Manpage of MOUNT


Section: Linux Programmer's Manual (2)
Updated: 2016-03-15


mount - mount filesystem  


#include <sys/mount.h>int mount(const char *source, const char *target,          const char *filesystemtype, unsigned long mountflags,          const void *data);


mount() 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.

Appropriate privilege (Linux: the CAP_SYS_ADMINcapability) is required to mount 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 filesystemtypeargument supported by the kernel are listed in /proc/filesystems(e.g., "minix", "ext2", "ext3", "jfs", "xfs", "reiserfs", "msdos", "proc", "nfs", "iso9660"). Further types may become available when the appropriate modules are loaded.

The mountflagsargument 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 in the low order 16 bits:

MS_BIND (Linux 2.4 onward)
Perform a bind mount, making a file or a directory subtree visible at another point within a filesystem. Bind mounts may cross filesystem boundaries and span chroot(2) jails. The filesystemtypeand dataarguments are ignored. Up until Linux 2.6.26, mountflagswas also ignored (the bind mount has the same mount options as the underlying mount point).
MS_DIRSYNC (since Linux 2.5.19)
Make directory changes on this filesystem synchronous. (This property can be obtained for individual directories or subtrees using chattr(1).)
MS_LAZYTIME (since Linux 4.0)
Reduce on-disk updates of inode timestamps (atime, mtime, ctime) by maintaining these changes only in memory. The on-disk timestamps are updated only when:
the inode needs to be updated for some change unrelated to file timestamps;
the application employs fsync(2), syncfs(2), or sync(2);
an undeleted inode is evicted from memory; or
more than 24 hours have passed since the inode was written to disk.
This mount option significantly reduces writes needed to update the inode's timestamps, especially mtime and atime. However, in the event of a system crash, the atime and mtime fields on disk might be out of date by up to 24 hours.

Examples of workloads where this option could be of significant benefit include frequent random writes to preallocated files, as well as cases where the MS_STRICTATIMEmount option is also enabled. (The advantage of combining MS_STRICTATIMEand MS_LAZYTIMEis that stat(2) will return the correctly updated atime, but the atime updates will be flushed to disk only in the cases listed above.)

Permit mandatory locking on files in this filesystem. (Mandatory locking must still be enabled on a per-file basis, as described in fcntl(2).) Since Linux 4.5, this mount option requires the CAP_SYS_ADMINcapability.
Move a subtree. sourcespecifies an existing mount point and targetspecifies the new location. The move is atomic: at no point is the subtree unmounted. The filesystemtype, mountflags, and dataarguments are ignored.
Do not update access times for (all types of) files on this filesystem.
Do not allow access to devices (special files) on this filesystem.
Do not update access times for directories on this filesystem. This flag provides a subset of the functionality provided by MS_NOATIME; that is, MS_NOATIMEimplies MS_NODIRATIME.
Do not allow programs to be executed from this filesystem.
Do not honor set-user-ID and set-group-ID bits when executing programs from this filesystem.
Mount filesystem read-only.
MS_RELATIME (since Linux 2.6.20)
When a file on this filesystem is accessed, update the file's last access time (atime) only if the current value of atime is less than or equal to the file's last modification time (mtime) or last status change time (ctime). This option is useful for programs, such as mutt(1), that need to know when a file has been read since it was last modified. Since Linux 2.6.30, the kernel defaults to the behavior provided by this flag (unless MS_NOATIMEwas specified), and the MS_STRICTATIMEflag is required to obtain traditional semantics. In addition, since Linux 2.6.30, the file's last access time is always updated if it is more than 1 day old.
Remount an existing mount. This allows you to change the mountflagsand dataof an existing mount without having to unmount and remount the filesystem. targetshould be the same value specified in the initial mount() call; sourceand filesystemtypeare ignored. The mountflagsand dataarguments should match the values used in the original mount() call, except for those parameters that are being deliberately changed.

The following mountflagscan be changed: MS_RDONLY, MS_SYNCHRONOUS, MS_MANDLOCK; before kernel 2.6.16, the following could also be changed: MS_NOATIMEand MS_NODIRATIME; and, additionally, before kernel 2.4.10, the following could also be changed: MS_NOSUID, MS_NODEV, MS_NOEXEC.

