dosfs.c

代码,来自google,关于FAT16/32文件系统编程源代码!适合初合者学习,有很全的注释!

/*
	DOSFS Embedded FAT-Compatible Filesystem
	(C) 2005 Lewin A.R.W. Edwards ([e-mail])

	You are permitted to modify and/or use this code in your own projects without
	payment of royalty, regardless of the license(s) you choose for those projects.

	You cannot re-copyright or restrict use of the code as released by Lewin Edwards.
*/

#include <string.h>
#include <stdlib.h>

#include "dosfs.h"

/*
	Get starting sector# of specified partition on drive #unit
	NOTE: This code ASSUMES an MBR on the disk.
	scratchsector should point to a SECTOR_SIZE scratch area
	Returns 0xffffffff for any error.
	If pactive is non-NULL, this function also returns the partition active flag.
	If pptype is non-NULL, this function also returns the partition type.
	If psize is non-NULL, this function also returns the partition size.
*/
uint32_t DFS_GetPtnStart(uint8_t unit, uint8_t *scratchsector, uint8_t pnum, uint8_t *pactive, uint8_t *pptype, uint32_t *psize)
{
	uint32_t result;
	PMBR mbr = (PMBR) scratchsector;

	// DOS ptable supports maximum 4 partitions
	if (pnum > 3)
		return DFS_ERRMISC;

	// Read MBR from target media
	if (DFS_ReadSector(unit,scratchsector,0,1)) {
		return DFS_ERRMISC;
	}

	result = (uint32_t) mbr->ptable[pnum].start_0 |
	  (((uint32_t) mbr->ptable[pnum].start_1) << 8) |
	  (((uint32_t) mbr->ptable[pnum].start_2) << 16) |
	  (((uint32_t) mbr->ptable[pnum].start_3) << 24);

	if (pactive)
		*pactive = mbr->ptable[pnum].active;

	if (pptype)
		*pptype = mbr->ptable[pnum].type;

	if (psize)
		*psize = (uint32_t) mbr->ptable[pnum].size_0 |
		  (((uint32_t) mbr->ptable[pnum].size_1) << 8) |
		  (((uint32_t) mbr->ptable[pnum].size_2) << 16) |
		  (((uint32_t) mbr->ptable[pnum].size_3) << 24);

	return result;
}


/*
	Retrieve volume info from BPB and store it in a VOLINFO structure
	You must provide the unit and starting sector of the filesystem, and
	a pointer to a sector buffer for scratch
	Attempts to read BPB and glean information about the FS from that.
	Returns 0 OK, nonzero for any error.
*/
uint32_t DFS_GetVolInfo(uint8_t unit, uint8_t *scratchsector, uint32_t startsector, PVOLINFO volinfo)
{
	PLBR lbr = (PLBR) scratchsector;
	volinfo->unit = unit;
	volinfo->startsector = startsector;

	if(DFS_ReadSector(unit,scratchsector,startsector,1))
		return DFS_ERRMISC;

// tag: OEMID, refer dosfs.h
//	strncpy(volinfo->oemid, lbr->oemid, 8);
//	volinfo->oemid[8] = 0;

	volinfo->secperclus = lbr->bpb.secperclus;
	volinfo->reservedsecs = (uint16_t) lbr->bpb.reserved_l |
		  (((uint16_t) lbr->bpb.reserved_h) << 8);

	volinfo->numsecs =  (uint16_t) lbr->bpb.sectors_s_l |
		  (((uint16_t) lbr->bpb.sectors_s_h) << 8);

	if (!volinfo->numsecs)
		volinfo->numsecs = (uint32_t) lbr->bpb.sectors_l_0 |
		  (((uint32_t) lbr->bpb.sectors_l_1) << 8) |
		  (((uint32_t) lbr->bpb.sectors_l_2) << 16) |
		  (((uint32_t) lbr->bpb.sectors_l_3) << 24);

	// If secperfat is 0, we must be in a FAT32 volume; get secperfat
	// from the FAT32 EBPB. The volume label and system ID string are also
	// in different locations for FAT12/16 vs FAT32.
	volinfo->secperfat =  (uint16_t) lbr->bpb.secperfat_l |
		  (((uint16_t) lbr->bpb.secperfat_h) << 8);
	if (!volinfo->secperfat) {
		volinfo->secperfat = (uint32_t) lbr->ebpb.ebpb32.fatsize_0 |
		  (((uint32_t) lbr->ebpb.ebpb32.fatsize_1) << 8) |
		  (((uint32_t) lbr->ebpb.ebpb32.fatsize_2) << 16) |
		  (((uint32_t) lbr->ebpb.ebpb32.fatsize_3) << 24);

		memcpy(volinfo->label, lbr->ebpb.ebpb32.label, 11);
		volinfo->label[11] = 0;
	
