C pthread_join return value

9
votes

I'm attempting to print a return value from pthread_join. I have the following code:

    for(j = 0 ; j < i ; ++j){
        pthread_join( tid[j], returnValue);  /* BLOCK */
        printf("%d\n",  (int)&&returnValue);
}

All of the threads are stored in the tid array and are created and returned correctly. At the end of each thread function I have the following line: 

pthread_exit((void *)buf.st_size);

I am attempting to return the size of some file I was reading. For some reason I cannot get it to print the correct value. It is more then likely the way I am attempting to dereference the void ** from the pthread_join function call, but I'm not too sure how to go about doing that. Thanks in advance for any help.

2
creturnpthreads

2 Answers

15
votes

You need to pass the address of a void * variable to pthread_join -- it will get filled in with the exit value. That void * then should be cast back to whatever type was originally stored into it by the pthread_exit call:

for(j = 0 ; j < i ; ++j) {
    void *returnValue;
    pthread_join( tid[j], &returnValue);  /* BLOCK */
    printf("%zd\n",  (size_t)(off_t)returnValue);
}
0
votes

This is working:

for(j = 0 ; j < i ; ++j) {
    int returnValue;
    pthread_join( tid[j], (void **)&returnValue);  /* BLOCK */
    printf("%d\n",  returnValue);
}