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] Operation mode */ );
FR_OK, FR_DISK_ERR, FR_INT_ERR, FR_INVALID_OBJECT, FR_DENIED, FR_TIMEOUT
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.
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 changes to the volume is performed.
The contiguous file would have an advantage at time-critical read/write operations. It eliminates some overheads in the file system and the storage media caused by random access due to fragmented file data. Especially FAT access for the contiguous file on the exFAT volume is completely eliminated and storage media will be accessed sequentially.
Also the contiguous file can be easily accessed directly via low-level disk functions. But this is not recommended in consideration for future compatibility.
Available when _USE_EXPAND == 1 and _FS_READONLY == 0.
/* 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->drv;
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);