#include <sys/mount.h> int umount(const char *target); int umount2(const char *target, int flags);
umount() and umount2() remove the attachment of the (topmost) filesystem mounted on target.
Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required to unmount filesystems.
Linux 2.1.116 added the umount2() system call, which, like umount(), unmounts a target, but allows additional flags controlling the behavior of the operation:
MNT_FORCE (since Linux 2.1.116)
Force unmount even if busy. This can cause data loss. (Only for NFS mounts.)
MNT_DETACH (since Linux 2.4.11)
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.
MNT_EXPIRE (since Linux 2.6.8)
Mark the mount point as expired. If a mount point is not currently in use, then an initial call to umount2() with this flag fails with the error EAGAIN, but marks the mount point as expired. The mount point remains expired as long as it isn't accessed by any process. A second umount2() call specifying MNT_EXPIRE unmounts an expired mount point. This flag cannot be specified with either MNT_FORCE or MNT_DETACH.
UMOUNT_NOFOLLOW (since Linux 2.6.34)
Don't dereference target if it is a symbolic link. This flag allows security problems to be avoided in set-user-ID-root programs that allow unprivileged users to unmount filesystems.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
The 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 Linux kernel source code for details.
A call to umount2() specifying MNT_EXPIRE successfully marked an unbusy filesystem as expired.
target could not be unmounted because it is busy.
target points outside the user address space.
target is not a mount point.
umount2() was called with MNT_EXPIRE and either MNT_DETACH or MNT_FORCE.
EINVAL (since Linux 2.6.34)
umount2() was called with an invalid flag value in flags.
A pathname was longer than MAXPATHLEN.
A pathname was empty or had a nonexistent component.
The kernel could not allocate a free page to copy filenames or data into.
The caller does not have the required privileges.
MNT_DETACH and MNT_EXPIRE are available in glibc since version 2.11.
These functions are Linux-specific and should not be used in programs intended to be portable.
The 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).
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