Provided by: freebsd-manpages_12.2-2_all 
NAME
mount, nmount, unmount — mount or dismount a file system
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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);
DESCRIPTION
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.
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.
RETURN VALUES
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.
ERRORS
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.
[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.
[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.
[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.
SEE ALSO
lsvfs(1), mksnap_ffs(8), mount(8), umount(8)
HISTORY
The mount() and unmount() functions appeared in Version 1 AT&T UNIX. The nmount() system
call first appeared in FreeBSD 5.0.
BUGS
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.