mount, umount -装载和卸载文件系统

内容简介

#include

int mount(const char *source, const char *target, const char *filesystemtype, unsigned longmountflags, const void *data);

int umount(const char *target);

int umount2(const char *target, intflags);

描述

mount() attaches the filesystem specified bysource(which is often a device name, but can also be a directory name or a dummy) to the directory specified bytarget.

umount() and umount2() remove the attachment of the (topmost) filesystem mounted on target.

Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required to 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 for libc4 and libc5 and in for glibc2) in the low order 16 bits:

标签

描述

MS_BIND

(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 spanchroot(2) jails. The filesystemtype, mountflags, and dataarguments are ignored.

MS_DIRSYNC (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).)

MS_MANDLOCK

Permit mandatory locking on files in this file system. (Mandatory locking must still be enabled on a per-file basis, as described infcntl(2).)

MS_MOVE

Move a subtree. source specifies an existing mount point andtarget specifies the new location. The move is atomic: at no point is the subtree unmounted. The filesystemtype, mountflags, and data arguments are ignored.

MS_NOATIME

Do not update access times for (all types of) files on this file system.

MS_NODEV

Do not allow access to devices (special files) on this file system.

MS_NODIRATIME

Do not update access times for directories on this file system.

MS_NOEXEC

Do not allow programs to be executed from this file system.

MS_NOSUID

Do not honour set-user-ID and set-group-ID bits when executing programs from this file system.

MS_RDONLY

Mount file system read-only.

MS_REMOUNT

Remount an existing mount. This is allows you to change themountflags 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.

The following mountflags can be changed: MS_RDONLY,MS_SYNCHRONOUS, MS_MANDLOCK; before kernel 2.6.16, the following could also be changed: MS_NOATIME andMS_NODIRATIME; and, additionally, before kernel 2.4, the following could also be changed: MS_NOSUID, MS_NODEV,MS_NOEXEC.

MS_SYNCHRONOUS

Make writes on this file system synchronous (as though theO_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. From kernel 2.6.16 onwards, MS_NOATIME andMS_NODIRATIME are also 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:

MNT_FORCE (since Linux 2.1.116)

Force unmount even if busy. (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 specifyingMNT_EXPIRE unmounts an expired mount point. This flag cannot be specified with either MNT_FORCE or MNT_DETACH.

返回值

On success, zero is returned. On error, -1 is returned, anderrnois set appropriately.

错误

下面给出的误差值，导致文件系统类型无关的错误。每个文件系统类型可能有自己特殊的错误和自己的特殊行为。详情请参阅内核源代码。

标签

描述

EACCES

A component of a path was not searchable. (See alsopath_resolution(2).) 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 theMS_NODEV option.

EAGAIN

A call to umount2() specifying MNT_EXPIRE successfully marked an unbusy file system as expired.

EBUSY

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.

EFAULT

One of the pointer arguments points outside the user address space.

EINVAL

source had an invalid superblock. Or, a remount (MS_REMOUNT) was attempted, but source was not already mounted on target. Or, a move (MS_MOVE) was attempted, butsource was not a mount point, or was ’/’. Or, an unmount was attempted, but target was not a mount point. Or, umount2() was called with MNT_EXPIRE and either MNT_DETACH orMNT_FORCE.

ELOOP

Too many link encountered during pathname resolution. Or, a move was attempted, while target is a descendant of source.

EMFILE

(In case no block device is required:) Table of dummy devices is full.

ENAMETOOLONG

A pathname was longer than MAXPATHLEN.

ENODEV

filesystemtype not configured in the kernel.

ENOENT

A pathname was empty or had a nonexistent component.

ENOMEM

The kernel could not allocate a free page to copy filenames or data into.

ENOTBLK

source is not a block device (and a device was required).

ENOTDIR

The second argument, or a prefix of the first argument, is not a directory.

ENXIO

The major number of the block device source is out of range.

EPERM

The caller does not have the required privileges.

遵循于

这些功能是Linux特有的，应在拟移植的程序不能使用。

历史

The originalumount() function was called asumount(device)and would return ENOTBLK when called with something other than a block device. In Linux 0.98p4 a callumount(dir)was added, in order to support anonymous devices. In Linux 2.3.99-pre7 the callumount(device)was removed, leaving onlyumount(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 .

Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program on a filesystem mounted with MS_NOSUID would fail with EPERM. Since Linux 2.4 the set-user-ID and set-group-ID bits are just silently ignored in this case.

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