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.\"     @(#)mount.2	8.2 (Berkeley) 12/11/93
.\"
.Dd December 11, 1993
.Dt MOUNT 2
.Os BSD 4
.Sh NAME
.Nm mount ,
.Nm fmount,
.Nm unmount
.Nd mount or dismount a filesystem
.Sh SYNOPSIS
.Fd #include <sys/param.h>
.Fd #include <sys/mount.h>
.Ft int
.Fn mount "const char *type" "const char *dir" "int flags" "void *data"
.Ft int
.Fn fmount "const char *type" "int fd" "int flags" "void *data"
.Ft int
.Fn unmount "const char *dir" "int flags"
.Sh DESCRIPTION
The
.Fn mount
function grafts
a filesystem object onto the system file tree
at the point
.Ar dir .
The argument
.Ar data
describes the filesystem object to be mounted.
The argument
.Ar type
tells the kernel how to interpret
.Ar data
(See
.Ar type
below).
The contents of the filesystem
become available through the new mount point
.Ar dir .
Any files in
.Ar dir
at the time
of a successful mount are swept under the carpet so to speak, and
are unavailable until the filesystem is unmounted.
.Pp
The following
.Ar flags
may be specified to
suppress default semantics which affect filesystem access.
.Bl -tag -width MNT_SYNCHRONOUS
.It Dv MNT_RDONLY
The filesystem should be treated as read-only;
Even the super-user may not write on it.
.It Dv MNT_NOEXEC
Do not allow files to be executed from the filesystem.
.It Dv MNT_NOSUID
Do not honor setuid or setgid bits on files when executing them.
.It Dv MNT_NODEV
Do not interpret special files on the filesystem.
.It Dv MNT_UNION
Union with underlying filesystem instead of obscuring it.
.It Dv MNT_SYNCHRONOUS
All I/O to the filesystem should be done synchronously.
.It Dv MNT_CPROTECT
Enable data protection on the filesystem if the filesystem is configured for it.
.El
.Pp
The flag
.Dv 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.
.Pp
The flag
.Dv MNT_RELOAD
causes the vfs subsystem to update its data structures pertaining to
the specified already mounted filesystem.
.Pp
The
.Fa type
argument defines the type of the filesystem.
.Pp
.Fa 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.
.Pp
The
.Fn fmount
function call is equivalent to the
.Fn mount
function call, except in the use of the second argument.
It takes an open file descriptor representing mount point
instead of the string literal containing full path to the mount
point in the filesystem hierarchy.
.Pp
The
.Fn unmount
function call disassociates the filesystem from the specified
mount point
.Fa dir .
.Pp
The
.Fa flags
argument may specify
.Dv 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.
.Sh RETURN VALUES
The
.Fn mount
and
.Fn fmount
return the value 0 if the mount was successful, otherwise -1 is returned
and the variable
.Va errno
is set to indicate the error.
.Pp
.Nm unmount
returns the value 0 if the unmount succeeded; otherwise -1 is returned
and the variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn mount
and
.Fn fmount
will fail when one of the following occurs:
.Bl -tag -width [ENAMETOOLONG]
.It Bq Er EPERM
The caller is not the super-user, and the device-node and the mountpoint
do not have adequate ownership and permissions.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded
.Dv {NAME_MAX}
characters, or an entire path name exceeded
.Dv {PATH_MAX}
characters.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating a pathname.
.It Bq Er ENOENT
A component of
.Fa dir
does not exist.
.It Bq Er ENOTDIR
A component of
.Ar name
is not a directory,
or a path prefix of
.Ar special
is not a directory.
.It Bq Er EINVAL
A pathname contains a character with the high-order bit set.
.It Bq Er EBUSY
Another process currently holds a reference to
.Fa dir .
.It Bq Er EFAULT
.Fa Dir
points outside the process's allocated address space.
.El
.Pp
.Nm unmount
may fail with one of the following errors:
.Bl -tag -width [ENAMETOOLONG]
.It Bq Er EPERM
The caller is not the super-user, and the
.Nm mount()
was not done by the user.
.It Bq Er EPERM
A system policy denied the operation.
.It Bq Er ENOTDIR
A component of the path is not a directory.
.It Bq Er EINVAL
The pathname contains a character with the high-order bit set.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded
.Dv {NAME_MAX}
characters, or an entire path name exceeded
.Dv {PATH_MAX}
characters.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EINVAL
The requested directory is not in the mount table.
.It Bq Er EBUSY
A process is holding a reference to a file located
on the filesystem.
.It Bq Er EIO
An I/O error occurred while writing cached filesystem information.
.It Bq Er EFAULT
.Fa Dir
points outside the process's allocated address space.
.El
.Sh SEE ALSO
.Xr mount 8 ,
.Xr unmount 8 ,
.Xr open 2
.Sh BUGS
Some of the error codes need translation to more obvious messages.
.Sh HISTORY
.Fn mount
and
.Fn unmount
function calls appeared in
.At v6 .
.Fn fmount
function call first appeared in macOS version 10.13.