Pointer to pointer confusion

Question

Edit: The reason queue is 2d is because I need a pointer of Command so that cmd can equal NULL. NULL == (void *). This is where I get confused though, and why I've come here. :)

To help try and figure out another problem I have in Python, I'm implementing a small test program in C. While I know a little, apparently I'm confused. I'm trying to write a simple queue to be used in asynchronous USB transfers. Something's not right with queue, as every command popped from the queue is the same. If I write queue[1024][0] as queue[1024][1] instead, the command alternates between two distinct commands, and the program crashes in command_thread_main. Apparently it doesn't notice that cmd should be NULL. Changing [1] any higher has no effect as far as I can tell. Any hints?

typedef struct Command {
    void (*cb) (char *data, int size);
    unsigned char *data;
    int size;
} Command;

struct Command queue[1024][0];

int queueEnd = 0;
int queueStart = 0;

static void queue_push(void (*cb), unsigned char *data, int size) {
    if (queueEnd >= 1024)
        return;
    queue[queueEnd]->cb = cb;
    queue[queueEnd]->data = data;
    queue[queueEnd]->size = size;
    queueEnd++;
}

struct Command * queue_pop(void) {
    if( queueStart > queueEnd )
        return NULL;
    return queue[queueStart++];
}

static void *command_thread_main(void *arg) {
    struct Command *cmd;
    while (!do_exit) {
        if(locked) continue;
        locked = 1;
        cmd = queue_pop();
        if(cmd != NULL)
            cmd->cb(cmd->data, cmd->size);
    }
}

Answer 1

I think you have a bug you need to fix before anything else. You have a 2D array of commands and have set one of those dimensions to zero!

struct Command queue[1024][0];

When you access queue you seem to treat it as a 1D structure. Should you declare it as :

struct Command queue[1024];

Answer 2

• Don't you mean struct Command queue[1024];? (That is, no [0] or [1] or whatever.)
• In queue_pop I think you should test for queueStart >= queueEnd.
• You should implement a circular array.

Right now you store the struct itself in an array, not pointers to it. That is sensible. You'll need to change -> to . though:

queue[queueEnd].cb = cb;
queue[queueEnd].data = data;
queue[queueEnd].size = size;

(And hence queue_pop should return a variable of type struct Command (not struct Command *), and the main code should also be updated accordingly.)

Of course you can also pass pointers around, but with such a small struct/queue there's no actual need.

Answer 3

Another problem is that you've declared queue as an array of structs, but you're using it as an array of pointers to structs by using the dereferencing -> operator instead of the membership . one.

I don't mean to sound snarky, but compiler warning flags (-Wall for gcc) are your friends.

Answer 4

As others have pointed out, that 2D queue is definitely wrong. You need a 1D queue, and I suspect that what you want is an array of pointers:

Command * queue[1024];

I reommend you go a way and think about the problem a bit, draw some diagrams and then come back with clearer code and question.