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

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 by f_lseek function, the file must be truncated prior to use this function and read/write pointer of the file stays at top of the file after the function. 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. (>= 4GiB on FAT volume)

When opt is 0, the function finds a contiguous data area and set it as suggested point for next allocation instead of allocating it to the file. The next cluster allocation is started at top of the contiguous area found by this function. Thus the write file is guaranteed be contiguous and no allocation delay until the size reaches that size at least unless any other operation to the volume with changes of FAT is performed.

The contiguous file would have an advantage at time-critical read/write operations. It reduces some overheads in the file system and the storage media caused by random access due to fragmented file data. Especially, at the exFAT volume, any FAT access for the contiguous file is completely eliminated and storage media will be accessed sequentially.

Also the contiguous file data can be easily accessed directly via low-level disk functions but it is not recommended in consideration for future compatibility.

    /* 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);
        ...
    }

    /* 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 */
        ...
        free(fp);
        ...
    }
    /* Now you have a contiguous file accessible with fp */

    /* Accessing the file data directly via low-level disk functions */

    /* Get physical location of the file data */
    drv = fp->obj.fs->drv;
    sect = 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, sect, 2048);