MS_SILENT (since Linux 2.6.17)
Suppress the display of certain (printk()) warning messages in the kernel log. This flag supersedes the misnamed and obsolete MS_VERBOSEflag (available since Linux 2.4.12), which has the same meaning.
MS_STRICTATIME (since Linux 2.6.30)
Always update the last access time (atime) when files on this filesystem are accessed. (This was the default behavior before Linux 2.6.30.) Specifying this flag overrides the effect of setting the MS_NOATIMEand MS_RELATIMEflags.
Make writes on this filesystem synchronous (as though the O_SYNCflag to open(2) was specified for all file opens to this filesystem).

From Linux 2.4 onward, the MS_NODEV, MS_NOEXEC, and MS_NOSUIDflags are settable on a per-mount-point basis. From kernel 2.6.16 onward, MS_NOATIMEand MS_NODIRATIMEare also settable on a per-mount-point basis. The MS_RELATIMEflag is also settable on a per-mount-point basis.

The dataargument is interpreted by the different filesystems. Typically it is a string of comma-separated options understood by this filesystem. See mount(8) for details of the options available for each filesystem type.  


On success, zero is returned. On error, -1 is returned, and errnois 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 component of a path was not searchable. (See also path_resolution(7).) Or, mounting a read-only filesystem was attempted without giving the MS_RDONLYflag. Or, the block device sourceis located on a filesystem mounted with the MS_NODEVoption.
sourceis already mounted. Or, it cannot be remounted read-only, because it still holds files open for writing. Or, it cannot be mounted on targetbecause targetis still busy (it is the working directory of some thread, the mount point of another device, has open files, etc.).
One of the pointer arguments points outside the user address space.
sourcehad an invalid superblock. Or, a remount (MS_REMOUNT) was attempted, but sourcewas not already mounted on target. Or, a move (MS_MOVE) was attempted, but sourcewas not a mount point, or was aq/aq.
Too many links encountered during pathname resolution. Or, a move was attempted, while targetis a descendant of source.
(In case no block device is required:) Table of dummy devices is full.
A pathname was longer than MAXPATHLEN.
filesystemtypenot configured in the kernel.
A pathname was empty or had a nonexistent component.
The kernel could not allocate a free page to copy filenames or data into.
sourceis not a block device (and a device was required).
target, or a prefix of source, is not a directory.
The major number of the block device sourceis out of range.
The caller does not have the required privileges.


The definitions of MS_DIRSYNC, MS_MOVE, MS_REC, MS_RELATIME, and MS_STRICTATIMEwere added to glibc headers in version 2.12.  


This function is Linux-specific and should not be used in programs intended to be portable.  


The original MS_SYNCflag was renamed MS_SYNCHRONOUSin 1.1.69 when a different MS_SYNCwas added to <mman.h>.

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

Per-process namespaces

Starting with kernel 2.4.19, Linux provides per-process mount namespaces. A mount namespace is the set of filesystem mounts that are visible to a process. Mount-point namespaces can be (and usually are) shared between multiple processes, and changes to the namespace (i.e., mounts and unmounts) by one process are visible to all other processes sharing the same namespace. (The pre-2.4.19 Linux situation can be considered as one in which a single namespace was shared by every process on the system.)

A child process created by fork(2) shares its parent's mount namespace; the mount namespace is preserved across an execve(2).

A process can obtain a private mount namespace if: it was created using the clone(2) CLONE_NEWNSflag, in which case its new namespace is initialized to be a copyof the namespace of the process that called clone(2); or it calls unshare(2) with the CLONE_NEWNSflag, which causes the caller's mount namespace to obtain a private copy of the namespace that it was previously sharing with other processes, so that future mounts and unmounts by the caller are invisible to other processes (except child processes that the caller subsequently creates) and vice versa.

The Linux-specific /proc/PID/mountsfile exposes the list of mount points in the mount namespace of the process with the specified ID; see proc(5) for details.  


umount(2), namespaces(7), path_resolution(7), lsblk(8), findmnt(8), mount(8), umount(8)



Per-process namespaces

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