pthread_join() and pthread_exit()

Question

I have a question about C concurrency programming.

In the pthread library, the prototype of pthread_join is

int pthread_join(pthread_t tid, void **ret);

and the prototype of pthread_exit is:

void pthread_exit(void *ret);

So I am confused that, why pthread_join takes the return value of the process as a pointer to a void pointer from reaped thread, but pthread_exit only takes a void pointer from the exited thread? I mean basically they are all return values from a thread, why there is a difference in type?

score 40 · Accepted Answer · edited Apr 17 '13 at 16:39

40

In pthread_exit, ret is an input parameter. You are simply passing the address of a variable to the function.

In pthread_join, ret is an output parameter. You get back a value from the function. Such value can, for example, be set to NULL.

Long explanation:

In pthread_join, you get back the address passed to pthread_exit by the finished thread. If you pass just a plain pointer, it is passed by value so you can't change where it is pointing to. To be able to change the value of the pointer passed to pthread_join, it must be passed as a pointer itself, that is, a pointer to a pointer.

edited Apr 17 '13 at 16:39

Eduardo Bezerra

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answered Dec 15 '11 at 01:16

stefanB

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But why define ret in `pthread_exit` a `void *` type, it'a always `NULL` or some other constant values – stonestrong Feb 13 '14 at 08:22

score 32 · Answer 2 · edited Jun 23 '20 at 15:04

It because every time

void pthread_exit(void *ret);

will be called from thread function so which ever you want to return simply its pointer pass with pthread_exit().

Now at

int pthread_join(pthread_t tid, void **ret);

will be always called from where thread is created so here to accept that returned pointer you need double pointer ..

i think this code will help you to understand this

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

void* thread_function(void *ignoredInThisExample)
{
    char *a = malloc(10);
    strcpy(a,"hello world");
    pthread_exit((void*)a);
}
int main()
{
    pthread_t thread_id;
    char *b;

    pthread_create (&thread_id, NULL,&thread_function, NULL);

    pthread_join(thread_id,(void**)&b); //here we are reciving one pointer 
                                        value so to use that we need double pointer 
    printf("b is %s\n",b); 
    free(b); // lets free the memory

}

Why do you have to free(b) if it's in main()? Is this because you allocated a on the heap? — M. Smith, Oct 01 '20 at 04:12

score 2 · Answer 3 · answered Dec 15 '11 at 01:13

2

The typical use is

void* ret = NULL;
pthread_t tid = something; /// change it suitably
if (pthread_join (tid, &ret)) 
   handle_error();
// do something with the return value ret

answered Dec 15 '11 at 01:13

Basile Starynkevitch

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pthread_join() and pthread_exit()

3 Answers3

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