// tag: OEMID, refer dosfs.h
//		memcpy(volinfo->system, lbr->ebpb.ebpb32.system, 8);
//		volinfo->system[8] = 0; 
	}
	else {
		memcpy(volinfo->label, lbr->ebpb.ebpb.label, 11);
		volinfo->label[11] = 0;
	
// tag: OEMID, refer dosfs.h
//		memcpy(volinfo->system, lbr->ebpb.ebpb.system, 8);
//		volinfo->system[8] = 0; 
	}

	// note: if rootentries is 0, we must be in a FAT32 volume.
	volinfo->rootentries =  (uint16_t) lbr->bpb.rootentries_l |
		  (((uint16_t) lbr->bpb.rootentries_h) << 8);

	// after extracting raw info we perform some useful precalculations
	volinfo->fat1 = startsector + volinfo->reservedsecs;

	// The calculation below is designed to round up the root directory size for FAT12/16
	// and to simply ignore the root directory for FAT32, since it's a normal, expandable
	// file in that situation.
	if (volinfo->rootentries) {
		volinfo->rootdir = volinfo->fat1 + (volinfo->secperfat * 2);
		volinfo->dataarea = volinfo->rootdir + (((volinfo->rootentries * 32) + (SECTOR_SIZE - 1)) / SECTOR_SIZE);
	}
	else {
		volinfo->dataarea = volinfo->fat1 + (volinfo->secperfat * 2);
		volinfo->rootdir = (uint32_t) lbr->ebpb.ebpb32.root_0 |
		  (((uint32_t) lbr->ebpb.ebpb32.root_1) << 8) |
		  (((uint32_t) lbr->ebpb.ebpb32.root_2) << 16) |
		  (((uint32_t) lbr->ebpb.ebpb32.root_3) << 24);
	}

	// Calculate number of clusters in data area and infer FAT type from this information.
	volinfo->numclusters = (volinfo->numsecs - volinfo->dataarea) / volinfo->secperclus;
	if (volinfo->numclusters < 4085)
		volinfo->filesystem = FAT12;
	else if (volinfo->numclusters < 65525)
		volinfo->filesystem = FAT16;
	else
		volinfo->filesystem = FAT32;

	return DFS_OK;
}

/*
	Fetch FAT entry for specified cluster number
	You must provide a scratch buffer for one sector (SECTOR_SIZE) and a populated VOLINFO
	Returns a FAT32 BAD_CLUSTER value for any error, otherwise the contents of the desired
	FAT entry.
	scratchcache should point to a UINT32. This variable caches the physical sector number
	last read into the scratch buffer for performance enhancement reasons.
*/
uint32_t DFS_GetFAT(PVOLINFO volinfo, uint8_t *scratch, uint32_t *scratchcache, uint32_t cluster)
{
	uint32_t offset, sector, result;

	if (volinfo->filesystem == FAT12) {
		offset = cluster + (cluster / 2);
	}
	else if (volinfo->filesystem == FAT16) {
		offset = cluster * 2;
	}
	else if (volinfo->filesystem == FAT32) {
		offset = cluster * 4;
	}
	else
		return 0x0ffffff7;	// FAT32 bad cluster	

	// at this point, offset is the BYTE offset of the desired sector from the start
	// of the FAT. Calculate the physical sector containing this FAT entry.
	sector = ldiv(offset, SECTOR_SIZE).quot + volinfo->fat1;

	// If this is not the same sector we last read, then read it into RAM
	if (sector != *scratchcache) {
		if(DFS_ReadSector(volinfo->unit, scratch, sector, 1)) {
			// avoid anyone assuming that this cache value is still valid, which
			// might cause disk corruption
			*scratchcache = 0;
			return 0x0ffffff7;	// FAT32 bad cluster	
		}
		*scratchcache = sector;
	}

