Why is retval a void** in pthread_join?

I am having a hard time understanding why pthread_join's retval argument is a void**. I have read the manpage and tried to wrap my head around it but I still cannot fully understand it. I couldn't convince myself that retval cannot be a void*. Could someone please enlighten me?

Thank you very much in advance!

It's because you are supposed to supply the address of a void* to pthread_join. pthread_join will then write the address supplied by pthread_exit(void*) into the variable (who's address you supplied).

Example scenario:

typedef struct {
    // members
} input_data;

typedef struct {
    // members
} output_data;

Starting thread side:

input_data id;
pthread_create(..., start_routine, &id);

void* start_routine(void *ptr) {
    input_data *id = ptr;
    output_data *od = malloc(sizeof *od);
    // use the input data `id`, populate the output data `od`.
    pthread_exit(od);
}

Joining side:

output_data *od;
pthread_join((void**) &od);
// use `od`
free(od);