Portable Network Graphics, PNG analyzer

// by petter wahlman, http://www.twitter.com/badeip
// display and verify the crc of PNG chunk data
// hex.h - available here: http://pastebin.com/yj3xsZLW
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <arpa/inet.h>

#include "hex.h"

unsigned long crc_table[256];

int crc_table_computed = 0;

void make_crc_table(void)
{
    unsigned long c;
    int n, k;

    for (n = 0; n < 256; n++) {
        c = (unsigned long) n;
        for (k = 0; k < 8; k++) {
            if (c & 1)
                c = 0xedb88320L ^ (c >> 1);
            else
                c = c >> 1;
        }
        crc_table[n] = c;
    }
    crc_table_computed = 1;
}

unsigned long update_crc(unsigned long crc, unsigned char *buf, int len)
{
    unsigned long c = crc;
    int n;

    if (!crc_table_computed)
        make_crc_table();
    for (n = 0; n < len; n++) {
        c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
    }
    return c;
}

unsigned long crc(unsigned char *buf, int len)
{
    return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;
}

int print_sections(char *mem, size_t size)
{
    char *ptr = mem;
    int i;

    i = 0;

    if ((*(unsigned int *)ptr != 0x474e5089) ||
        (*(unsigned int *)(ptr + sizeof(int)) != 0x0a1a0a0d)) {
        fprintf(stderr, "error, not a valid png-image\n");
        return 1;
    }

    i += 8;

    do {
        unsigned int section_len;
        unsigned int org_crc, calc_crc;

        printf("file size:      0x%08zx\n", size);

        section_len = ntohl(*(unsigned int *)&ptr[i]);
        printf("section offset: 0x%08x\n", i);

        printf("section length: 0x%08x\n", section_len);
        i += 4;

        printf("section name:   %c%c%c%c\n",
            isalpha(*(ptr + i + 0)) ? *(ptr + i + 0) : '?',
            isalpha(*(ptr + i + 1)) ? *(ptr + i + 1) : '?',
            isalpha(*(ptr + i + 2)) ? *(ptr + i + 2) : '?',
            isalpha(*(ptr + i + 3)) ? *(ptr + i + 3) : '?');
        i += 4;

        calc_crc = ntohl(crc((unsigned char *)&mem[i - 4], section_len + 4));
        printf("calculated crc: 0x%08x\n", calc_crc);

        org_crc = *(unsigned int *)&ptr[i + section_len];
        printf("crc:            0x%08x\n", org_crc);

        if (org_crc != calc_crc) {
            fprintf(stderr, "error, incorrect CRC\n");
            return 1;
        }

        printf("data:\n");
        print_hex(mem + i, section_len, 0);
        i += section_len;

        i += 4; // skip past crc (printed above)

        printf("\n");
    } while(i < size);

    return 0;
}

int main(int argc, char **argv)
{
    int fd;
    char *map;
    char *png;
    struct stat st;
    int i;

    map = NULL;
    if (argc < 2) {
        argc = 2;
        argv[1] = "challenge.png";
    }
    for (i = 1; i < argc; i++) {
        png = argv[i];
        printf("filename:       %s\n", png);
        fd = open(png, O_RDONLY);
        if (-1 == fd) {
            perror(png);
            return 1;
        }

        fstat(fd, &st);

        map = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
        close(fd);
        print_sections(map, st.st_size);
    }
    if (isatty(fileno(stdin)))
        munmap(map, st.st_size);

    return 0;
}