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From: "thomas.izard at silkan dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/65485] New: Use openmp in dynamic library but not in calling program causes segfault at the end of execution
Date: Fri, 20 Mar 2015 08:58:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65485

            Bug ID: 65485
           Summary: Use openmp in dynamic library but not in calling
                    program causes segfault at the end of execution
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thomas.izard at silkan dot com

Consider:
#include "omp.h"
#include <iostream>
extern "C" {
int test() { 
  int N = omp_get_max_threads();
#pragma omp parallel num_threads(N)
  {
    std::cout << omp_get_thread_num() << std::endl;
  }
  return 0;
}
};

Compiled with g++ -fopenmp -shared -fPIC -Wall -Wextra -o shared.so shared.cpp
(no warnings/no errors).

and :

#include <iostream>
#include <dlfcn.h>
int main() {
  void* handle = dlopen("./shared.so", RTLD_NOW);
  if (!handle) {
    std::cerr << "can not open shared.so" << std::endl;
    return 1;
  }
  int(*f)() =  (int(*)()) dlsym(handle,"test");
  if (!f) {
    std::cerr << "can not find 'test' symbol in shared.so" << std::endl;
    return 1;
  }
  (*f)();
  if (dlclose(handle)) {
    std::cerr << "can not close shared.so" << std::endl;
    return 1;
  }  
  return 0;
}

compiled with:
 g++ -Wall -Wextra -o main main.cpp -ldl (no warnings/no errors).

The main program loads the dynamic library and calls the "test" function. It
does not directly use openmp.

The problem is that a segmentation fault happen at the very end of the program.
According to valgrind, at this point some threads are still active.

Adding a -fopenmp flag while compiling the main program does not fix the
problem. It seems that in this case the compiled code is not linked to GOMP
(confirmed by ldd).

The only workaround is to actually add an OpenMP directive (or a call to an
OpenMP runtime function) in the main.cpp file. 

Same issue with (at least): g++-4.6.4, g++-4.7, g++-4.8.2

Systems used : 
Linux 3.2.0-77-generic #114-Ubuntu SMP (Ubuntu 64 12.04)
Linux 3.13.0-43-generic #72-Ubuntu SMP (Ubuntu 64 server 14.04)

Thomas