FatFs - f

Parameters

path

Pointer to the null-terminated string specifies the logical drive to be formatted. If it has no drive number in it, it means to specify the default drive. The logical drive may or may not have been mounted for the format process.

opt

Specifies the structure holding format options. If a null pointer is given, it gives the function all options in default value. The format option structure has five members described below:

BYTE fmt: Specifies combination of FAT type flags, FM_FAT, FM_FAT32, FM_EXFAT and bitwise-or of these three, FM_ANY. FM_EXFAT is ignored when exFAT is not enabled. These flags specify which FAT type to be created on the volume. If two or more types are specified, one out of them will be selected depends on the volume size and au_size. The flag FM_SFD specifies to create the volume on the drive in SFD format. The default value is FM_ANY.
DWORD au_size: Specifies size of the allocation unit (cluter) in unit of byte. The valid value is power of 2 between the sector size and 128 * sector size inclusive for FAT/FAT32 volume and up to 16 MB for exFAT volume. If a zero (also default value) or any invalid value is given, the default allocation unit size depends on the volume size is used.
UINT n_align: Specifies alignment of the volume data area (file allocation pool, usually erase block boundary of flash media) in unit of sector. The valid value for this member is between 1 and 32768 inclusive in power of 2. If a zero (also default value) or any invalid value is given, the function obtains the block size from lower layer via disk_ioctl function.
BYTE n_fat: Specifies number of FAT copies on the FAT/FAT32 volume. Valid value for this member is 1 or 2. The default value (0) and any invaid value is considered as 1. If the FAT type is exFAT, this value has no effect.
UINT n_root: Specifies number of root directory entries on the FAT volume. Valid value for this member is up to 32768 and aligned to sector size / 32. The default value (0) and any invaid value is considered as 512. If the FAT type is FAT32 or exFAT, this value has no effect.

work

Pointer to the working buffer used for the format process. When a null pointer is given with FF_USE_LFN == 3, the function obtains a memory block in this function for the working buffer.

len

Size of the working buffer in unit of byte. It needs to be the sector size of the corresponding physical drive at least. Plenty of working buffer reduces number of write transactions to the drive and the format process will finish quickly.

Return Values

FR_OK, FR_DISK_ERR, FR_NOT_READY, FR_WRITE_PROTECTED, FR_INVALID_DRIVE, FR_MKFS_ABORTED, FR_INVALID_PARAMETER, FR_NOT_ENOUGH_CORE

Description

The FAT sub-type, FAT12/FAT16/FAT32, of FAT volume except exFAT is determined by only number of clusters on the volume and nothing else, according to the FAT specification issued by Microsoft. Thus the FAT sub-type of created volume depends on the volume size and the cluster size. In case of the combination of FAT type and cluter size specified by argument cannot be valid on the volume, the function will fail with FR_MKFS_ABORTED.

The allocation unit, also knows as cluster, is a unit of disk space allocation for files. When the size of allocation unit is 32768 bytes, a file with 100 bytes in size occupies 32768 bytes of disk space. The space efficiency of disk usage gets worse as increasing size of allocation unit, but, on the other hand, the read/write performance increases. Therefore the size of allocation unit is a trade-off between space efficiency and performance. For the large storages in GB order, 32768 bytes or larger (this is automatically selected by default) is recommended for most case unless extremely many small files are created on a volume.

When the logical drive to be formatted is associated with a physical drive (FF_MULTI_PARTITION = 0) and FM_SFD flag is not specified, a partition occupies entire disk space is created and then the FAT volume is created in the partition. When FM_SFD flag is specified, the FAT volume is created without any disk partitioning.

When the logical drive to be formatted is associated with a specific partition by multiple partition feature (FF_MULTI_PARTITION = 1), the FAT volume is created on the partition specified by the volume mapping table and FM_SFD flag is ignored. The physical drive needs to be partitioned with f_fdisk function or any partitioning tool prior to create the FAT volume with this function.

There are three disk partitioning formats, MBR, GPT and SFD. The MBR format (aka FDISK format) is usually used for harddisk, memory card and U disk. It can divide a physical drive into one or more partitions with a partition table. The GPT (GUID Partition Table) is a newly defined patitioning format for large storage devices. FatFs suppors the GPT only when 64-bit LBA is enabled. SFD (super-floppy disk) is non-partitioned disk format. The FAT volume occupies the entire physical drive without any disk partitioning. It is usually used for floppy disk, optical disk and most super-floppy media. Some combination of systems and media support only either partitioned format or non-partitioned format and the other is not supported.

QuickInfo

Available when FF_FS_READOLNY == 0 and FF_USE_MKFS == 1.

Example

/* Format default drive and create a file */
int main (void)
{
    FATFS fs;           /* Filesystem object */
    FIL fil;            /* File object */
    FRESULT res;        /* API result code */
    UINT bw;            /* Bytes written */
    BYTE work[FF_MAX_SS]; /* Work area (larger is better for processing time) */


    /* Format the default drive with default parameters */
    res = f_mkfs("", 0, work, sizeof work);
    if (res) ...

    /* Gives a work area to the default drive */
    f_mount(&fs, "", 0);

    /* Create a file as new */
    res = f_open(&fil, "hello.txt", FA_CREATE_NEW | FA_WRITE);
    if (res) ...

    /* Write a message */
    f_write(&fil, "Hello, World!\r\n", 15, &bw);
    if (bw != 15) ...

    /* Close the file */
    f_close(&fil);

    /* Unregister work area */
    f_mount(0, "", 0);

    ...

f_mkfs

Parameters

Return Values

Description

QuickInfo

Example

See Also