umount(2) -- Linux man page
NAMEmount, umount - mount and unmount filesystems
int mount(const char *source, const char *target, const char *filesystemtype, unsigned long mountflags, const void *data);
int umount(const char *target);
DESCRIPTIONmount attaches the filesystem specified by source (which is often a device name, but can also be a directory name or a dummy) to the directory specified by target.
umount and umount2 remove the attachment of the (topmost) filesystem mounted on target.
Only the super-user may mount and unmount filesystems. Since Linux 2.4 a single filesystem can be visible at multiple mount points, and multiple mounts can be stacked on the same mount point.
Values for the filesystemtype argument supported by the kernel are listed in /proc/filesystems (like "minix", "ext2", "msdos", "proc", "nfs", "iso9660" etc.). Further types may become available when the appropriate modules are loaded.
The mountflags argument may have the magic number 0xC0ED (MS_MGC_VAL) in the top 16 bits (this was required in kernel versions prior to 2.4, but is no longer required and ignored if specified), and various mount flags (as defined in <linux/fs.h> for libc4 and libc5 and in <sys/mount.h> for glibc2) in the low order 16 bits:
- (Linux 2.4 onwards) Perform a bind mount, making a file or a directory subtree visible at another point within a file system. Bind mounts may cross file system boundaries and span chroot(2) jails. The filesystemtype, mountflags, and data arguments are ignored.
- (Since Linux 2.5.19.) Make directory changes on this file system synchronous. (This property can be obtained for individual directories or subtrees using chattr(8).)
- Permit mandatory locking on files in this file system. (Mandatory locking must still be enabled on a per-file basis, as described in fcntl(2).)
- Move a subtree. source specifies an existing mount point and target specifies the new location. The move is atomic: at no point is the subtree unmounted. The filesystemtype, mountflags, and data arguments are ignored.
- Do not update access times for (all types of) files on this file system.
- Do not allow access to devices (special files) on this file system.
- Do not update access times for directories on this file system.
- Do not allow programs to be executed from this file system.
- Do not honour set-UID and set-GID bits when executing programs from this file system.
- Mount file system read-only.
- Remount an existing mount. This is allows you to change the mountflags and data of an existing mount without having to unmount and remount the file system. source and target should be the same values specified in the initial mount() call; filesystemtype is ignored.
- Make writes on this file system synchronous (as though the O_SYNC flag to open(2) was specified for all file opens to this file system).
From Linux 2.4 onwards, the MS_NODEV, MS_NOEXEC, and MS_NOSUID flags are settable on a per-mount point basis.
The data argument is interpreted by the different file systems. Typically it is a string of comma-separated options understood by this file system. See mount(8) for details of the options available for each filesystem type.
Linux 2.1.116 added the umount2() system call, which, like umount(), unmounts a target, but allows additional flags controlling the behaviour of the operation:
- Force unmount even if busy. (Since 2.1.116. Only for NFS mounts.)
- Perform a lazy unmount: make the mount point unavailable for new accesses, and actually perform the unmount when the mount point ceases to be busy. (Since 2.4.11.)
RETURN VALUEOn success, zero is returned. On error, -1 is returned, and errno is set appropriately.
ERRORSThe error values given below result from filesystem type independent errors. Each filesystem type may have its own special errors and its own special behavior. See the kernel source code for details.
- The user is not the super-user.
- Filesystemtype not configured in the kernel.
- Source is not a block device (and a device was required).
- Source is already mounted. Or, it cannot be remounted read-only, because it still holds files open for writing. Or, it cannot be mounted on target because target is still busy (it is the working directory of some task, the mount point of another device, has open files, etc.). Or, it could not be unmounted because it is busy.
- Source had an invalid superblock. Or, a remount was attempted, while source was not already mounted on target. Or, a move was attempted, while source was not a mount point, or was '/'. Or, an umount was attempted, while target was not a mount point.
- The second argument, or a prefix of the first argument, is not a directory.
- One of the pointer arguments points outside the user address space.
- The kernel could not allocate a free page to copy filenames or data into.
- A pathname was longer than MAXPATHLEN.
- A pathname was empty or had a nonexistent component.
- Too many link encountered during pathname resolution. Or, a move was attempted, while target is a descendant of source.
A component of a path was not searchable.
Or, mounting a read-only filesystem was attempted without giving the MS_RDONLY flag.
Or, the block device Source is located on a filesystem mounted with the MS_NODEV option.
- The major number of the block device source is out of range.
- (In case no block device is required:) Table of dummy devices is full.
CONFORMING TOThese functions are Linux-specific and should not be used in programs intended to be portable.
HISTORYThe original umount function was called as umount(device) and would return ENOTBLK when called with something other than a block device. In Linux 0.98p4 a call umount(dir) was added, in order to support anonymous devices. In Linux 2.3.99-pre7 the call umount(device) was removed, leaving only umount(dir) (since now devices can be mounted in more than one place, so specifying the device does not suffice).
The original MS_SYNC flag was renamed MS_SYNCHRONOUS in 1.1.69 when a different MS_SYNC was added to <mman.h>.
Before Linux 2.4 an attempt to execute a set-UID or set-GID program on a filesystem mounted with MS_NOSUID would fail with EPERM. Since Linux 2.4 the set-UID and set-GID bits are just silently ignored in this case.
SEE ALSOmount(8), umount(8)