Simple C FIFO Queues (aka Ring Buffers)

main.c

#define QUEUE_SIZE 16

int main(int argc, char** argv) {
    queue_t example = {0, 0, QUEUE_SIZE, malloc(sizeof(void*) * QUEUE_SIZE)};
  
    // Write until queue is full
    for (int i=0; i<QUEUE_SIZE; i++) {
        int res = queue_write(&example, (void*)(i+1));
        assert((i == QUEUE_SIZE - 1) ? res == -1: res == 0);
    }
    // Read until queue is empty
    for (int i=0; i<QUEUE_SIZE; i++) {
        void* handle = queue_read(&example);
        assert((i == QUEUE_SIZE - 1) ? handle == NULL: handle == (i+1));
    }
}

safe_queue.c

// A simple fifo queue (or ring buffer) in c.
// This implementation \should be\ "thread safe" for single producer/consumer with atomic writes of size_t.
// This is because the head and tail "pointers" are only written by the producer and consumer respectively.
// Demonstrated with void pointers and no memory management.
// Note that empty is head==tail, thus only QUEUE_SIZE-1 entries may be used.

#include <stdlib.h>
#include <assert.h>

typedef struct {
    size_t head;
    size_t tail;
    size_t size;
    void** data;
} queue_t;

void* queue_read(queue_t *queue) {
    if (queue->tail == queue->head) {
        return NULL;
    }
    void* handle = queue->data[queue->tail];
    queue->data[queue->tail] = NULL;
    queue->tail = (queue->tail + 1) % queue->size;
    return handle;
}

int queue_write(queue_t *queue, void* handle) {
    if (((queue->head + 1) % queue->size) == queue->tail) {
        return -1;
    }
    queue->data[queue->head] = handle;
    queue->head = (queue->head + 1) % queue->size;
    return 0;
}

yolo_queue.c

// A simple and terrifying fifo queue in C.
// This is "thread safe" in the same manner as the previous example.
// It's also an "optimisation" to add length, save space and improve speed.
// Don't let the head/tail "pointers" overflow.

#include <stdlib.h>
#include <assert.h>

typedef struct {
  size_t head;
  size_t tail;
  void** data;
} queue_t;

size_t queue_count(queue_t *queue) {
  return queue->head - queue->tail;
}

void* queue_read(queue_t *queue) {
  if (queue->head == queue->tail) {
    return NULL;
  }
  void* handle = queue->data[queue->tail % queue->size];
  queue->tail ++;
  return handle;
}

int queue_write(queue_t *queue, void* handle) {
  if ((queue->head - queue->tail) == queue->size) {
    return -1;
  }
  queue->data[queue->head % queue->size] = handle;
  queue->head ++;
}

typedef struct {
size_t head;
size_t tail; //missing size_t size?
void** data;
} queue_t;

typedef struct { size_t head; size_t tail; //missing size_t size? void** data; } queue_t;

yeah, seems like OP missed that