Global dispatch queue (single instance) in C. Set a global queue once and use it anywhere the header file is imported.

GlobalQueue.c

#include "GlobalQueue.h"

dispatch_queue_t queue;

dispatch_queue_t queue_ref(void) {
    if (!queue) {
        queue = dispatch_queue_create_with_target("GlobalDispatchQueue", DISPATCH_QUEUE_SERIAL, dispatch_get_main_queue());
    }
    
    return queue;
}

GlobalQueue.h

#ifndef GlobalQueue_h
#define GlobalQueue_h

#include <stdio.h>
#include <dispatch/dispatch.h>

extern dispatch_queue_t queue;

extern dispatch_queue_t queue_ref(void);

#endif /* GlobalQueue_h */

// To use:
// 1. #include "GlobalQueue.h" in any .m or .c source file
// 2. Call queue_ref() to use the dispatch queue
//    Example:
//    dispatch_queue_t serial_queue_with_queue_target = dispatch_queue_create_with_target("serial_queue_with_queue_target", DISPATCH_QUEUE_SERIAL, queue_ref());
//    
//    Note:
//    Once queue_ref() has been called, the queue can be used via the queue variable in all source files
//    Example:
//
//    Example:
//    dispatch_queue_t serial_queue_with_queue_target = dispatch_queue_create_with_target("serial_queue_with_queue_target", DISPATCH_QUEUE_SERIAL, queue));
//