http://www.openedv.com/posts/list/27427.htm
- //计算根目录起始扇区、数据起始扇区(物理扇区地址)
- fatend = fs->sects_fat * fs->n_fats + fs->fatbase;
- if (fat == FS_FAT32) {
- fs->dirbase = LD_DWORD(&(fs->win[0x2C])); /* Directory start cluster */
- fs->database = fatend; /* Data start sector (physical) */
- } else {
- fs->dirbase = fatend; /* Directory start sector (physical) */
- fs->database = fs->n_rootdir / 16 + fatend; /* Data start sector (physical) */
- }
- //最大簇号
- fs->max_clust = /* Maximum cluster number */
- (LD_DWORD(&(fs->win[0x20])) - fs->database + sect) / fs->sects_clust + 2;
- return FR_OK;
- }
- /*-----------------------*/
- /* Open or Create a File */
- FRESULT f_open (
- FIL *fp, /*
 ointer to the buffer of new file object to create */ - const char *path, /* ointer to the file path */
- BYTE mode /* Access mode and file open mode flags */
- )
- {
- FRESULT res;
- BYTE *dir;
- DIR dirscan;
- char fn[8+3+1]; //8.3 DOS文件名
- FATFS *fs = FatFs;
- /*首先初始化SD卡，检测文件系统类型，初始化FATFS对象*/
- if ((res = check_mounted()) != FR_OK) return res;
- #ifndef _FS_READONLY
- //如果磁盘设置为写保护，则返回错误码:FR_WRITE_PROTECTED
- if ((mode & (FA_WRITE|FA_CREATE_ALWAYS)) && (disk_status() & STA_PROTECT))
- return FR_WRITE_PROTECTED;
- #endif
- //根据用户提供的文件路径path，将文件名对应的目录项及其整个扇区读取到win[]中，
- //并填充目录项dirscan、标准格式的文件名fn，以及目录项在win[]中的字节偏移量dir
- res = trace_path(&dirscan, fn, path, &dir); /* Trace the file path */
- #ifndef _FS_READONLY
- /* Create or Open a File */
- if (mode & (FA_CREATE_ALWAYS|FA_OPEN_ALWAYS)) {
- DWORD dw;
- //如果文件不存在，则强制新建文件
- if (res != FR_OK) { /* No file, create new */
- mode |= FA_CREATE_ALWAYS;
- if (res != FR_NO_FILE) return res;
- dir = reserve_direntry(&dirscan); /* Reserve a directory entry */
- if (dir == NULL) return FR_DENIED;
- memcpy(dir, fn, 8+3); /* Initialize the new entry */
- *(dir+12) = fn[11];
- memset(dir+13, 0, 32-13);
- }
- else { /* File already exists */
- if ((dir == NULL) || (*(dir+11) & (AM_RDO|AM_DIR))) /* Could not overwrite (R/O or DIR) */
- return FR_DENIED;
- //如果文件存在，但又以FA_CREATE_ALWAYS方式打开文件，则重写文件
- if (mode & FA_CREATE_ALWAYS) { /* Resize it to zero */
- dw = fs->winsect; /* Remove the cluster chain */
- if (!remove_chain(((DWORD)LD_WORD(dir+20) << 16) | LD_WORD(dir+26))
- || !move_window(dw) )
- return FR_RW_ERROR;
- ST_WORD(dir+20, 0); ST_WORD(dir+26, 0); /* cluster = 0 */
- ST_DWORD(dir+28, 0); /* size = 0 */
- }
- }
- //如果是强制新建文件操作，则还需更新时间和日期
- if (mode & FA_CREATE_ALWAYS) {
- *(dir+11) = AM_ARC;
- dw = get_fattime();
- ST_DWORD(dir+14, dw); /* Created time */
- ST_DWORD(dir+22, dw); /* Updated time */
- fs->dirtyflag = 1;
- }
- }
- /* Open a File */
- else {
- #endif
- if (res != FR_OK) return res; /* Trace failed */
- //如果打开的是一个目录文件，则返回错误码:FR_NO_FILE
- if ((dir == NULL) || (*(dir+11) & AM_DIR)) /* It is a directory */
- return FR_NO_FILE;
- #ifndef _FS_READONLY
- //如果以FA_WRITE方式打开Read-Only属性的文件，则返回错误码:FR_DENIED
- if ((mode & FA_WRITE) && (*(dir+11) & AM_RDO)) /* R/O violation */
- return FR_DENIED;
- }
- #endif
- //填充FIL文件结构体参数
- #ifdef _FS_READONLY
- fp->flag = mode & (FA_UNBUFFERED|FA_READ);
- #else
- fp->flag = mode & (FA_UNBUFFERED|FA_WRITE|FA_READ);
- fp->dir_sect = fs->winsect; /* ointer to the directory entry */
- fp->dir_ptr = dir;
- #endif
- fp->org_clust = ((DWORD)LD_WORD(dir+20) << 16) | LD_WORD(dir+26); /* File start cluster */
- fp->fsize = LD_DWORD(dir+28); /* File size */
- fp->fptr = 0; /* File ptr */
- //这一步很重要，它将直接导致f_read和f_write操作中的逻辑顺序
- fp->sect_clust = 1; /* Sector counter */
- fs->files++;
- return FR_OK;
- }
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