ff141eb21e
Added information in the documentation about problems with and fixes for the bad CRC and bad iTXt chunk situations.
154 lines
3.8 KiB
C
154 lines
3.8 KiB
C
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/* png-fix-itxt version 1.0.0
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*
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* Copyright 2013 Glenn Randers-Pehrson
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* Last changed in libpng 1.6.3 [(PENDING RELEASE)]
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*
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* This code is released under the libpng license.
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* For conditions of distribution and use, see the disclaimer
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* and license in png.h
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*
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* Usage:
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*
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* png-fix-itxt.exe < bad.png > good.png
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*
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* Fixes a PNG file written with libpng-1.6.0 or 1.6.1 that has one or more
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* uncompressed iTXt chunks. Assumes that the actual length is greater
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* than or equal to the value in the length byte, and that the CRC is
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* correct for the actual length. This program hunts for the CRC and
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* adjusts the length byte accordingly. It is not an error to process a
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* PNG file that has no iTXt chunks or one that has valid iTXt chunks;
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* such files will simply be copied.
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*
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* Requires zlib (for crc32 and Z_NULL); build with
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*
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* gcc -O -o png-fix-itxt png-fix-itxt.c -lz
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*
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* If you need to handle iTXt chunks larger than 500000 kbytes you must
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* rebuild png-fix-itxt with a larger values of MAX_LENGTH (or a smaller value
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* if you know you will never encounter such huge iTXt chunks).
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*/
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#include <stdio.h>
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#include <zlib.h>
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#define MAX_LENGTH 500000
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#define GETBREAK ((unsigned char)(inchar=getchar())); if (inchar == EOF) break
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int
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main(void)
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{
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unsigned int i;
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unsigned char buf[MAX_LENGTH];
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unsigned long crc;
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unsigned char c;
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int inchar;
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/* Skip 8-byte signature */
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for (i=8; i; i--)
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{
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c=GETBREAK;
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putchar(c);
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}
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if (inchar != EOF)
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for (;;)
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{
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/* Read the length */
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unsigned long length; /* must be 32 bits! */
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c=GETBREAK; buf[0] = c; length = c; length <<= 8;
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c=GETBREAK; buf[1] = c; length += c; length <<= 8;
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c=GETBREAK; buf[2] = c; length += c; length <<= 8;
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c=GETBREAK; buf[3] = c; length += c;
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/* Read the chunkname */
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c=GETBREAK; buf[4] = c;
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c=GETBREAK; buf[5] = c;
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c=GETBREAK; buf[6] = c;
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c=GETBREAK; buf[7] = c;
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/* The iTXt chunk type expressed as integers is (105, 84, 88, 116) */
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if (buf[4] == 105 && buf[5] == 84 && buf[6] == 88 && buf[7] == 116)
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{
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if (length >= MAX_LENGTH-12)
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break; /* To do: handle this more gracefully */
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/* Initialize the CRC */
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crc = crc32(0, Z_NULL, 0);
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/* Copy the data bytes */
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for (i=8; i < length + 12; i++)
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{
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c=GETBREAK; buf[i] = c;
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}
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/* Calculate the CRC */
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crc = crc32(crc, buf+4, (uInt)length+4);
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for (;;)
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{
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/* Check the CRC */
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if (((crc >> 24) & 0xff) == buf[length+8] &&
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((crc >> 16) & 0xff) == buf[length+9] &&
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((crc >> 8) & 0xff) == buf[length+10] &&
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((crc ) & 0xff) == buf[length+11])
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break;
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length++;
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if (length >= MAX_LENGTH-12)
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break;
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c=GETBREAK;
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buf[length+11]=c;
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/* Update the CRC */
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crc = crc32(crc, buf+7+length, 1);
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}
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/* Update length bytes */
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buf[0] = (length << 24) & 0xff;
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buf[1] = (length << 16) & 0xff;
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buf[2] = (length << 8) & 0xff;
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buf[3] = (length ) & 0xff;
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/* Write the fixed iTXt chunk (length, name, data, crc) */
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for (i=0; i<length+12; i++)
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putchar(buf[i]);
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}
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else
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{
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/* Copy bytes that were already read (length and chunk name) */
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for (i=0; i<8; i++)
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putchar(buf[i]);
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/* Copy data bytes and CRC */
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for (i=8; i< length+12; i++)
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{
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c=GETBREAK;
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putchar(c);
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}
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if (inchar == EOF)
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{
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break;
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}
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/* The IEND chunk type expressed as integers is (73, 69, 78, 68) */
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if (buf[4] == 73 && buf[5] == 69 && buf[6] == 78 && buf[7] == 68)
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break;
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}
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if (inchar == EOF)
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break;
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if (buf[4] == 73 && buf[5] == 69 && buf[6] == 78 && buf[7] == 68)
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break;
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}
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return 0;
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}
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