Fixed size ring buffer queue in C

rbq.c

#include <stdlib.h>

typedef int rbq_val_t;
struct rbq_t {
    rbq_val_t *buffer;
    size_t capacity, size;
    size_t p;
};

void init_rbq(struct rbq_t *Q, size_t capacity) {
    Q->buffer = calloc(capacity, sizeof(rbq_val_t));
    Q->capacity = capacity;
    Q->p = 0;
    Q->size = 0;
}

void free_rbq(struct rbq_t *Q) {
    free(Q->buffer);
    Q->capacity = 0;
    Q->size = 0;
    Q->p = 0;
    Q->buffer = NULL;
}

void append(struct rbq_t *Q, rbq_val_t v) {
    Q->buffer[(Q->p + Q->size++) % Q->capacity] = v;
}

rbq_val_t pop(struct rbq_t *Q) {
    return Q->buffer[(Q->p + --Q->size)];
}

void append_left(struct rbq_t *Q, rbq_val_t v) {
    Q->p = (Q->p + Q->capacity - 1) % Q->capacity;
    Q->size++;
    Q->buffer[Q->p] = v;
}

rbq_val_t pop_left(struct rbq_t *Q) {
    size_t pp = Q->p;
    Q->p = (Q->p + 1) % Q->capacity;
    Q->size--;
    return Q->buffer[pp];
}