FatFs - f_expand

f_expand

The f_expand function prepares or allocates a contiguous data area to the file.

FRESULT f_expand (
  FIL*    fp,  /* [IN] File object */
  FSIZE_t fsz, /* [IN] File size expanded to */
  BYTE    opt  /* [IN] Allocation mode */
);

Parameters

fp: Pointer to the open file object.
fsz: Number of bytes in size to prepare or allocate for the file. 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.
opt: Allocation mode. Prepare to allocate (0) or Allocate now (1).

Return Values

FR_OK, FR_DISK_ERR, FR_INT_ERR, FR_INVALID_OBJECT, FR_DENIED, FR_TIMEOUT

Description

The f_expand function prepares or allocates a contiguous data area to the file. When opt is 1, the function allocates a contiguous data area to the file. Unlike expansion of file size by f_lseek function, the file must be truncated prior to use this function and read/write pointer of the file stays at offset 0 after the function call. The file content allocated with this function is undefined because no data is written to the file in this process. The function can fail with FR_DENIED due to some reasons below.

No free contiguous space was found.
Size of the file was not zero.
The file has been opened in read-only mode.
Not allowable file size. (>= 4 GB on FAT volume)

When opt is 0, the function finds a contiguous data area and set it as suggested point for allocation. The subsequent cluster allocation begins at top of the contiguous area found by this function. Thus the write file is guaranteed be contiguous and without allocation delay until the file size reaches that size unless any other changes to the volume is performed.

The contiguous file has an advantage at time-critical read/write operations. It eliminates some overheads in the filesystem and the storage device caused by random access due to fragmented file data.

Also the contiguous file can be easily accessed directly via low-level disk functions. But this is not recommended in consideration for portability and future compatibility. If the file is not confirmed be contiguous, use this function to examine if it is contiguous or not.

QuickInfo

Available when FF_USE_EXPAND == 1 and FF_FS_READONLY == 0.

Example

    /* Creating a contiguous file */

    /* Create a new file */
    res = f_open(fp = malloc(sizeof (FIL)), "file.dat", FA_WRITE|FA_CREATE_ALWAYS);
    if (res) { /* Check if the file has been opened */
        free(fp);
        die("Failed to open the file.");
    }

    /* Alloacte a 100 MiB of contiguous area to the file */
    res = f_expand(fp, 104857600, 1);
    if (res) { /* Check if the file has been expanded */
        f_close(fp);
        free(fp);
        die("Failed to allocate contiguous area.");
    }

    /* Now you have a contiguous file accessible with fp */

    /* Accessing the contiguous file via low-level disk functions */

    /* Get physical location of the file data */
    drv = fp->obj.fs->pdrv;
    lba = fp->obj.fs->database + fp->obj.fs->csize * (fp->obj.sclust - 2);

    /* Write 2048 sectors from top of the file at a time */
    res = disk_write(drv, buffer, lba, 2048);