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/* png-fix-itxt version 1.0.0
*
* Copyright 2015 Glenn Randers-Pehrson
* Last changed in libpng 1.6.18 [July 23, 2015]
*
* This code is released under the libpng license.
* For conditions of distribution and use, see the disclaimer
* and license in png.h
*
* Usage:
*
* png-fix-itxt < bad.png > good.png
*
* Fixes a PNG file written with libpng-1.6.0 or 1.6.1 that has one or more
* uncompressed iTXt chunks. Assumes that the actual length is greater
* than or equal to the value in the length byte, and that the CRC is
* correct for the actual length. This program hunts for the CRC and
* adjusts the length byte accordingly. It is not an error to process a
* PNG file that has no iTXt chunks or one that has valid iTXt chunks;
* such files will simply be copied.
*
* Requires zlib (for crc32 and Z_NULL); build with
*
* gcc -O -o png-fix-itxt png-fix-itxt.c -lz
*
* If you need to handle iTXt chunks larger than 500000 kbytes you must
* rebuild png-fix-itxt with a larger values of MAX_LENGTH (or a smaller value
* if you know you will never encounter such huge iTXt chunks).
*/
#include <stdio.h>
#include <zlib.h>
#define MAX_LENGTH 500000
/* Read one character (inchar), also return octet (c), break if EOF */
#define GETBREAK inchar=getchar(); \
c=(inchar & 0xffU);\
if (inchar != c) break
int
main(void)
{
unsigned int i;
unsigned char buf[MAX_LENGTH];
unsigned long crc;
unsigned char c;
int inchar;
/* Skip 8-byte signature */
for (i=8; i; i--)
{
GETBREAK;
putchar(c);
}
if (inchar == c) /* !EOF */
for (;;)
{
/* Read the length */
unsigned long length; /* must be 32 bits! */
GETBREAK; buf[0] = c; length = c; length <<= 8;
GETBREAK; buf[1] = c; length += c; length <<= 8;
GETBREAK; buf[2] = c; length += c; length <<= 8;
GETBREAK; buf[3] = c; length += c;
/* Read the chunkname */
GETBREAK; buf[4] = c;
GETBREAK; buf[5] = c;
GETBREAK; buf[6] = c;
GETBREAK; buf[7] = c;
/* The iTXt chunk type expressed as integers is (105, 84, 88, 116) */
if (buf[4] == 105 && buf[5] == 84 && buf[6] == 88 && buf[7] == 116)
{
if (length >= MAX_LENGTH-12)
break; /* To do: handle this more gracefully */
/* Initialize the CRC */
crc = crc32(0, Z_NULL, 0);
/* Copy the data bytes */
for (i=8; i < length + 12; i++)
{
GETBREAK; buf[i] = c;
}
if (inchar != c) /* EOF */
break;
/* Calculate the CRC */
crc = crc32(crc, buf+4, (uInt)length+4);
for (;;)
{
/* Check the CRC */
if (((crc >> 24) & 0xffU) == buf[length+8] &&
((crc >> 16) & 0xffU) == buf[length+9] &&
((crc >> 8) & 0xffU) == buf[length+10] &&
((crc ) & 0xffU) == buf[length+11])
break;
length++;
if (length >= MAX_LENGTH-12)
break;
GETBREAK;
buf[length+11] = c;
/* Update the CRC */
crc = crc32(crc, buf+7+length, 1);
}
if (inchar != c) /* EOF */
break;
/* Update length bytes */
buf[0] = (unsigned char)((length >> 24) & 0xffU);
buf[1] = (unsigned char)((length >> 16) & 0xffU);
buf[2] = (unsigned char)((length >> 8) & 0xffU);
buf[3] = (unsigned char)((length ) & 0xffU);
/* Write the fixed iTXt chunk (length, name, data, crc) */
for (i=0; i<length+12; i++)
putchar(buf[i]);
}
else
{
if (inchar != c) /* EOF */
break;
/* Copy bytes that were already read (length and chunk name) */
for (i=0; i<8; i++)
putchar(buf[i]);
/* Copy data bytes and CRC */
for (i=8; i< length+12; i++)
{
GETBREAK;
putchar(c);
}
if (inchar != c) /* EOF */
{
break;
}
/* The IEND chunk type expressed as integers is (73, 69, 78, 68) */
if (buf[4] == 73 && buf[5] == 69 && buf[6] == 78 && buf[7] == 68)
break;
}
if (inchar != c) /* EOF */
break;
if (buf[4] == 73 && buf[5] == 69 && buf[6] == 78 && buf[7] == 68)
break;
}
return 0;
}
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