A simple array/buffer datastructure

array.h

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

// Dynamic Array:
//  A stretchy buffer datastructure and a macro interface for both the buffers and static arrays.
//  Structure idea by Sean Barrett:
//  https://github.com/nothings/stb/blob/master/deprecated/stretchy_buffer.h

typedef struct BufHdr BufHdr;
typedef struct ArrHdr ArrHdr;

struct BufHdr {
	size_t len;
	size_t cap;
	size_t size;
	char buf[];
};

struct ArrHdr {
	size_t len;
	size_t cap;
	char buf[];
};

struct BufHdr* bhdr (const void* buf) {
	BufHdr* hdr = (BufHdr*)((char*)buf - offsetof(BufHdr, buf));
	if (hdr->len == hdr->cap) {
		hdr->cap <<= 1;
		realloc(hdr, offsetof(BufHdr, buf) + hdr->size * hdr->cap); 
	}
	return hdr;
}

void* buf_init (size_t size) {
	size_t cap = 8;
	BufHdr* hdr = malloc(offsetof(BufHdr, buf) + size * cap);
	hdr->len = 0;
	hdr->cap = cap;
	hdr->size = size;
	return hdr->buf;
}

void buf_free (const void* buf) {
	free(bhdr(buf));
	buf = 0;
}

// Buffer interface

#define bdel(b) buf_free(b)
#define bset(s) buf_init(s)
#define bcap(b) bhdr(b)->cap
#define blen(b) bhdr(b)->len
#define bput(b, ...) b[bhdr(b)->len++] = (__VA_ARGS__)
#define bclr(b) bhdr(b)->len = 0

// Array interface

#define ahdr(a) ((ArrHdr*)((char*)a - offsetof(ArrHdr, buf)))
#define aset(a, s) (char[(sizeof(*a)*s)+offsetof(ArrHdr, buf)]){0}; ahdr(a)->len = 0; ahdr(a)->cap = s
#define acap(a) ahdr(a)->cap
#define alen(a) ahdr(a)->len
#define aput(a, ...) a[ahdr(a)->len++] = (__VA_ARGS__)
#define aclr(a) ahdr(a)->len = 0

// Example

#if 0
int main (int argc, char** argv) {

	// Buffer
	{
		int* b = bset(sizeof(int));
		for (int i = 0; i < 10; i++) {
			bput(b, i);
		}
		for (int i = 0; i < blen(b); i++) {
			assert(b[i] == i);
		}
		bclr(b);
		assert(blen(b) == 0);
		bdel(b);
	}

	// Array
	{
		int* a = (int*)aset(a, 8);
		for (int i = 0; i < acap(a); i++) {
			aput(a, i);
		}
		for (int i = 0; i < acap(a); i++) {
			assert(a[i] == i);
		}
		assert(alen(a) == acap(a));
		aclr(a);
		assert(alen(a) == 0);
	}
	return 0;
}
#endif