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?
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.