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 mounted.
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 mkfile(1M).
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 information.
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 long.
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.
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 location.
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.
Usual mount point for mnttab file system
Header file that contains IOCTL definitions
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.