dynamic array in C (stretchy buffer technique by stb)

array.c

#include <stdio.h>
#include <stdlib.h>

//--------------------------------------------------------------------------------------------------
// Dynamic array public API.

#define array_count(a)     ((a) ? ARRAY_HEADER(a)->len : 0)
#define array_cap(a)       ((a) ? ARRAY_HEADER(a)->cap : 0)
#define array_free(a)      ((a) ? (free(ARRAY_HEADER(a)), (a)=NULL, 0) : 0)
#define array_add(a, val)  (array__maybegrow(a, 1), (a)[ARRAY_HEADER(a)->len++] = (val))

//--------------------------------------------------------------------------------------------------
// Dynamic array "private" implementation.

#define ARRAY_HEADER(a)    ((ArrayHeader*)a - 1)

typedef struct ArrayHeader
{
	int len;
	int cap;
} ArrayHeader;

static void* array_grow_impl(void* buf, size_t element_size, size_t add)
{
	size_t new_len = buf ? ARRAY_HEADER(buf)->len + add : add;
	size_t new_cap = buf ? ARRAY_HEADER(buf)->cap * 2 : 16;
	if (new_cap < new_len) new_cap = new_len;
	size_t size = sizeof(ArrayHeader) + new_cap * element_size;
	ArrayHeader* new_hdr;
	if (buf) {
		new_hdr = (ArrayHeader*)realloc(ARRAY_HEADER(buf), size);
	} else {
		new_hdr = (ArrayHeader*)malloc(size);
		new_hdr->len = 0;
	}
	new_hdr->cap = (int)new_cap;
	return (void*)(new_hdr + 1);
}
#define array__needgrow(a, n)  ((a)==NULL || ARRAY_HEADER(a)->len + (n) > ARRAY_HEADER(a)->cap)
#define array__maybegrow(a, n) (array__needgrow(a, (n)) ? (*(void**)&(a)=array_grow_impl(a, sizeof(*(a)), n)) : 0)

//--------------------------------------------------------------------------------------------------
// Demo.

int main()
{
	int* a = NULL;

	for (int i = 0; i < 100; ++i) {
		array_add(a, i);
	}

	for (int i = 0; i < 100; ++i) {
		printf("%d\n", a[i]);
	}

	printf("len=%d cap=%d\n", array_count(a), array_cap(a));
	array_free(a);

	return 0;
}