Incrementing variable's value with multithreading

Question

I tried to write a Code with 2 threads, which should increment the 2 numbers x y to 100, everytime an increment happens, it should be printed out.

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

void *inc_x(void *x_void_ptr){
   int *x_ptr = (int *)x_void_ptr;
   while (++(*x_ptr)<100) {
      printf("x increment to %d \n",*x_ptr);
      *x_ptr++;
   }
   return NULL;

}

void *inc_y(void *y_void_ptr){
   int *y_ptr = (int *)y_void_ptr;
   while (++(*y_ptr)<100) {
       printf("y increment to %d \n",*y_ptr);
       *y_ptr++;
   }
   return NULL;
}

int main()
{
    int x = 0;
    int y = 0;
    printf("x: 0 , y : 0\n");
    pthread_t inc_x_thread, inc_y_thread;
    if (pthread_create(&inc_x_thread, NULL, inc_x, &x)) {
        fprintf(stderr, "Error creating thread \n");
        return 1;
    }
    if (pthread_create(&inc_y_thread, NULL, inc_y, &y)) {
        fprintf(stderr, "Error creating thread \n");
        return 2;
     }
    printf("%d , %d" , x,y);

    return 0;
}

but my x,y values were not incremented. Can someone tell me why ? Thanks (I am new to C by the way).

Answer 1

use following in main before checking reflected values of x and y:

    pthread_join(inc_x_thread, NULL);
    pthread_join(inc_y_thread, NULL);

and

use like this in both functions while incrementing : (*y_ptr)++;

    void *inc_y(void *y_void_ptr){
    int *y_ptr = (int *)y_void_ptr;
    while (++(*y_ptr)<100) {
    printf("y increment to %d \n",*y_ptr);
    (*y_ptr)++;  // correction here
    }
    return NULL;
    }

Answer 2

You can try to use mutex, what will stop another thread until finish that task.

do something like that.

init mutex variable

pthread_mutex_t mutex;
pthread_mutex_init(&mutex, NULL);

Then inside function

 void *inc_x(void *x_void_ptr){
     pthread_mutex_lock(&mutex);

     //do something what will shared with another threads.

     pthread_mutex_unlock(&mutex);
     ...
 }

Then destroy mutex

pthread_mutex_destroy(&mutex);