MOUNT(2)

NAME
mount, unmount - mount or dismount a filesystem

LIBRARY
libc

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);

DESCRIPTION
The mount() 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.

By default only the super-user may call the mount() function. This restriction can be removed by setting the sysctl vfs.usermount to a non-zero value.

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

MNT_RDONLY
    The filesystem 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 filesystem to read/write.

MNT_NOEXEC
    Do not allow files to be executed from the filesystem.

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_NODEV
    Do not interpret special files on the filesystem. This flag is set automatically when the caller is not the super-user.

MNT_SUIDDIR
    Directories with the SUID bit set chown new files to their own owner.

MNT_SYNCHRONOUS
    All I/O to the filesystem should be done synchronously.

MNT_ASYNC
    All I/O to the filesystem should be done asynchronously.

MNT_FORCE
    Force a read-write mount even if the filesystem appears to be unclean. Dangerous.

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 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, many filesystems 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 filesystem.

The type argument names the filesystem. The types of filesystems known to the system can be obtained with lsvfs(1).

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 lsvfs(1). Thus the NFS filesystem is described by the mount_nfs(8) manual page.

The unmount() 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 or made read-only (if MNT_UPDATE and MNT_RDONLY are also specified) 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 MNT_SUIDDIR option 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.

RETURN VALUES

ERRORS
The mount() function will fail when one of the following occurs:

[EPERM]
    The caller is neither the super-user nor the owner of dir, or dir belongs to a filesystem that does not support stacked mounts.

[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]
    Dir points outside the process's allocated address space.

The following errors can occur for a ufs filesystem mount:

[ENODEV]
    A component of ufs_args fspec does not exist.

[ENOTBLK]
    Fspec 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 filesystem 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 filesystem.

[EIO]
    An I/O error occurred while reading the super block or cylinder group information.

[EFAULT]
    Fspec points outside the process's allocated address space.

The following errors can occur for a nfs filesystem 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 following errors can occur for a mfs filesystem mount:

[EMFILE]
    No space remains in the mount table.

[EINVAL]
    The super block for the filesystem 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 filesystem.

[EIO]
    A paging error occurred while reading the super block or cylinder group information.

[EFAULT]
    Name points outside the process's allocated address space.

The unmount() function 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.

[ENOTDIR]
    A component of the path is not a directory.

[ENAMETOOLONG]
    A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.

[ELOOP]
    Too many symbolic links were encountered in translating the pathname.

[EINVAL]
    The requested directory is not in the mount table.

[EBUSY]
    A process is holding a reference to a file located on the filesystem.

[EIO]
    An I/O error occurred while writing cached filesystem information.

[EFAULT]
    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.

SEE ALSO
lsvfs(1), mountctl(2), mfs(8), mount(8), sysctl(8), umount(8)

HISTORY
Mount() and unmount() function calls appeared in AT&T UNIX.

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