	// At this point, we "merely" need to extract the relevant entry.
	// This is easy for FAT16 and FAT32, but a royal PITA for FAT12 as a single entry
	// may span a sector boundary. The normal way around this is always to read two
	// FAT sectors, but that luxury is (by design intent) unavailable to DOSFS.
	offset = ldiv(offset, SECTOR_SIZE).rem;

	if (volinfo->filesystem == FAT12) {
		// Special case for sector boundary - Store last byte of current sector.
		// Then read in the next sector and put the first byte of that sector into
		// the high byte of result.
		if (offset == SECTOR_SIZE - 1) {
			result = (uint32_t) scratch[offset];
			sector++;
			if(DFS_ReadSector(volinfo->unit, scratch, sector, 1)) {
				// avoid anyone assuming that this cache value is still valid, which
				// might cause disk corruption
				*scratchcache = 0;
				return 0x0ffffff7;	// FAT32 bad cluster	
			}
			*scratchcache = sector;
			// Thanks to Claudio Leonel for pointing out this missing line.
			result |= ((uint32_t) scratch[0]) << 8;
		}
		else {
			result = (uint32_t) scratch[offset] |
			  ((uint32_t) scratch[offset+1]) << 8;
		}
		if (cluster & 1)
			result = result >> 4;
		else
			result = result & 0xfff;
	}
	else if (volinfo->filesystem == FAT16) {
		result = (uint32_t) scratch[offset] |
		  ((uint32_t) scratch[offset+1]) << 8;
	}
	else if (volinfo->filesystem == FAT32) {
		result = ((uint32_t) scratch[offset] |
		  ((uint32_t) scratch[offset+1]) << 8 |
		  ((uint32_t) scratch[offset+2]) << 16 |
		  ((uint32_t) scratch[offset+3]) << 24) & 0x0fffffff;
	}
	else
		result = 0x0ffffff7;	// FAT32 bad cluster	
	return result;
}


/*
	Set FAT entry for specified cluster number
	You must provide a scratch buffer for one sector (SECTOR_SIZE) and a populated VOLINFO
	Returns DFS_ERRMISC for any error, otherwise DFS_OK
	scratchcache should point to a UINT32. This variable caches the physical sector number
	last read into the scratch buffer for performance enhancement reasons.

	NOTE: This code is HIGHLY WRITE-INEFFICIENT, particularly for flash media. Considerable
	performance gains can be realized by caching the sector. However this is difficult to
	achieve on FAT12 without requiring 2 sector buffers of scratch space, and it is a design
	requirement of this code to operate on a single 512-byte scratch.

	If you are operating DOSFS over flash, you are strongly advised to implement a writeback
	cache in your physical I/O driver. This will speed up your code significantly and will
	also conserve power and flash write life.
*/
uint32_t DFS_SetFAT(PVOLINFO volinfo, uint8_t *scratch, uint32_t *scratchcache, uint32_t cluster, uint32_t new_contents)
{
	uint32_t offset, sector, result;
	if (volinfo->filesystem == FAT12) {
		offset = cluster + (cluster / 2);
		new_contents &=0xfff;
	}
	else if (volinfo->filesystem == FAT16) {
		offset = cluster * 2;
		new_contents &=0xffff;
	}
	else if (volinfo->filesystem == FAT32) {
		offset = cluster * 4;
		new_contents &=0x0fffffff;	// FAT32 is really "FAT28"
	}
	else
		return DFS_ERRMISC;	

	// at this point, offset is the BYTE offset of the desired sector from the start
	// of the FAT. Calculate the physical sector containing this FAT entry.
	sector = ldiv(offset, SECTOR_SIZE).quot + volinfo->fat1;

	// If this is not the same sector we last read, then read it into RAM
	if (sector != *scratchcache) {
		if(DFS_ReadSector(volinfo->unit, scratch, sector, 1)) {
			// avoid anyone assuming that this cache value is still valid, which
			// might cause disk corruption
			*scratchcache = 0;
			return DFS_ERRMISC;
		}
		*scratchcache = sector;
	}

	// At this point, we "merely" need to extract the relevant entry.
	// This is easy for FAT16 and FAT32, but a royal PITA for FAT12 as a single entry
	// may span a sector boundary. The normal way around this is always to read two
	// FAT sectors, but that luxury is (by design intent) unavailable to DOSFS.
	offset = ldiv(offset, SECTOR_SIZE).rem;

	if (volinfo->filesystem == FAT12) {

		// If this is an odd cluster, pre-shift the desired new contents 4 bits to
		// make the calculations below simpler
		if (cluster & 1)
			new_contents = new_contents << 4;

