gcc zero-length array extension demo

buffer_test.c

#include <stdio.h>
#include <malloc.h>

struct static_buffer {
  size_t size;
  char buffer[0];
};

struct static_buffer* alloc_buffer(size_t size)
{
  struct static_buffer *buf;
  size_t alloc_size;

  alloc_size = sizeof(struct static_buffer) + size;
  printf("sizeof(struct static_buffer) = %lu\n"
    "size = %lu\n"
    "Allocating %lu bytes\n",
    sizeof(struct static_buffer),
    size,
    alloc_size);
  buf = (struct static_buffer*)malloc(alloc_size);

  if (!buf)
    return NULL;

  buf->size = size;
  return buf;
}

int main()
{
  int i;
  struct static_buffer *my_buf;

  // Alloc buffer to store 32 bytes
  my_buf = alloc_buffer(0x20);

  // Fill with marker
  for (i = 0; i < my_buf->size; ++i)
    my_buf->buffer[i] = i % 0xFF;

  // Dump buffer object
  puts("Buffer:");
  for (i = 0; i < sizeof(struct static_buffer) + 0x20; ++i)
  {
    printf("%02x ", ((unsigned char*)my_buf)[i]);
    if (i % 0xF == 0xE)
      putchar('\n');
  }
  putchar('\n');
}