fatfs文件系统详解之f_mount函数分析

前序

上一篇分析了格式化一个磁盘的时候发生了什么，在格式化一个磁盘之后，就要将磁盘进行挂载，“挂载”这个词听起来很抽象，但是在软件代码上，到底发生了什么？

 

分析假设

（1）假设一个磁盘就一个分区。

（2）只分析FAT32文件系统相关的代码。

（3）函数的大部分分析，都写入代码注释中。

 

f_mount()函数全部代码

为了方便分析，排除视觉障碍，已经删除了不在假设范围内代码。

FRESULT f_mount (
	FATFS* fs,			/* fat文件系统描述的一个结构体,需要用户进行分配,然后会被记录在全局的FATFS[]这个数组中 */
	const TCHAR* path,	/* 物理磁盘 */
	BYTE opt			/* 挂载选项,1-代表立马挂载,从代码分析中知,这个值必须传入1 */
)
{
	FATFS *cfs;
	int vol;
	FRESULT res;
	const TCHAR *rp = path;


	/* 获取驱动号,为了找到FatFs[]数组中的空闲项 */
	vol = get_ldnumber(&rp);
	if (vol < 0) return FR_INVALID_DRIVE;
	cfs = FatFs[vol];					/* Pointer to fs object */

	if (cfs) {
#if _FS_LOCK != 0
		clear_lock(cfs);
#endif
#if _FS_REENTRANT						/* Discard sync object of the current volume */
		if (!ff_del_syncobj(cfs->sobj)) return FR_INT_ERR;
#endif
		cfs->fs_type = 0;				/* Clear old fs object */
	}

	if (fs) {
		fs->fs_type = 0;				/* Clear new fs object */
#if _FS_REENTRANT						/* Create sync object for the new volume */
		if (!ff_cre_syncobj((BYTE)vol, &fs->sobj)) return FR_INT_ERR;
#endif
	}

    /* 把用户分配的FATFS结构图放入全局数组指针中 */
	FatFs[vol] = fs;					/* Register new fs object */

	if (!fs || opt != 1) return FR_OK;	/* Do not mount now, it will be mounted later */
    
    /* 这个函数才是挂载时的关键所在 */
	res = find_volume(&path, &fs, 0);	/* Force mounted the volume */
	LEAVE_FF(fs, res);
}

从f_mount函数分析中可知,find_volume()函数才是挂载的核心代码，

static
FRESULT find_volume (	/* FR_OK(0): successful, !=0: any error occurred */
	const TCHAR** path,	/* 硬件磁盘,与f_mount函数的参数是一致的 */
	FATFS** rfs,		/* 与f_mount函数的fs参数是一致的 */
	BYTE mode			/* 这个传入的是0 */
)
{
	BYTE fmt, *pt;
	int vol;
	DSTATUS stat;
	DWORD bsect, fasize, tsect, sysect, nclst, szbfat, br[4];
	WORD nrsv;
	FATFS *fs;
	UINT i;


	/* Get logical drive number */
	*rfs = 0;
	vol = get_ldnumber(path);
	if (vol < 0) return FR_INVALID_DRIVE;

	/* Check if the file system object is valid or not */
    /* 1.找到用户分配的FATFS结构体 */
	fs = FatFs[vol];					/* Get pointer to the file system object */
	if (!fs) return FR_NOT_ENABLED;		/* Is the file system object available? */

	ENTER_FF(fs);						/* Lock the volume */
	*rfs = fs;							/* Return pointer to the file system object */

	mode &= (BYTE)~FA_READ;				/* Desired access mode, write access or not */

    /* 判定磁盘当前状态,如果磁盘被初始化过,那么就判定是挂载过了,直接返回OK */
	if (fs->fs_type) {					/* If the volume has been mounted */
		stat = disk_status(fs->drv);
		if (!(stat & STA_NOINIT)) {		/* and the physical drive is kept initialized */
			if (!_FS_READONLY && mode && (stat & STA_PROTECT)) {	/* Check write protection if needed */
				return FR_WRITE_PROTECTED;
			}
			return FR_OK;				/* The file system object is valid */
		}
	}

	/* The file system object is not valid. */
	/* Following code attempts to mount the volume. (analyze BPB and initialize the fs object) */

