png.c

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

typedef struct {
  int chunks, w, h;
} info_t;

const char expected_header[8] = {0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a};

extern int chunk_callback(uint32_t type, uint32_t len, void *p);

uint32_t flip_endian(uint32_t val) {
  val = ((val << 8) & 0xff00ff00) | ((val >> 8) & 0xff00ff ); 
  return (val << 16) | ((val >> 16) & 0xffff);
}

char *parse_png(void *p, uint32_t plen, info_t *out) {
  if (plen < 8 || memcmp(p, expected_header, 8)) {
    return "invalid header";
  }

  int found_header = 0;
  out->chunks = 0;

  void *curr = p + 8;
  while (curr - p < plen) {
    uint32_t *lp = (uint32_t *) curr;
    uint32_t len = flip_endian(lp[0]);
    if ((curr - p) + (len + 12) > plen) {
      return "chunk too large";
    }

    if (!memcmp(curr + 4, "IHDR", 4)) {
      if (len != 13) {
        return "IHDR has wrong size";
      }
      found_header = 1;
      out->w = lp[2];
      out->h = lp[3];
    }

    // demonstrate malloc/free: copy chunk so caller can change it
    void *copy = malloc(len);
    if (!copy) {
      return "couldn't malloc for copy";
    }
    memcpy(copy, curr + 8, len);
    chunk_callback(lp[1], len, copy);
    free(copy);

    curr += (len + 12);  // chunk length plus 3 x 32-bit ints
    ++out->chunks;
  }
  if (!found_header) {
    return "header not found";
  }

  return NULL;
}