Question

I got an error message of Segmentation fault (core dump) when I try to run this code.
Note: This is a really long program (almost 600 lines) so I only posted the ones that I ‘think’ is related. Let me know if more needed? Thanks in advance 🙂

#define CONSTANT 4

int main()
{
  pthread_t tid[CONSTANT];
  int i, check;
  for( i = 0; i < CONSTANT; i++ )
  {
     check = pthread_create( &tid[i], NULL, tFunction, (void *) CONSTANT );
  }
}

void * tFunction ( void * param )
{
  int num = * (int *) param;  /* Seg fault line */
}

Answer 1

What you’re doing is:

check = pthread_create( &tid[i], NULL, tFunction,(void *) 4);

And treating the 4th argument as an int *, which it obviously isn’t. When you dereference the address 4 in tFunction you get a segfault.

If you want to pass a pointer to an int with a value of 4, pass the address of an int variable, ie:

#include <pthread.h>

#define CONSTANT 4

void * tFunction ( void * param )
{
  int num = * (int *) param;  /* Seg fault line */
}

int main(void)
{
  int arg = CONSTANT;
  pthread_t tid[CONSTANT];
  int i, check;
  for( i = 0; i < CONSTANT; i++ )
  {
     check = pthread_create( &tid[i], NULL, tFunction, (void *) &arg );
  }

  return 0;
}

EDIT: pthread_join is going to be useful so that you can wait for your threads to terminate before exiting your program.

EDIT2: If you haven’t read the comments: You should ensure that if your passing a local variable (as with this example which was meant to show a very minor change to his code to get it working) that any new threads finish before the variable goes out of scope with the use of pthread_join, or to dynamically allocate memory for the variable on the heap.