Can't multithread but works with a for loop or single thread

Question

I'm trying to get multiple threads to perform parallel calculations on a 2D array. The user specifies how many threads they want so on a 25*25 2d array if the user wants 5 threads then each thread performs calculation on 125 elements. (for simplicity I hardcoded these number just to try and get program to work under these conditions).

The code works for 1 thread and when I simulated with a for loop everything works correctly. It's a conways game of life program. With 1 thread or forloop calling gen function 5 times the programs works fine. It prints out the grids properly. With 5 threads it just prints out once and program ends

I can't test inside the threads because printf doesn't work in threads. I've spent hours on this and I can't figure it out.

int N; 
int **gridA;// odd generations
int **gridB;//even
int T = 5;//number of threads

int main ( int argc, char *argv[] ){
  int i,j;
  const int STACK_SIZE = 65536;
  char *stack;
  char *stackTop[t];
  pid_t cret[t], wret;
  N = 25;//array size [25][25];
  //initialize stack
   stack = malloc(STACK_SIZE);
  for(i = 0; i < T; i++){
    stackTop[i] = stack + STACK_SIZE;
  }

//initilize arrays and load gridA with input

    while(1){}
      for(i=0; i < T; i++) cret[i]=clone(generateNext, stackTop[i], CLONE_VM|SIGCHLD, (void*)i);//thread code
      for(i=0; i < T; i++) waitpid(cret[i],&status,0);//wait for threads to finish       

   //for(i=0; i < T; i++){generateNext((void*)i);} Simulate threads, works like this
    if(toggle){//grids used interchangeably. 
  print_array(gridA);
        toggle = 0;
    } else {
        print_array(gridB);
        toggle = 1;
    }
  }
}

//figures out the next generation
void generateNext(void *p){
//finds out the two points each thread will calculate for by using p
//eg first thread: gridA[0][24] to [4][24] 2nd thread: [5][25] to 9[25]
//then finds neighbours and changes state of each element accordingly

}

Answer 1

while(1){} is a bad idea. Please provide the code that actually causes problems.

That said clone() and waitpid() are not really for multithreading but for multiprocessing. Since processes have separate memory spaces, this can't work. Please check out any pthread tutorial to get you started with multithreading.