## f_lseek

The f_lseek function moves the file read/write pointer of an open file object. It can also be used to expand the file size (cluster pre-allocation).

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

#### Parameters

fp
  Pointer to the open file object.
ofs
  Byte offset from top of the file.

#### Return Values

FR_OK, FR_DISK_ERR, FR_INT_ERR, FR_INVALID_OBJECT, FR_TIMEOUT

#### 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. This is suitable to pre-allocate a cluster chain quickly, for fast write operation. 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 current 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 insufficient free space on the volume to expand the file.

The fast seek feature 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.

The fast seek feature is enabled when the member `cltbl` in the file object is not NULL. The CLMT must be created in the `DWORD` array prior to use the fast seek feature. 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 into 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.

#### QuickInfo

Available when `_FS_MINIMIZE <= 2`. To use fast seek feature, `_USE_FASTSEEK` needs to be set 1.

#### Example

        /* 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 */
        ...                                      /* DATA_START and write block size should be aligned to sector boundary */

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

        res = f_close(fp);

    /* Using fast seek feature */

        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 feature (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 */

#### See Also

`f_open, f_truncate, FIL`

Return