source: bootcd/isolinux/syslinux-6.03/libfat/fatchain.c

/* ----------------------------------------------------------------------- *
 *
 *   Copyright 2004-2008 H. Peter Anvin - All Rights Reserved
 *
 *   This program is free software; you can redistribute it and/or modify
 *   it under the terms of the GNU General Public License as published by
 *   the Free Software Foundation, Inc., 53 Temple Place Ste 330,
 *   Boston MA 02111-1307, USA; either version 2 of the License, or
 *   (at your option) any later version; incorporated herein by reference.
 *
 * ----------------------------------------------------------------------- */

/*
 * fatchain.c
 *
 * Follow a FAT chain
 */

#include "libfatint.h"
#include "ulint.h"

/*
 * Convert a cluster number (or 0 for the root directory) to a
 * sector number.  Return -1 on failure.
 */
libfat_sector_t libfat_clustertosector(const struct libfat_filesystem *fs,
                                       int32_t cluster)
{
    if (cluster == 0)
        cluster = fs->rootcluster;

    if (cluster == 0)
        return fs->rootdir;
    else if (cluster < 2 || cluster >= fs->endcluster)
        return -1;
    else
        return fs->data + ((libfat_sector_t) (cluster - 2) << fs->clustshift);
}

/*
 * Get the next sector of either the root directory or a FAT chain.
 * Returns 0 on end of file and -1 on error.
 */

libfat_sector_t libfat_nextsector(struct libfat_filesystem * fs,
                                  libfat_sector_t s)
{
    int32_t cluster, nextcluster;
    uint32_t fatoffset;
    libfat_sector_t fatsect;
    uint8_t *fsdata;
    uint32_t clustmask = fs->clustsize - 1;
    libfat_sector_t rs;

    if (s < fs->data) {
        if (s < fs->rootdir)
            return -1;

        /* Root directory */
        s++;
        return (s < fs->data) ? s : 0;
    }

    rs = s - fs->data;

    if (~rs & clustmask)
        return s + 1;           /* Next sector in cluster */

    cluster = 2 + (rs >> fs->clustshift);

    if (cluster >= fs->endcluster)
        return -1;

    switch (fs->fat_type) {
    case FAT12:
        /* Get first byte */
        fatoffset = cluster + (cluster >> 1);
        fatsect = fs->fat + (fatoffset >> LIBFAT_SECTOR_SHIFT);
        fsdata = libfat_get_sector(fs, fatsect);
        if (!fsdata)
            return -1;
        nextcluster = fsdata[fatoffset & LIBFAT_SECTOR_MASK];

        /* Get second byte */
        fatoffset++;
        fatsect = fs->fat + (fatoffset >> LIBFAT_SECTOR_SHIFT);
        fsdata = libfat_get_sector(fs, fatsect);
        if (!fsdata)
            return -1;
        nextcluster |= fsdata[fatoffset & LIBFAT_SECTOR_MASK] << 8;

        /* Extract the FAT entry */
        if (cluster & 1)
            nextcluster >>= 4;
        else
            nextcluster &= 0x0FFF;

        if (nextcluster >= 0x0FF8)
            return 0;
        break;

    case FAT16:
        fatoffset = cluster << 1;
        fatsect = fs->fat + (fatoffset >> LIBFAT_SECTOR_SHIFT);
        fsdata = libfat_get_sector(fs, fatsect);
        if (!fsdata)
            return -1;
        nextcluster =
            read16((le16_t *) & fsdata[fatoffset & LIBFAT_SECTOR_MASK]);

        if (nextcluster >= 0x0FFF8)
            return 0;
        break;

    case FAT28:
        fatoffset = cluster << 2;
        fatsect = fs->fat + (fatoffset >> LIBFAT_SECTOR_SHIFT);
        fsdata = libfat_get_sector(fs, fatsect);
        if (!fsdata)
            return -1;
        nextcluster =
            read32((le32_t *) & fsdata[fatoffset & LIBFAT_SECTOR_MASK]);
        nextcluster &= 0x0FFFFFFF;

        if (nextcluster >= 0x0FFFFFF8)
            return 0;
        break;

    default:
        return -1;              /* WTF? */
    }

    return libfat_clustertosector(fs, nextcluster);
}