mount
, nmount
,
unmount
—
mount or dismount a file system
Standard C Library (libc, -lc)
#include <sys/param.h>
#include <sys/mount.h>
int
mount
(const
char *type, const char
*dir, int flags,
void *data);
int
unmount
(const
char *dir, int
flags);
#include
<sys/uio.h>
int
nmount
(struct
iovec *iov, u_int
niov, int
flags);
The mount
() system call grafts a file system object onto
the system file tree at the point dir. The argument
data describes the file system object to be mounted. The
argument type tells the kernel how to interpret
data (See type below). The
contents of the file system 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 file system is unmounted.
The nmount
() system call behaves similarly
to mount
(), except that the mount options (file
system type name, device to mount, mount-point name, etc.) are passed as an
array of name-value pairs in the array iov, containing
niov elements. The following options are required by
all file systems:
fstype |
file system type name (e.g.,
“procfs ”) |
fspath |
mount point pathname (e.g.,
“/proc ”) |
Depending on the file system type, other options may be recognized
or required; for example, most disk-based file systems require a
“from
” option containing the pathname
of a special device in addition to the options listed above.
By default only the super-user may call the
mount
() system call. This restriction can be removed
by setting the vfs.usermount
sysctl(8)
variable to a non-zero value; see the BUGS section for more information.
The following flags may be specified to
suppress default semantics which affect file system access.
MNT_RDONLY
- The file system should be treated as read-only; even the super-user may
not write on it. Specifying MNT_UPDATE without this option will upgrade a
read-only file system to read/write.
MNT_NOEXEC
- Do not allow files to be executed from the file system.
MNT_NOSUID
- Do not honor setuid or setgid bits on files when executing them. This flag
is set automatically when the caller is not the super-user.
MNT_NOATIME
- Disable update of file access times.
MNT_SNAPSHOT
- Create a snapshot of the file system. This is currently only supported on
UFS2 file systems, see
mksnap_ffs(8)
for more information.
MNT_SUIDDIR
- Directories with the SUID bit set chown new files to their own owner. This
flag requires the SUIDDIR option to have been compiled into the kernel to
have any effect. See the
mount(8)
and
chmod(2)
pages for more information.
MNT_SYNCHRONOUS
- All I/O to the file system should be done synchronously.
MNT_ASYNC
- All I/O to the file system should be done asynchronously.
MNT_FORCE
- Force a read-write mount even if the file system appears to be unclean.
Dangerous. Together with
MNT_UPDATE
and
MNT_RDONLY
, specify that the file system is to be
forcibly downgraded to a read-only mount even if some files are open for
writing.
MNT_NOCLUSTERR
- Disable read clustering.
MNT_NOCLUSTERW
- Disable write clustering.
MNT_NOCOVER
- Do not mount over the root of another mount point.
MNT_EMPTYDIR
- Require an empty directory for the mount point directory.
The flag MNT_UPDATE
indicates that the
mount command is being applied to an already mounted file system. This
allows the mount flags to be changed without requiring that the file system
be unmounted and remounted. Some file systems may not allow all flags to be
changed. For example, many file systems will not allow a change from
read-write to read-only.
The flag MNT_RELOAD
causes the vfs
subsystem to update its data structures pertaining to the specified already
mounted file system.
The type argument names the file system. The
types of file systems known to the system can be obtained with
lsvfs(1).
The data argument 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 file
system. By convention file system manual pages are named by prefixing
``mount_'' to the name of the file system as returned by
lsvfs(1).
Thus the NFS file system is described by the
mount_nfs(8)
manual page. It should be noted that a manual page for default file systems,
known as UFS and UFS2, does not exist.
The unmount
() system call disassociates
the file system from the specified mount point
dir.
The flags argument may include
MNT_FORCE
to specify that the file system 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 file system is later remounted.
If the MNT_BYFSID
flag is specified,
dir should instead be a file system ID encoded as
“FSID
:val0:val1”,
where val0 and val1 are the
contents of the fsid_t val[]
array in decimal. The file system that has the specified file system ID will
be unmounted.
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable
errno is set to indicate the error.
The mount
() and nmount
() system
calls will fail when one of the following occurs:
- [
EPERM
]
- The caller is neither the super-user nor the owner of
dir.
- [
ENAMETOOLONG
]
- A component of a pathname exceeded 255 characters, or the entire length of
a path name exceeded 1023 characters.
- [
ELOOP
]
- Too many symbolic links were encountered in translating a pathname.
- [
ENOENT
]
- A component of dir does not exist.
- [
ENOTDIR
]
- A component of name is not a directory, or a path
prefix of special is not a directory.
- [
EBUSY
]
- Another process currently holds a reference to
dir.
- [
EBUSY
]
- The
MNT_NOCOVER
option was given, and the
requested mount point is already the root of another mount point.
- [
EFAULT
]
- The dir argument points outside the process's
allocated address space.
- [
EIO
]
- An I/O error occurred while reading data from
special.
- [
EINTEGRITY
]
- The backing store for special detected corrupted
data while reading.
The following errors can occur for a ufs file
system mount:
- [
ENODEV
]
- A component of ufs_args fspec does not exist.
- [
ENOTBLK
]
- The fspec argument is not a block device.
- [
ENOTEMPTY
]
- The
MNT_EMPTYDIR
option was specified, and the
requested mount point is not an empty directory.
- [
ENXIO
]
- The major device number of fspec is out of range
(this indicates no device driver exists for the associated hardware).
- [
EBUSY
]
- fspec is already mounted.
- [
EMFILE
]
- No space remains in the mount table.
- [
EINVAL
]
- The super block for the file system had a bad magic number or an out of
range block size.
- [
EINTEGRITY
]
- The super block for the file system had a bad check hash. The check hash
can usually be corrected by running
fsck(8).
- [
ENOMEM
]
- Not enough memory was available to read the cylinder group information for
the file system.
- [
EIO
]
- An I/O error occurred while reading the super block or cylinder group
information.
- [
EFAULT
]
- The fspec argument points outside the process's
allocated address space.
The following errors can occur for a nfs file
system mount:
- [
ETIMEDOUT
]
- Nfs timed out trying to contact the server.
- [
EFAULT
]
- Some part of the information described by nfs_args points outside the
process's allocated address space.
The unmount
() system call may fail with
one of the following errors:
- [
EPERM
]
- The caller is neither the super-user nor the user who issued the
corresponding
mount
() call.
- [
ENAMETOOLONG
]
- The length of the path name exceeded 1023 characters.
- [
EINVAL
]
- The requested directory is not in the mount table.
- [
ENOENT
]
- The file system ID specified using
MNT_BYFSID
was
not found in the mount table.
- [
EINVAL
]
- The file system ID specified using
MNT_BYFSID
could not be decoded.
- [
EINVAL
]
- The specified file system is the root file system.
- [
EBUSY
]
- A process is holding a reference to a file located on the file
system.
- [
EIO
]
- An I/O error occurred while writing cached file system information.
- [
EFAULT
]
- The dir argument points outside the process's
allocated address space.
The mount
() and unmount
()
functions appeared in Version 1 AT&T UNIX.
The nmount
() system call first appeared in
FreeBSD 5.0.
Some of the error codes need translation to more obvious messages.
Allowing untrusted users to mount arbitrary media, e.g. by
enabling vfs.usermount, should not be considered safe.
Most file systems in FreeBSD were not built to
safeguard against malicious devices.