    /* 2.进行FATFS结构体填充 */
	fs->fs_type = 0;					/* Clear the file system object */
	fs->drv = LD2PD(vol);				/* Bind the logical drive and a physical drive */

    /* 2.1初始化磁盘 */
	stat = disk_initialize(fs->drv);	/* Initialize the physical drive */
	if (stat & STA_NOINIT) { 			/* Check if the initialization succeeded */
		return FR_NOT_READY;			/* Failed to initialize due to no medium or hard error */
	}
	if (!_FS_READONLY && mode && (stat & STA_PROTECT)) { /* Check disk write protection if needed */
		return FR_WRITE_PROTECTED;
	}
	/* Find an FAT partition on the drive. Supports only generic partitioning, FDISK and SFD. */
	bsect = 0;

    /* 2.2 check_fs()函数会把磁盘的第1个扇区(就是MBR)读入到fs->win[]数组中, 判断MBR是否是合法的MBR*/
	fmt = check_fs(fs, bsect);			/* Load sector 0 and check if it is an FAT-VBR as SFD */

    /* 2.3 fmt=0,说明是FAT32文件系统 */
	if (fmt == 2 || (fmt < 2 && LD2PT(vol) != 0)) {	/* Not an FAT-VBR or forced partition number */
		for (i = 0; i < 4; i++) {			/* Get partition offset */
			pt = fs->win + (MBR_Table + i * SZ_PTE);
			br[i] = pt[PTE_System] ? ld_dword(pt + PTE_StLba) : 0;
		}
		i = LD2PT(vol);						/* Partition number: 0:auto, 1-4:forced */
		if (i) i--;
		do {								/* Find an FAT volume */
			bsect = br[i];
			fmt = bsect ? check_fs(fs, bsect) : 3;	/* Check the partition */
		} while (!LD2PT(vol) && fmt >= 2 && ++i < 4);
	}
	if (fmt == 4) return FR_DISK_ERR;		/* An error occured in the disk I/O layer */
	if (fmt >= 2) return FR_NO_FILESYSTEM;	/* No FAT volume is found */

	/* An FAT volume is found. Following code initializes the file system object */

	{
	    /* 读取MBR中的BPB_BytsPerSec域,判断扇区大小是否是512(SS(fs)) */
		if (ld_word(fs->win + BPB_BytsPerSec) != SS(fs)) return FR_NO_FILESYSTEM;	/* (BPB_BytsPerSec must be equal to the physical sector size) */

        /* 2.4获取FAT表的大小、FAT表的个数、每个簇的扇区数、根目录项数(对于FAT32不存在这个)、磁盘簇的个数、MBR、FAT表、数据区的位置、 */
		fasize = ld_word(fs->win + BPB_FATSz16);			/* Number of sectors per FAT */
		if (fasize == 0) fasize = ld_dword(fs->win + BPB_FATSz32);
		fs->fsize = fasize;

		fs->n_fats = fs->win[BPB_NumFATs];					/* Number of FATs */
		if (fs->n_fats != 1 && fs->n_fats != 2) return FR_NO_FILESYSTEM;	/* (Must be 1 or 2) */
		fasize *= fs->n_fats;								/* Number of sectors for FAT area */

		fs->csize = fs->win[BPB_SecPerClus];				/* Cluster size */
		if (fs->csize == 0 || (fs->csize & (fs->csize - 1))) return FR_NO_FILESYSTEM;	/* (Must be power of 2) */

		fs->n_rootdir = ld_word(fs->win + BPB_RootEntCnt);	/* Number of root directory entries */
		if (fs->n_rootdir % (SS(fs) / SZDIRE)) return FR_NO_FILESYSTEM;	/* (Must be sector aligned) */

		tsect = ld_word(fs->win + BPB_TotSec16);			/* Number of sectors on the volume */
		if (tsect == 0) tsect = ld_dword(fs->win + BPB_TotSec32);

		nrsv = ld_word(fs->win + BPB_RsvdSecCnt);			/* Number of reserved sectors */
		if (nrsv == 0) return FR_NO_FILESYSTEM;				/* (Must not be 0) */

