MNTTAB(5) File Formats and Configurations MNTTAB(5)


mnttab - mounted file system table


The file /etc/mnttab is really a file system that provides read-only
access to the table of mounted file systems for the current host.
/etc/mnttab is read by programs using the routines described in
getmntent(3C). Mounting a file system adds an entry to this table.
Unmounting removes an entry from this table. Remounting a file system
causes the information in the mounted file system table to be updated to
reflect any changes caused by the remount. The list is maintained by the
kernel in order of mount time. That is, the first mounted file system is
first in the list and the most recently mounted file system is last. When
mounted on a mount point the file system appears as a regular file
containing the current mnttab information.

Each entry is a line of fields separated by TABs in the form:

special mount_point fstype options time


The name of the resource that has been mounted.

The pathname of the directory on which the filesystem is

The file system type of the mounted file system.

The mount options. See respective mount file system man
page in the See Also section below.

The time at which the file system was mounted.

Examples of entries for the special field include the pathname of a
block-special device, the name of a remote file system in the form of
host:pathname, or the name of a swap file, for example, a file made with


The following ioctl(2) calls are supported:

Returns the count of mounted resources in the
current snapshot in the uint32_t pointed to by arg.

Returns an array of uint32_t's that is twice as long
as the length returned by MNTIOC_NMNTS. Each pair of
numbers is the major and minor device number for the
file system at the corresponding line in the
current /etc/mnttab snapshot. arg points to the
memory buffer to receive the device number

Sets a tag word into the options list for a mounted
file system. A tag is a notation that will appear in
the options string of a mounted file system but it
is not recognized or interpreted by the file system
code. arg points to a filled in mnttagdesc
structure, as shown in the following example:

uint_t mtd_major; /* major number for mounted fs */
uint_t mtd_minor; /* minor number for mounted fs */
char *mtd_mntpt; /* mount point of file system */
char *mtd_tag; /* tag to set/clear */

If the tag already exists then it is marked as set
but not re-added. Tags can be at most MAX_MNTOPT_TAG

Use of this ioctl is restricted to processes with
the {PRIV_SYS_MOUNT} privilege.

Marks a tag in the options list for a mounted file
system as not set. arg points to the same structure
as MNTIOC_SETTAG, which identifies the file system
and tag to be cleared.

Use of this ioctl is restricted to processes with
the {PRIV_SYS_MOUNT} privilege.


The arg pointer in an MNTIOC_ ioctl call pointed to an
inaccessible memory location or a character pointer in a
mnttagdesc structure pointed to an inaccessible memory

The tag specified in a MNTIOC_SETTAG call already exists
as a file system option, or the tag specified in a
MNTIOC_CLRTAG call does not exist.

The tag specified in a MNTIOC_SETTAG call is too long or
the tag would make the total length of the option string
for the mounted file system too long.

The calling process does not have {PRIV_SYS_MOUNT}
privilege and either a MNTIOC_SETTAG or MNTIOC_CLRTAG
call was made.


Usual mount point for mnttab file system

Header file that contains IOCTL definitions


ioctl(2), poll(2), read(2), stat(2), getmntent(3C), mkfile(8), mount(8),
mount_hsfs(8), mount_nfs(8), mount_pcfs(8), mount_ufs(8)


The mnttab file system provides the previously undocumented dev=xxx
option in the option string for each mounted file system. This is
provided for legacy applications that might have been using the
dev=information option.

Using dev=option in applications is strongly discouraged. The device
number string represents a 32-bit quantity and might not contain correct
information in 64-bit environments.

Applications requiring device number information for mounted file systems
should use the getextmntent(3C) interface, which functions properly in
either 32- or 64-bit environments.


The snapshot of the mnttab information is taken any time a read(2) is
performed at offset 0 (the beginning) of the mnttab file. The file
modification time returned by stat(2) for the mnttab file is the time of
the last change to mounted file system information. A poll(2) system call
requesting a POLLRDBAND event can be used to block and wait for the
system's mounted file system information to be different from the most
recent snapshot since the mnttab file was opened.

illumos September 8, 2015 MNTTAB(5)