stretchy buffer

buffer.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stddef.h>
#include <assert.h>
#include <ctype.h>
#include <string.h>
#include <stdarg.h>

#define MAX(x, y) ((x) >= (y) ? (x) : (y))

typedef struct BufHdr {
	size_t len;
	size_t cap;
	char buf[0];
} BufHdr;
/*            Sean Barrett's stretchy buffer              */
/*backend*/                 
#define buf__hdr(b) ((BufHdr *)((char *)b - offsetof(BufHdr, buf)))
#define buf__fits(b, n) (buf_len(b) + (n) <= buf_cap(b))
#define buf__fit(b, n) (buf__fits(b, n) ? 0 : ((b) = buf__grow((b), buf_len(b) + (n), sizeof(*(b)))))
/* api */
#define buf_len(b) ((b) ? buf__hdr(b)->len : 0)
#define buf_cap(b) ((b) ? buf__hdr(b)->cap : 0)
#define buf_push(b, x) (buf__fit((b), 1), (b)[buf__hdr(b)->len++] = (x))
#define buf_free(b) ((b) ? (free(buf__hdr(b)), (b) = NULL) : 0)

void *buf__grow(const void *buf, size_t new_len, size_t elem_size) {
	size_t new_cap = MAX(1 + 2 * buf_cap(buf), new_len);
	assert(new_len <= new_cap);
	size_t new_size = offsetof(BufHdr, buf) + new_cap*elem_size;
	BufHdr *new_hdr;
	if (buf) {
		new_hdr = realloc(buf__hdr(buf), new_size);
	}
	else {
		new_hdr = malloc(new_size);
		new_hdr->len = 0;
	}
	new_hdr->cap = new_cap;
	return new_hdr->buf;
}