C编程-pthread_create（）循环索引作为参数

Question

I have following c code that has some problems.

I want to realize a matrix multiplication (1dim arrays) with threads

pthread_t thread[ARRAY_SIZE];


void* mmult(void* r){
   int row = (intptr_t) r;         
   int index;                                
   int column, tmp, size = ARRAY_SIZE;
   for(column = 0; column < size; column++){ 
      tmp = 0;
      for(index = 0; index < size; index++){
         tmp += MA[row * size + index]  *  MBT[column * size + index];
      }
      MC[row * size + column] = tmp;
   }

   pthread_exit(NULL);      


}
int main(void) {
   int size = ARRAY_SIZE, row, column;
   struct timeval start, end;
   double exectime;

   init_matrix(MA); //function to create Matrix
   init_matrix(MB);



   for(row = 0; row < size; row++){ /* create Array-Threads */

      if(pthread_create(&thread[row], NULL, &mmult,&row) != 0){ //also doesnt work with (void*) &row
         perror("Fehler beim Erstellen eines Feld-Threads");
         exit(EXIT_FAILURE);
      }
    }

   int i;
   for(i =0; row < size; i++){
      status = pthread_join(thread[i], NULL);
   }


   return 0;
}

I have Matrix MA and Matrix MB and Matrix MBT where the values are switched to multiply it with MA .

The size of my Matrix is defined by ARRAY_SIZE * ARRAY_SIZE

I am trying to create a thread for every row of the matrix and multiply row with column in my function mmult .

The error message I get is : Segmentation fault (core dumped)

Answer 1

Even though you haven't created a Minimal, Complete, and Verifiable Example to show us (which you really should have) it's easy to guess what the problem is.

Take a closer look at how you create your thread:

pthread_create(&thread[row], NULL, &mmult,&row)

Here you pass a pointer to the variable row . Now for how you use the argument in the thread function:

int row = (intptr_t) r;

But r is a pointer to an int , it's not the value of the current "row" when creating the thread.

It would be more correct to do

int row = *(int *) r;

But this will lead to another problem: All thread function will be called with the very same pointer, all pointing to the very same place. And since you have no control over when threads start to run, you don't know if all threads will get different values for row or if there might be duplicates.

To solve this you need to pass the value of the row variable when creating the thread. This is one of the few places where it's considered okay to cast a value to a pointer. It actually needs two casts:

pthread_create(&thread[row], NULL, &mmult,(void *) (intptr_t) row)

Now you need to do the opposite when getting the value inside the thread:

int row = (int) (intptr_t) r;

Answer 2

Your function is already a void * , there's no need to pass it by reference. You're passing a reference to the void * instead of passing void * itself. Instead try pthread_create(&thread[row], NULL, mmult, &row)

Answer 3

This is the syntax of the function pthread_create .

int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine) (void *), void *arg);

The first argument is of type pthread_t and represents the thread id although there might be some other errors.