		// Special case for sector boundary
		if (offset == SECTOR_SIZE - 1) {

			// Odd cluster: High 12 bits being set
			if (cluster & 1) {
				scratch[offset] = (scratch[offset] & 0x0f) | new_contents & 0xf0;
			}
			// Even cluster: Low 12 bits being set
			else {
				scratch[offset] = new_contents & 0xff;
			}
			result = DFS_WriteSector(volinfo->unit, scratch, *scratchcache, 1);
			// mirror the FAT into copy 2
			if (DFS_OK == result)
				result = DFS_WriteSector(volinfo->unit, scratch, (*scratchcache)+volinfo->secperfat, 1);

			// If we wrote that sector OK, then read in the subsequent sector
			// and poke the first byte with the remainder of this FAT entry.
			if (DFS_OK == result) {
				*scratchcache++;
				result = DFS_ReadSector(volinfo->unit, scratch, *scratchcache, 1);
				if (DFS_OK == result) {
					// Odd cluster: High 12 bits being set
					if (cluster & 1) {
						scratch[0] = new_contents & 0xff00;
					}
					// Even cluster: Low 12 bits being set
					else {
						scratch[0] = (scratch[0] & 0xf0) | new_contents & 0x0f;
					}
					result = DFS_WriteSector(volinfo->unit, scratch, *scratchcache, 1);
					// mirror the FAT into copy 2
					if (DFS_OK == result)
						result = DFS_WriteSector(volinfo->unit, scratch, (*scratchcache)+volinfo->secperfat, 1);
				}
				else {
					// avoid anyone assuming that this cache value is still valid, which
					// might cause disk corruption
					*scratchcache = 0;
				}
			}
		} // if (offset == SECTOR_SIZE - 1)

		// Not a sector boundary. But we still have to worry about if it's an odd
		// or even cluster number.
		else {
			// Odd cluster: High 12 bits being set
			if (cluster & 1) {
				scratch[offset] = (scratch[offset] & 0x0f) | new_contents & 0xf0;
				scratch[offset+1] = new_contents & 0xff00;
			}
			// Even cluster: Low 12 bits being set
			else {
				scratch[offset] = new_contents & 0xff;
				scratch[offset+1] = (scratch[offset+1] & 0xf0) | new_contents & 0x0f;
			}
			result = DFS_WriteSector(volinfo->unit, scratch, *scratchcache, 1);
			// mirror the FAT into copy 2
			if (DFS_OK == result)
				result = DFS_WriteSector(volinfo->unit, scratch, (*scratchcache)+volinfo->secperfat, 1);
		}
	}
	else if (volinfo->filesystem == FAT16) {
		scratch[offset] = (new_contents & 0xff);
		scratch[offset+1] = (new_contents & 0xff00) >> 8;
		result = DFS_WriteSector(volinfo->unit, scratch, *scratchcache, 1);
		// mirror the FAT into copy 2
		if (DFS_OK == result)
			result = DFS_WriteSector(volinfo->unit, scratch, (*scratchcache)+volinfo->secperfat, 1);
	}
	else if (volinfo->filesystem == FAT32) {
		scratch[offset] = (new_contents & 0xff);
		scratch[offset+1] = (new_contents & 0xff00) >> 8;
		scratch[offset+2] = (new_contents & 0xff0000) >> 16;
		scratch[offset+3] = (scratch[offset+3] & 0xf0) | ((new_contents & 0x0f000000) >> 24);
		// Note well from the above: Per Microsoft's guidelines we preserve the upper
		// 4 bits of the FAT32 cluster value. It's unclear what these bits will be used
		// for; in every example I've encountered they are always zero.
		result = DFS_WriteSector(volinfo->unit, scratch, *scratchcache, 1);
		// mirror the FAT into copy 2
		if (DFS_OK == result)
			result = DFS_WriteSector(volinfo->unit, scratch, (*scratchcache)+volinfo->secperfat, 1);
	}
	else
		result = DFS_ERRMISC;

	return result;
}

/*
	Convert a filename element from canonical (8.3) to directory entry (11) form
	src must point to the first non-separator character.
	dest must point to a 12-byte buffer.
*/
uint8_t *DFS_CanonicalToDir(uint8_t *dest, uint8_t *src)
{
	uint8_t *destptr = dest;

	memset(dest, ' ', 11);
	dest[11] = 0;

	while (*src && (*src != DIR_SEPARATOR) && (destptr - dest < 11)) {
		if (*src >= 'a' && *src <='z') {
			*destptr++ = (*src - 'a') + 'A';
			src++;
		}
		else if (*src == '.') {
			src++;
			destptr = dest + 8;
		}
		else {
			*destptr++ = *src++;
		}
	}