Monotonic token bucket rate limiter in portable C

#define _POSIX_C_SOURCE 200809L
#include <time.h>
typedef struct { double tokens, rate, cap; struct timespec last; } Bucket;
void tb_init(Bucket *b, double rate, double cap) {
    b->tokens = b->cap = cap;
    b->rate = rate;
    clock_gettime(CLOCK_MONOTONIC, &b->last);
}
int tb_allow(Bucket *b, double cost) {
    struct timespec now;
    clock_gettime(CLOCK_MONOTONIC, &now);
    double elapsed = (now.tv_sec - b->last.tv_sec)
        + (now.tv_nsec - b->last.tv_nsec) / 1e9;
    b->last = now;
    b->tokens += elapsed * b->rate;
    if (b->tokens > b->cap) b->tokens = b->cap;
    if (b->tokens < cost) return 0;
    b->tokens -= cost;
    return 1;
}