		/* Determine the FAT sub type */
		sysect = nrsv + fasize + fs->n_rootdir / (SS(fs) / SZDIRE);	/* RSV + FAT + DIR */
		if (tsect < sysect) return FR_NO_FILESYSTEM;		/* (Invalid volume size) */
		nclst = (tsect - sysect) / fs->csize;				/* Number of clusters */
		if (nclst == 0) return FR_NO_FILESYSTEM;			/* (Invalid volume size) */
		fmt = FS_FAT32;
		if (nclst <= MAX_FAT16) fmt = FS_FAT16;
		if (nclst <= MAX_FAT12) fmt = FS_FAT12;

		/* Boundaries and Limits */
		fs->n_fatent = nclst + 2;							/* Number of FAT entries */
		fs->volbase = bsect;								/* Volume start sector */
		fs->fatbase = bsect + nrsv; 						/* FAT start sector */
		fs->database = bsect + sysect;						/* Data start sector */
		if (fmt == FS_FAT32) {
			if (ld_word(fs->win + BPB_FSVer32) != 0) return FR_NO_FILESYSTEM;	/* (Must be FAT32 revision 0.0) */
			if (fs->n_rootdir) return FR_NO_FILESYSTEM;		/* (BPB_RootEntCnt must be 0) */
			fs->dirbase = ld_dword(fs->win + BPB_RootClus32);	/* Root directory start cluster */
			szbfat = fs->n_fatent * 4;						/* (Needed FAT size) */
		} else {
			if (fs->n_rootdir == 0)	return FR_NO_FILESYSTEM;/* (BPB_RootEntCnt must not be 0) */
			fs->dirbase = fs->fatbase + fasize;				/* Root directory start sector */
			szbfat = (fmt == FS_FAT16) ?					/* (Needed FAT size) */
				fs->n_fatent * 2 : fs->n_fatent * 3 / 2 + (fs->n_fatent & 1);
		}
		if (fs->fsize < (szbfat + (SS(fs) - 1)) / SS(fs)) return FR_NO_FILESYSTEM;	/* (BPB_FATSz must not be less than the size needed) */

#if !_FS_READONLY
		/* Get FSINFO if available */
		fs->last_clst = fs->free_clst = 0xFFFFFFFF;		/* Initialize cluster allocation information */
		fs->fsi_flag = 0x80;
#if (_FS_NOFSINFO & 3) != 3
        /* 2.5判断是否存在FSINFO扇区,如果存在则读出可用簇数、下一个可用簇号到FATFS结构体中 */
		if (fmt == FS_FAT32				/* Enable FSINFO only if FAT32 and BPB_FSInfo32 == 1 */
			&& ld_word(fs->win + BPB_FSInfo32) == 1
			&& move_window(fs, bsect + 1) == FR_OK)
		{
			fs->fsi_flag = 0;
			if (ld_word(fs->win + BS_55AA) == 0xAA55	/* Load FSINFO data if available */
				&& ld_dword(fs->win + FSI_LeadSig) == 0x41615252
				&& ld_dword(fs->win + FSI_StrucSig) == 0x61417272)
			{
#if (_FS_NOFSINFO & 1) == 0
				fs->free_clst = ld_dword(fs->win + FSI_Free_Count);
#endif
#if (_FS_NOFSINFO & 2) == 0
				fs->last_clst = ld_dword(fs->win + FSI_Nxt_Free);
#endif
			}
		}
#endif	/* (_FS_NOFSINFO & 3) != 3 */
#endif	/* !_FS_READONLY */
	}

	fs->fs_type = fmt;	/* FAT sub-type */
	fs->id = ++Fsid;	/* File system mount ID */
#if _USE_LFN == 1
	fs->lfnbuf = LfnBuf;	/* Static LFN working buffer */
#if _FS_EXFAT
	fs->dirbuf = DirBuf;	/* Static directory block working buuffer */
#endif
#endif
#if _FS_RPATH != 0
	fs->cdir = 0;		/* Initialize current directory */
#endif
#if _FS_LOCK != 0		/* Clear file lock semaphores */
	clear_lock(fs);
#endif
	return FR_OK;
}

总结

f_mount函数就是读出MBR扇区的内容放入FATFS结构图中，供以后使用。