Question

I am learning multithreading performance in C. When I tried to write a sample code, I bumped into a problem:

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

typedef struct{
    int a;
    char b;
    } args;

void* some_func (void* arg)
{
    args *argsa = malloc(sizeof(args));
//copy the content of arg to argsa, 
//so changes to arg in main would not affect argsa
    *argsa = *(args*) arg; 
    int i = 10;
    for (; i > 0; i--)
    {
        usleep (1); //to give other threads chances to cut in
        printf ("This is from the thread %d\n", argsa->a);
    }
    free (argsa);
}
int main()

{
    pthread_t thread[3];
    args ss;
    int index = 0;
    ss.b = 's';
    for (; index <3 ; index++)
    {
        ss.a = index;
        if (pthread_create (thread+index, NULL, some_func, (void*)&ss ))
        {
            usleep(10);
            printf ("something is wrong creating the thread"); 
        }
    }
        pthread_join ( thread[0], NULL);
        pthread_join ( thread[1], NULL);
        pthread_join ( thread[2], NULL);
    return 0;
}

I know char b in the struct is useless, but I just want to practice passing a structure.
I expect the code to print out “This is from the thread x”, where x is 0, 1 or 2, alternatively. However, the code currently only gives me “This is from the thread 2” 30 times. I believe there is something wrong with

*argsa = *(args*) arg; 

But I can’t find a way to solve this and get the desired output.

Any help would be appreciated!

Answer 1

Because you are passing the same pointer to all the threads. By the time thread 0 has started, you have already incremented the value of ss.a to 1 (and then 2).

This is a bit more correct:

void* some_func (void* arg)
{
    args *argsa = (args*)arg;
    int i;
    for (i = 0; i < 10; i++)
    {
        usleep (1); //to give other threads chances to cut in
        printf ("This is from the thread %d\n", argsa->a);
    }
}
int main()
{
    pthread_t thread[3];
    args ss[3];
    int index;
    for (index = 0; index < 3; index++)
    {
        ss[index].a = index;

        if (pthread_create(&thread[index], NULL, some_func, &ss[index] ))
        {
            printf ("something is wrong creating the thread"); 
        }
    }
    pthread_join ( thread[0], NULL);
    pthread_join ( thread[1], NULL);
    pthread_join ( thread[2], NULL);
    return 0;
}