MantisBT - CMake

View Issue Details

Project

Category

View Status

Date Submitted

Last Update

0015392

CMake

public

2015-02-09 10:58

2016-06-10 14:31

Reporter

Mathieu Malaterre

Assigned To

Kitware Robot

Priority

normal

Severity

minor

Reproducibility

have not tried

Status

closed

Resolution

moved

Platform

OS Version

Product Version

CMake 3.0.2

Target Version

Fixed in Version

Summary

0015392: Unclear documentation for module: FindOpenMP.cmake

Description

The FindOpenMP module states:

[...]
       The following variables are set:

          OpenMP_C_FLAGS - flags to add to the C compiler for OpenMP support
[...]

Instead is should be stated that the OpenMP flags should be passed both to the C compiler and the linker.

Eg, the following will not work (linker step will fail)

cmake_minimum_required(VERSION 2.8)
project(omp C)
find_package(OpenMP REQUIRED)
# OPENMP_FOUND

# Not OK
# pass OpenMP flag to C compiler (only)
set_property(SOURCE tomp.c
  PROPERTY COMPILE_FLAGS ${OpenMP_C_FLAGS}
  )

# OK
# pass OpenMP CFLAGS to C compiler and linker:
# set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")

add_executable(tomp tomp.c)

Steps To Reproduce

$ cat CMakeLists.txt
cmake_minimum_required(VERSION 2.8)
project(omp C)
find_package(OpenMP REQUIRED)
# OPENMP_FOUND

# Not OK
# pass OpenMP flag to C compiler (only)
set_property(SOURCE tomp.c
  PROPERTY COMPILE_FLAGS ${OpenMP_C_FLAGS}
  )

# OK
# pass OpenMP CFLAGS to C compiler and linker:
# set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")

add_executable(tomp tomp.c)

$ cat tomp.c
#include <math.h>

int main()
{
  const int size = 256;
  double table[size];

  int n;
#pragma omp parallel for
  for(n=0; n<size; ++n)
    table[n] = 0;

  return table[size-1];
}

$ cmake . && make
[...]
Linking C executable tomp
CMakeFiles/tomp.dir/tomp.c.o: In function `main':
tomp.c:(.text+0x87): undefined reference to `GOMP_parallel_start'
tomp.c:(.text+0x98): undefined reference to `GOMP_parallel_end'
CMakeFiles/tomp.dir/tomp.c.o: In function `main._omp_fn.0':
tomp.c:(.text+0xe3): undefined reference to `omp_get_num_threads'
tomp.c:(.text+0xeb): undefined reference to `omp_get_thread_num'
collect2: error: ld returned 1 exit status
make[2]: *** [tomp] Error 1
make[1]: *** [CMakeFiles/tomp.dir/all] Error 2
make: *** [all] Error 2

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