Implementing a generic Priority Queue in C (using heaps)

This article considers you are already familiar with the concept of a Priority Queue.

It’s a Queue with a twist. You push() elements like in a standard Queue, but when it comes to pop() them out you take the element with the “highest” priority.

The implementation is going to use a Binary Heap.

The following code is also available on github:

git clone


#ifndef __PQUEUE__H__
#define __PQUEUE__H__

* Debugging macro .
* Checks for a NULL pointer, and prints the error message, source file and
* line via 'stderr' .
* If the check fails the program exits with error code (-1) .
#define NP_CHECK(ptr) \
    { \
        if (NULL == (ptr)) { \
            fprintf(stderr, "%s:%d NULL POINTER: %s n", \
                __FILE__, __LINE__, #ptr); \
            exit(-1); \
        } \
    } \

#define DEBUG(msg) fprintf(stderr, "%s:%d %s", __FILE__, __LINE__, (msg))

* Priority Queue Structure
typedef struct PQueue_s {
    /* The actual size of heap at a certain time */
    size_t size;
    /* The amount of allocated memory for the heap */
    size_t capacity;
    /* An array of (void*), the actual max-heap */
    void **data;
    /* A pointer to a comparator function, used to prioritize elements */
    int (*cmp)(const void *d1, const void *d2);
} PQueue;

/** Allocates memory for a new Priority Queue .
Needs a pointer to a comparator function, thus establishing priorities .
PQueue *pqueue_new(int (*cmp)(const void *d1, const void *d2),
                   size_t capacity);

/** De-allocates memory for a given Priority Queue */
void pqueue_delete(PQueue *q);

/** Add an element inside the Priority Queue */
void pqueue_enqueue(PQueue *q, const void *data);

/** Removes the element with the greatest priority from within the Queue */
void *pqueue_dequeue(PQueue *q);