Multiple arguments to function called by pthread_create()?

Question

I need to pass multiple arguments to a function that I would like to call on a separate thread. I've read that the typical way to do this is to define a struct, pass the function a pointer to that, and dereference it for the arguments. However, I am unable to get this to work:

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

struct arg_struct {
    int arg1;
    int arg2;
};

void *print_the_arguments(void *arguments)
{
    struct arg_struct *args = (struct arg_struct *)args;
    printf("%d\n", args -> arg1);
    printf("%d\n", args -> arg2);
    pthread_exit(NULL);
    return NULL;
}

int main()
{
    pthread_t some_thread;
    struct arg_struct args;
    args.arg1 = 5;
    args.arg2 = 7;

    if (pthread_create(&some_thread, NULL, &print_the_arguments, (void *)&args) != 0) {
        printf("Uh-oh!\n");
        return -1;
    }

    return pthread_join(some_thread, NULL); /* Wait until thread is finished */
}

The output for this should be:

5
7

But when I run it I actually get:

141921115
-1947974263

Anyone know what I'm doing wrong?

Answer 1

Because you say

struct arg_struct *args = (struct arg_struct *)args;

instead of

struct arg_struct *args = arguments;

Answer 2

use

struct arg_struct *args = (struct arg_struct *)arguments;

in place of

struct arg_struct *args = (struct arg_struct *)args;

Answer 3

main() has it's own thread and stack variables. either allocate memory for 'args' in the heap or make it global:

struct arg_struct {
    int arg1;
    int arg2;
}args;

//declares args as global out of main()

Then of course change the references from args->arg1 to args.arg1 etc..

Answer 4

Use:

struct arg_struct *args = malloc(sizeof(struct arg_struct));

And pass this arguments like this:

pthread_create(&tr, NULL, print_the_arguments, (void *)args);

Don't forget free args! ;)