FRESULT f_lseek (
  FIL*    fp,  /* [IN] File object */
  FSIZE_t ofs  /* [IN] File read/write pointer */
);

Description

The f_lseek function moves the file read/write pointer of an open file. The offset can be specified in only origin from top of the file. When an offset beyond the file size is specified at write mode, the file size is expanded to the specified offset. The file data in the expanded area is undefined because no data is written to the file in this process. This is suitable to pre-allocate a cluster chain quickly, for fast write operation. When a contiguous data area needs to be allocated to the file, use f_expand function instead. After the f_lseek function succeeded, the current read/write pointer should be checked in order to make sure the read/write pointer has been moved correctry. In case of the read/write pointer is not the expected value, either of followings has been occured.

• End of file. The specified ofs was clipped at end of the file because the file has been opened in read-only mode.
• Disk full. There is no free space on the volume to expand the file.

The fast seek function enables fast backward/long seek operations without FAT access by using an on-memory CLMT (cluster link map table). It is applied to f_read and f_write function as well, however, the file size cannot be expanded by f_write, f_lseek function while the file is in fast seek mode.

The fast seek function is enabled when the member cltbl in the file object is not NULL. The CLMT must be created into the DWORD array prior to use the fast seek function. To create the CLMT, set address of the DWORD array to the member cltbl in the open file object, set the size of array in unit of items to the first item and call the f_lseek function with ofs = CREATE_LINKMAP. After the function succeeded and CLMT is created, no FAT access is occured in subsequent f_read, f_write, f_lseek function to the file. The number of items used or required is returned into the first item of the array. The number of items to be used is (number of the file fragments + 1) * 2. For example, when the file is fragmented in 5, 12 items in the array will be used. If the function failed with FR_NOT_ENOUGH_CORE, the given array size is insufficient for the file.

    /* Open file */
    fp = malloc(sizeof (FIL));
    res = f_open(fp, "file.dat", FA_READ|FA_WRITE);
    if (res) ...

    /* Move to offset of 5000 from top of the file */
    res = f_lseek(fp, 5000);

    /* Move to end of the file to append data */
    res = f_lseek(fp, f_size(fp));

    /* Forward 3000 bytes */
    res = f_lseek(fp, f_tell(fp) + 3000);

    /* Rewind 2000 bytes (take care on wraparound) */
    res = f_lseek(fp, f_tell(fp) - 2000);

/* Cluster pre-allocation (to prevent buffer overrun on streaming write) */

    res = f_open(fp, recfile, FA_CREATE_NEW | FA_WRITE);   /* Create a file */

    res = f_lseek(fp, PRE_SIZE);             /* Expand file size (cluster pre-allocation) */
    if (res || f_tell(fp) != PRE_SIZE) ...   /* Check if the file has been expanded */

    res = f_lseek(fp, DATA_START);           /* Record data stream WITHOUT cluster allocation delay */
    ...                                      /* Write operation should be aligned to sector boundary to optimize the write throughput */

    res = f_truncate(fp);                    /* Truncate unused area */
    res = f_lseek(fp, 0);                    /* Put file header */
    ...

    res = f_close(fp);

/* Using fast seek function */

    DWORD clmt[SZ_TBL];                    /* Cluster link map table buffer */

    res = f_open(fp, fname, FA_READ | FA_WRITE);   /* Open a file */

    res = f_lseek(fp, ofs1);               /* This is normal seek (cltbl is nulled on file open) */

    fp->cltbl = clmt;                      /* Enable fast seek function (cltbl != NULL) */
    clmt[0] = SZ_TBL;                      /* Set table size */
    res = f_lseek(fp, CREATE_LINKMAP);     /* Create CLMT */
    ...

    res = f_lseek(fp, ofs2);               /* This is fast seek */