Question

I am making a message queue API in C, but I am having a problem with the receive method. I send a char* message (“BOB”) to the message queue and it is successfully stored. I then try to receive the message but that fails.

Inside mq_receive() the correct message is dequeued and ret_val->buf points to 0x012f5754 (“BOB”). Next, msg_ptr (which is originally 0x00000000) is assigned 0x012f5754. Everything works as expected until the program returns to main(). In main(), receive_message is still NULL. I was expecting it to point to the first character of BOB which is 0x012f5754. What am I doing wrong? Thanks.

//main.c
main(){
    char* receive_message = NULL;
    //message queue init ...
    mq_send(msq_id, "BOB"); //this works correctly
    mq_receive(msq_id, receive_message);      
    printf("return value: %p\n", receive_message); 
}

//message_queue.c
mqd_t mq_receive(mqd_t mqdes, char *msg_ptr)
{
    queue_t* ret_val;
    q_attr* attr_ptr = (q_attr*)mqdes;
    ret_val = dequeue(attr_ptr);
    //all this works ret_val->buf points to BOB
    msg_ptr = ret_val->buf;
    return mqdes;
}

Answer 1

Arguments are passed by value in C, you need to pass the address of receive_message to mq_receive():

mqd_t mq_receive(mqd_t mqdes, char **msg_ptr)
{
    queue_t* ret_val;
    q_attr* attr_ptr = (q_attr*)mqdes;
    ret_val = dequeue(attr_ptr);
    //all this works ret_val->buf points to BOB
    *msg_ptr = ret_val->buf;
    return mqdes;
}

and invoke it:

mq_receive(msq_id, &receive_message);