queue.h

// Ray Lee 2022 August 22  ~  license CC0

#ifndef circular_queue_h
#define circular_queue_h


#include <stdint.h>
#include <stddef.h>

/* There are two ways a circular queue can deal with a request to enqueue
   items when it's full. Either we drop the request on the ground, or we
   overwrite old items in the back of the queue to make room for the new
   ones. Below enqueue(q, ...) implements the latter behavior. Check for
   queue_full(q) before enqueue for the former.

   The implementation below lets head and tail be free running. This lets
   us easily distinguish queue empty (head==tail) vs queue full, at the
   cost of always using a modulo operator to find the correct position in
   the backing array. Declaring circular queues of length 2^n is efficient.
*/

#define length_(arr) (sizeof(arr)/sizeof(arr[0]))

#define queue(type, size) \
	struct __queue_of_##size_##type { \
		size_t head, tail; \
		type entry[size]; \
	}

#define queue_count(q) ((q).tail-(q).head)
#define queue_empty(q) (queue_count(q)==0)
#define queue_full(q)  (queue_count(q)==length_((q).entry))
#define queue_size(q)  (length_((q).entry))
#define queue_free(q)  (length_((q).entry)-queue_count(q))

#define enqueue(q, ...) \
	do { \
		__typeof__((q).entry[0]) args[] = {__VA_ARGS__}; \
		for (int i_=0; i_<length_(args); i_++) { \
			(q).entry[(q).tail++ % length_((q).entry)] = args[i_]; \
		} \
		if (queue_count(q) > length_((q).entry)) \
			(q).head = (q).tail - length_((q).entry); \
	} while (0)

#define dequeue(q) ( queue_empty(q) ? NULL : &((q).entry[(q).head++ % length_((q).entry)]) )
#define queue_peek(q) ((q).entry[(q).head])


#endif

queue_example.c

// qtest.c
#include <stdio.h>
#include "queue.h"

void queue_wrap_example() {
	queue(int, 10) q = {};
	for (int i=0; i<38; i++) {
		enqueue(q, i);
	}
	while (!queue_empty(q)) {
		printf("%d\n", *dequeue(q));
	}
}

typedef queue(char, 32) flight_recorder_t;

void record(flight_recorder_t *fr, char *msg) {
	while (*msg) {
		enqueue(*fr, *msg++);
	}
}

void replay(flight_recorder_t *fr) {
	while (!queue_empty(*fr)) {
		printf("%c", *dequeue(*fr));
	}
}

void queue_string_example() {
	flight_recorder_t fr = {};
	record(&fr, "Hello, world!");
	record(&fr, " This is a test.");
	record(&fr, " 1234567890");
	record(&fr, " huzzah!\n");
	replay(&fr);
}

int main() {
	queue_wrap_example();
	queue_string_example();
	return 0;
}