo -lomp -o main

but I get the following error message:

/usr/bin/ld: cannot find -lomp: No such file or directory collect2: error: ld returned 1 exit status make: *** [makefile:12: prob_8] Error 1

Does anyone have a fix for this?

[anderkve]

Hi @vebjoro!

First, generally the way to use openmp is not to add -lomp, but rather use the compiler flag -fopenmp, see the examples here: https://github.com/anderkve/FYS3150/tree/master/code_examples/omp_parallelization (OpenMP support is sort of a built-in feature of most compilers.)

If that doesn't work, here are some questions and things you can try:

• Which compiler version are you using? Try running g++ --version.

• Have you tried explicitly installing the libomp-dev package? If not, try sudo apt install libomp-dev

• To find the path of an installed libomp.so library, you can try this: First, run the command

  apt list --installed | grep "libomp"

  Then you might get a list looking something like this:

  libomp-14-dev/jammy,now 1:14.0.0-1ubuntu1 amd64 [installed,automatic]
  libomp-dev/jammy,now 1:14.0-55~exp2 amd64 [installed]
  libomp5-14/jammy,now 1:14.0.0-1ubuntu1 amd64 [installed,automatic]

  Now try running commands like

  dpkg -L libomp-14-dev | grep ".so"

  and similar for other variants of the libomp package names found above. That should give output of this sort:

  /usr/lib/llvm-14/lib/libarcher.so
  /usr/lib/llvm-14/lib/libompd.so
  /usr/lib/llvm-14/lib/libomptarget.rtl.amdgpu.so
  /usr/lib/llvm-14/lib/libomptarget.rtl.cuda.so
  /usr/lib/llvm-14/lib/libomptarget.rtl.x86_64.so
  /usr/lib/llvm-14/lib/libomptarget.so
  /usr/lib/llvm-14/lib/libgomp.so
  /usr/lib/llvm-14/lib/libiomp5.so
  /usr/lib/llvm-14/lib/libomp.so
  /usr/lib/x86_64-linux-gnu/libiomp5.so
  /usr/lib/x86_64-linux-gnu/libomp5.so

  So here it looks like the libomp library lives in the folder /usr/lib/llvm-14/lib. So that means you could try to add -L /usr/lib/llvm-14/lib to your g++ command, i.e. g++ prob_8.o state.o -larmadillo -L /usr/lib/llvm-14/lib -lomp -o main.