Question

I have this piece of code that is giving me trouble.
I know all the threads are reading the same struct. But I have no idea how to fix this.

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

typedef struct {
  int a,b;
} s_param;

void *
threadfunc(void *parm)
{
  s_param *param2 = parm; 
  printf("ID:%d and v:%d\n",param2->a,param2->b);
  pthread_exit(NULL);
}

int main(int argc, char **argv)
{
  pthread_t thread[3];
  int rc=0,i;
  void * status;

  for(i=0; i<3 ; ++i){
    s_param param;
    param.b=10;
    param.a=i;
    rc = pthread_create(&thread[i], NULL, threadfunc, &param ); // !!!!
    if(rc){
      exit(1);
    }
  }  

  for(i=0; i<3 ; ++i){
    pthread_join(thread[i],&status);
  }
  return 0;
}

output:

ID:2 and v:10
ID:2 and v:10
ID:2 and v:10

and what I need:

ID:0 and v:10
ID:1 and v:10
ID:2 and v:10

Answer 1

The scope of the sparam in the for loop is static within that loop. When you set .a and .b you are writing to the same struct over and over, and all three threads are getting a pointer to that same single struct.

Instead, you could create three separate instances of the struct by making an array of them like so..

int main(int argc, char **argv)
{
  pthread_t thread[3];
  s_param param[3];
  int rc=0,i;
  void * status;

  for(i=0; i<3 ; ++i){
    param[i].b=10;
    param[i].a=i;
    rc = pthread_create(&thread[i], NULL, threadfunc, &param[i] ); // !!!!
    if(rc){
      exit(1);
    }
  } 
... etc

Should mention that creating the structs (and the threads) in an array like this is only feasible since you clearly do a join() with the main thread. If you didn’t do that join, you would be advised to either statically allocate the structs outside of the main function, or malloc them from the heap, because as soon as the entry thread exits the main function, the stack frame containing the array will become invalid and will soon be overwritten in an unpredictable way.