Updated: April 19, 2023

Format a DOS (FAT-12/16/32) filesystem (QNX Neutrino)


mkdosfs [-lMp] [-C|c size] [-e number]
        [-F type] [-f number] [-h number] 
        [-I vol_id] [-L vol_label] [-m media] 
        [-O oem_label] [-R|r] [-S size] [-s size] 
        device | mountpoint | file

Runs on:

QNX Neutrino


-C size
The minimum default cluster size. Don't specify both -C and -c.
-c size
The cluster size for the filesystem; the default is determined by disk geometry. Don't specify both -c and -C.
-e number
The number of root directory entries (FAT12/16 only); the default is 512.
-F type
The FAT type (12, 16, or 32).
-f number
The number of FAT tables to write; the default is 2.
-h number
The number of “hidden sectors”; the default is determined by disk geometry.
-I vol_id
The volume ID/serial number; the default is based on the current time.
-L vol_label
Specify a volume label; the default is none.
Make a LOST.DIR directory (for use by chkdosfs when repairing a filesystem).
Don't mirror FAT (FAT32 only).
-m media
Media descriptor byte; the default is 0xF0 or, if there are hidden sectors, 0xF8.
-O oem_label
Set the OEM label; the default is:
For a FAT12/FAT16 filesystem.
For a FAT32 filesystem.
(QNX Neutrino 6.6 or later) Update the partition descriptor on unmounted device targets.
Preserve the size and content of existing reserved sectors (reformat).
The number of “reserved sectors”; the default is determined by FAT type.
-S size
Sector size for the filesystem; the default is determined by disk geometry.
-s size
Set the size (number of sectors) in the filesystem; the default is determined by disk geometry.
The device name to host the DOS filesystem (e.g., /dev/hd0t11).
The mountpoint of the DOS filesystem (e.g., /fs/hd0-dos).
A regular file to contain the DOS filesystem image.


The mkdosfs utility formats a DOS filesystem on the specified target (typically a disk device or partition).

If you don't specify essential user options such as FAT type and cluster size, mkdosfs formats the DOS filesystem using the most suitable options for the size and disk geometry of the host. You can override this default auto-configuration and force a particular format to be used by setting the options you need.

Summary of filesystem commands

The following table shows the shared objects and related commands for the filesystems:

Partition type Filesystem Shared object Initialize with: Check with:
1, 4, or 6 DOS mkdosfs chkdosfs
7 Windows NTa N/A N/A
11, 12, or 14 FAT32 mkdosfs chkdosfs
77, 78, or 79 QNX 4 N/A N/A N/A
131 Linux (Ext2) N/A N/A
175 Apple Macintosh HFS or HFS Plusa N/A N/A
177, 178, or 179 Power-Safe mkqnx6fs chkqnx6fsb
185 QNX Trusted Disk mkqfs N/A

a Read-only.

b Not usually necessary.

For more information, see the Filesystems chapter of the System Architecture guide.


mkdosfs /dev/hd0t6

Format complete: FAT16 (4096-byte clusters), 201888 kB available.

Exit status:

The filesystem was constructed without error.
The filesystem wasn't constructed. This may be due to an error or inconsistency with the user options or because of a nonrecoverable error, such as disk I/O or insufficient memory.

Contributing author:

Robert Nordier


This utility is based on software from Robert Nordier; for licensing information, see Licensing information in Typographical Conventions, Support, and Licensing.


The mkdosfs utility destroys or overwrites any existing filesystem on the target.