Quick tool to list and dump data chunks from a PNG file

depng.c

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

int main(int argc, char** argv)
{
  if(argc < 2)
    return 1;

  FILE* f = fopen(argv[1], "rb");

  //Move to start of first chunk
  fseek(f, 8, SEEK_SET);

  int index = 0;
  int to_dump = -1;

  if(argc == 3)
    to_dump = atoi(argv[2]);

  while(1) {
    struct {
      uint32_t len;
      char type[5];
    } chunk;

    fread(&chunk, 4, 2, f);
    chunk.len = be32toh(chunk.len);
    chunk.type[4] = 0;

    // Listing chunks
    if(to_dump < 0)
      printf("CHUNK[%d] at 0x%x type %s, length %x\n", index, ftell(f), chunk.type, chunk.len);

    if(to_dump == index) {
      //Dump the given chunk's data
      char* buf = malloc(chunk.len);
      fread(buf, chunk.len, 1, f);
      fwrite(buf, chunk.len, 1, stdout);
      return 0;
    }

    if(strcmp(chunk.type, "IEND") == 0)
      break;

    fseek(f, chunk.len + 4 /* checksum */, SEEK_CUR);
    index += 1;
  }

  return 0;
}