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MOUNT(2)		    BSD 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 inter‐
     pret 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, con‐
     taining 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) vari‐
     able 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 execut‐
		      ing them.	 This flag is set automatically when the call‐
		      er 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 asyn‐

     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 struc‐
     tures 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 spe‐
     cific 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 speci‐
     fied 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 spe‐
     cial 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 speci‐
     fied 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 charac‐

     [ELOOP]		Too many symbolic links were encountered in translat‐
			ing a pathname.

     [ENOENT]		A component of dir does not exist.

     [ENOTDIR]		A component of name is not a directory, or a path pre‐
			fix of special is not a directory.

     [EBUSY]		Another process currently holds a reference to dir.

     [EFAULT]		The dir argument points outside the process's allo‐
			cated 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 allo‐
			cated 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 allo‐
			cated address space.

     A ufs mount can also fail if the maximum number of file systems are cur‐
     rently mounted.

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

     The mount() and unmount() functions appeared in Version 6 AT&T UNIX.

     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.

BSD			       February 23, 2005			   BSD

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