Command Section
MOUNT(2)                  FreeBSD System Calls Manual                 MOUNT(2)

     mount, nmount, unmount - mount or dismount a file system

     Standard C Library (libc, -lc)

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

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

     unmount(const char *dir, int flags);

     #include <sys/uio.h>

     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

           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

     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

     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

     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

     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

     The mount() and nmount() system calls will fail when one of the following

     [EPERM]            The caller is neither the super-user nor the owner of

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

     [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.

     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

     [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

     [EFAULT]           The dir argument points outside the process's
                        allocated address space.

     lsvfs(1), mksnap_ffs(8), mount(8), umount(8)

     The mount() and unmount() functions appeared in Version 6 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.

FreeBSD 11.1-RELEASE-p4         April 13, 2015         FreeBSD 11.1-RELEASE-p4
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