arr.c

#include <stdalign.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
  size_t count;
  size_t capacity;
} Header;

#define ARR_INIT_CAPACITY 8
#define ARR_GROWTH_FACTOR 1.5

#define ARR_HDR_SIZE                             \
  ((sizeof(Header) + alignof(max_align_t) - 1) & \
   ~(alignof(max_align_t) - 1))

#define arr__hdr(arr) \
  ((Header*)((char*)(arr) - ARR_HDR_SIZE))

#define arr_len(arr) \
  ((arr) ? arr__hdr(arr)->count : 0)

#define arr_free(arr)      \
  do {                     \
    if (arr) {             \
      free(arr__hdr(arr)); \
      (arr) = NULL;        \
    }                      \
  } while (0)

static int arr__push_impl(
  void** arr,
  size_t elem_size,
  const void* value
) {
  if (*arr == NULL) {
    size_t cap = ARR_INIT_CAPACITY;
    if (cap == 0 || cap > SIZE_MAX / elem_size) return 0;

    size_t bytes = ARR_HDR_SIZE + elem_size * cap;
    Header* h = malloc(bytes);
    if (!h) {
      return 0;
    }

    h->count = 0;
    h->capacity = cap;
    *arr = (char*)h + ARR_HDR_SIZE;
  }

  Header* h = arr__hdr(*arr);

  if (h->count >= h->capacity) {
    size_t newcap = h->capacity * ARR_GROWTH_FACTOR;

    if (newcap <= h->capacity ||
        newcap > SIZE_MAX / elem_size) {
      return 0;
    }

    size_t bytes = ARR_HDR_SIZE + elem_size * newcap;
    Header* newh = realloc(h, bytes);
    if (!newh) {
      return 0;
    }

    newh->capacity = newcap;
    *arr = (char*)newh + ARR_HDR_SIZE;
    h = newh;
  }

  memcpy(
    (char*)(*arr) + elem_size * h->count,
    value,
    elem_size
  );

  h->count++;
  return 1;
}

#define arr_push(arr, value)   \
  arr__push_impl(              \
    (void**)&(arr),            \
    sizeof(*(arr)),            \
    &(typeof(*(arr))){(value)} \
  )

int main(void) {
  uint32_t* numbers = {};

  for (size_t i = 0; i < 256; i++) {
    if (!arr_push(numbers, i * i)) {
      fprintf(stderr, "Failed to push %zu\n", i * i);
      return 1;
    }
  }

  for (size_t i = 0; i < arr_len(numbers); i++) {
    printf("%u ", numbers[i]);
  }

  printf("\n");

  arr_free(numbers);
  return 0;
}