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MOUNT(2) System Calls Manual MOUNT(2)

mount, unmount — mount or dismount a filesystem

#include <sys/param.h>
#include <sys/mount.h>

int
mount(int type, const char *dir, int flags, caddr_t data);

int
unmount(const char *dir, int flags);

The () function grafts a filesystem object onto the system file tree at the point dir. The argument data describes the filesystem object to be mounted. The argument type tells the kernel how to interpret data (See type below). The contents of the filesystem become available through the new mount point dir. Any files in dir at the time of a successful mount are swept under the carpet so to speak, and are unavailable until the filesystem is unmounted.

The following flags may be specified to suppress default semantics which affect filesystem access.

The filesystem should be treated as read-only; Even the super-user may not write on it.
Do not allow files to be executed from the filesystem.
Do not honor setuid or setgid bits on files when executing them.
Do not interpret special files on the filesystem.
All I/O to the filesystem should be done synchronously.

The flag MNT_UPDATE indicates that the mount command is being applied to an already mounted filesystem. This allows the mount flags to be changed without requiring that the filesystem be unmounted and remounted. Some filesystems may not allow all flags to be changed. For example, most filesystems will not allow a change from read-write to read-only.

The type argument names the filesystem. The types of filesystems known to the system can be obtained with sysctl(8) by using the command:

sysctl vfs

Data is a pointer to a structure that contains the type specific arguments to mount. The format for these argument structures is described in the manual page for each filesystem. By convention filesystem manual pages are named by prefixing ``mount_'' to the name of the filesystem as returned by sysctl(8). Thus the NFS filesystem is described by the mount_nfs(8) manual page.

The () function call disassociates the filesystem from the specified mount point dir.

The flags argument may specify MNT_FORCE to specify that the filesystem should be forcibly unmounted even if files are still active. Active special devices continue to work, but any further accesses to any other active files result in errors even if the filesystem is later remounted.

The mount() returns the value 0 if the mount was successful, otherwise -1 is returned and the variable errno is set to indicate the error.

Umount returns the value 0 if the umount succeeded; otherwise -1 is returned and the variable errno is set to indicate the error.

Mount() will fail when one of the following occurs:

[]
The caller is not the super-user.
[]
A component of a pathname exceeded 255 characters, or the entire length of a path name exceeded 1023 characters.
[]
Too many symbolic links were encountered in translating a pathname.
[]
A component of dir does not exist.
[]
A component of name is not a directory, or a path prefix of special is not a directory.
[]
A pathname contains a character with the high-order bit set.
[]
Another process currently holds a reference to dir.
[]
Dir points outside the process's allocated address space.

The following errors can occur for a ufs filesystem mount:

[]
A component of ufs_args fspec does not exist.
[]
Fspec is not a block device.
[]
The major device number of fspec is out of range (this indicates no device driver exists for the associated hardware).
[]
Fspec is already mounted.
[]
No space remains in the mount table.
[]
The super block for the filesystem had a bad magic number or an out of range block size.
[]
Not enough memory was available to read the cylinder group information for the filesystem.
[]
An I/O error occurred while reading the super block or cylinder group information.
[]
Fspec points outside the process's allocated address space.

The following errors can occur for a filesystem mount:

[]
timed out trying to contact the server.
[]
Some part of the information described by nfs_args points outside the process's allocated address space.

The following errors can occur for a mfs filesystem mount:

[]
No space remains in the mount table.
[]
The super block for the filesystem had a bad magic number or an out of range block size.
[]
Not enough memory was available to read the cylinder group information for the filesystem.
[]
A paging error occurred while reading the super block or cylinder group information.
[]
points outside the process's allocated address space.

Umount may fail with one of the following errors:

[]
The caller is not the super-user.
[]
A component of the path is not a directory.
[]
The pathname contains a character with the high-order bit set.
[]
A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
[]
Too many symbolic links were encountered in translating the pathname.
[]
The requested directory is not in the mount table.
[]
A process is holding a reference to a file located on the filesystem.
[]
An I/O error occurred while writing cached filesystem information.
[]
Dir points outside the process's allocated address space.

A ufs or mfs mount can also fail if the maximum number of filesystems are currently mounted.

mount(8), umount(8), sysctl(8)

Some of the error codes need translation to more obvious messages.

Mount() and umount() function calls appeared in Version 6 AT&T UNIX.

BSD 4 May 24, 1995 MOUNT(2)