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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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related to

0015393

closed

Kitware Robot

undefined reference to `GOMP_parallel'

Notes
(0042709) Kitware Robot (administrator) 2016-06-10 14:29	Resolving issue as `moved`. This issue tracker is no longer used. Further discussion of this issue may take place in the current CMake Issues page linked in the banner at the top of this page.

Issue History
Date Modified	Username	Field	Change
2015-02-09 10:58	Mathieu Malaterre	New Issue
2015-02-10 09:01	Brad King	Relationship added	related to 0015393
2016-06-10 14:29	Kitware Robot	Note Added: 0042709
2016-06-10 14:29	Kitware Robot	Status	new => resolved
2016-06-10 14:29	Kitware Robot	Resolution	open => moved
2016-06-10 14:29	Kitware Robot	Assigned To	=> Kitware Robot
2016-06-10 14:31	Kitware Robot	Status	resolved => closed