FatFs - f_lseek

## 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 */
      FSIZE_t ofs  /* [IN] File read/write pointer */
    );

#### Parameters

fp
  Pointer to the open file object.
ofs
  Byte offset from top of the file to set read/write pointer. The data type `FSIZE_t` is an alias of either `DWORD`(32-bit) or `QWORD`(64-bit) depends on the configuration option `FF_FS_EXFAT`.

#### Return Values

FR_OK, FR_DISK_ERR, FR_INT_ERR, FR_INVALID_OBJECT, FR_TIMEOUT

#### Description

File read/write ponter in the open file object points the data byte to be read/written at next read/write operation. It advances as the number of bytes read/written. The `f_lseek` function moves the file read/write pointer without any read/write operation to 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 data area to the file 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.

#### QuickInfo

Available when `FF_FS_MINIMIZE <= 2`. To use fast seek function, `FF_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 */
        ...                                      /* 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 */

#### See Also

`f_open, f_truncate, f_expand, FIL`

Return