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?

Answer 1

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.

Answer 2

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  }