Manpage of MOUNT
MOUNTSection: Linux Programmer's Manual (2)
NAMEmount - mount filesystem
#include <sys/mount.h>int mount(const char *source, const char *target, const char *filesystemtype, unsigned long mountflags, const void *data);
DESCRIPTIONmount() attaches the filesystem specified by source(which is often a pathname referring to a device, but can also be the pathname of a directory or file, or a dummy string) to the location (a directory or file) specified by the pathname in target.
Appropriate privilege (Linux: the CAP_SYS_ADMINcapability) is required to mount filesystems.
Values for the filesystemtypeargument supported by the kernel are listed in /proc/filesystems(e.g., "btrfs", "ext4", "jfs", "xfs", "vfat", "fuse", "tmpfs", "cgroup", "proc", "mqueue", "nfs", "cifs", "iso9660"). Further types may become available when the appropriate modules are loaded.
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.
A call to mount() performs one of a number of general types of operation, depending on the bits specified in mountflags. The choice of which operation to perform is determined by testing the bits set in mountflags, with the tests being conducted in the order listed here:
- Remount an existing mount: mountflagsincludes MS_REMOUNT.
- Create a bind mount: mountflagsincludes MS_BIND.
- Change the propagation type of an existing mount: mountflagsincludes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE.
- Move an existing mount to a new location: mountflagsincludes MS_MOVE.
- Create a new mount: mountflagsincludes none of the above flags.
Additional mount flagsThe list below describes the additional flags that can be specified in mountflags. Note that some operation types ignore some or all of these flags, as described later in this page.
- 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:
- 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.
- 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 or file capabilities when executing programs from this filesystem.
- Mount filesystem read-only.
- MS_REC (since Linux 2.4.11)
- Used in conjunction with MS_BINDto create a recursive bind mount, and in conjunction with the propagation type flags to recursively change the propagation type of all of the mounts in a subtree. See below for further details.
- 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.
- 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. Since Linux 2.6.16, MS_RDONLYcan be set or cleared on a per-mount-point basis as well as on the underlying filesystem. The mounted filesystem will be writable only if neither the filesystem nor the mountpoint are flagged as read-only.
Remounting an existing mountAn existing mount may be remounted by specifying MS_REMOUNTin mountflags. 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.
The sourceand filesystemtypearguments are ignored.
The mountflagsand dataarguments should match the values used in the original mount() call, except for those parameters that are being deliberately changed. Another exception is that MS_BINDhas a different meaning for remount, and it should be included only if explicitly desired.
The following mountflagscan be changed: MS_LAZYTIME, MS_MANDLOCK, MS_NOATIME, MS_NODEV, MS_NODIRATIME, MS_NOEXEC, MS_NOSUID, MS_RELATIME, MS_RDONLY, and MS_SYNCHRONOUS. Attempts to change the setting of the MS_DIRSYNCflag during a remount are silently ignored.
Since Linux 3.17, if none of MS_NOATIME, MS_NODIRATIME, MS_RELATIME, or MS_STRICTATIMEis specified in mountflags, then the remount operation preserves the existing values of these flags (rather than defaulting to MS_RELATIME).
Since Linux 2.6.26, this flag can be used with MS_BINDto modify only the per-mount-point flags. This is particularly useful for setting or clearing the "read-only" flag on a mount point without changing the underlying filesystem. Specifying mountflagsas:
MS_REMOUNT | MS_BIND | MS_RDONLY
Creating a bind mountIf mountflagsincludes MS_BIND(available since Linux 2.4), then perform a bind mount. A bind mount makes a file or a directory subtree visible at another point within the single directory hierarchy. Bind mounts may cross filesystem boundaries and span chroot(2) jails.
The filesystemtypeand dataarguments are ignored.
The remaining bits in the mountflagsargument are also ignored, with the exception of MS_REC. (The bind mount has the same mount options as the underlying mount point.) However, see the discussion of remounting above, for a method of making an existing bind mount read-only.
By default, when a directory is bind mounted, only that directory is mounted; if there are any submounts under the directory tree, they are not bind mounted. If the MS_RECflag is also specified, then a recursive bind mount operation is performed: all submounts under the sourcesubtree (other than unbindable mounts) are also bind mounted at the corresponding location in the targetsubtree.
Changing the propagation type of an existing mountIf mountflagsincludes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE(all available since Linux 2.6.15), then the propagation type of an existing mount is changed. If more than one of these flags is specified, an error results.
The only flags that can be used with changing the propagation type are MS_RECand MS_SILENT.
The source, filesystemtype, and dataarguments are ignored.
The meanings of the propagation type flags are as follows:
- Make this mount point shared. Mount and unmount events immediately under this mount point will propagate to the other mount points that are members of this mount's peer group. Propagation here means that the same mount or unmount will automatically occur under all of the other mount points in the peer group. Conversely, mount and unmount events that take place under peer mount points will propagate to this mount point.
- Make this mount point private. Mount and unmount events do not propagate into or out of this mount point.
- If this is a shared mount point that is a member of a peer group that contains other members, convert it to a slave mount. If this is a shared mount point that is a member of a peer group that contains no other members, convert it to a private mount. Otherwise, the propagation type of the mount point is left unchanged.
When a mount point is a slave, mount and unmount events propagate into this mount point from the (master) shared peer group of which it was formerly a member. Mount and unmount events under this mount point do not propagate to any peer.
A mount point can be the slave of another peer group while at the same time sharing mount and unmount events with a peer group of which it is a member.
- Make this mount unbindable. This is like a private mount, and in addition this mount can't be bind mounted. When a recursive bind mount (mount() with the MS_BINDand MS_RECflags) is performed on a directory subtree, any bind mounts within the subtree are automatically pruned (i.e., not replicated) when replicating that subtree to produce the target subtree.
By default, changing the propagation type affects only the targetmount point. If the MS_RECflag is also specified in mountflags, then the propagation type of all mount points under targetis also changed.
For further details regarding mount propagation types (including the default propagation type assigned to new mounts), see mount_namespaces(7).
Moving a mountIf mountflagscontains the flag MS_MOVE(available since Linux 2.4.18), then move a subtree: sourcespecifies an existing mount point and targetspecifies the new location to which that mount point is to be relocated. The move is atomic: at no point is the subtree unmounted.
Creating a new mount pointIf none of MS_REMOUNT, MS_BIND, MS_MOVE, MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLEis specified in mountflags, then mount() performs its default action: creating a new mount point. sourcespecifies the source for the new mount point, and targetspecifies the directory at which to create the mount point.
RETURN VALUEOn success, zero is returned. On error, -1 is returned, and errnois 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 Linux kernel source code for details.
- A component of a path was not searchable. (See also path_resolution(7).)
- Mounting a read-only filesystem was attempted without giving the MS_RDONLYflag.
- The block device sourceis located on a filesystem mounted with the MS_NODEVoption.
- sourceis already mounted.
- sourcecannot be remounted read-only, because it still holds files open for writing.
- sourcecannot 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.
- A remount operation (MS_REMOUNT) was attempted, but sourcewas not already mounted on target.
- A move operation (MS_MOVE) was attempted, but sourcewas not a mount point, or was aq/aq.
- mountflagsincludes more than one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE.
- mountflagsincludes MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLEand also includes a flag other than MS_RECor MS_SILENT.
- An attempt was made to bind mount an unbindable mount.
- Too many links encountered during pathname resolution.
- A move operation was attempted, and 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.
VERSIONSThe definitions of MS_DIRSYNC, MS_MOVE, MS_PRIVATE, MS_REC, MS_RELATIME, MS_SHARED, MS_SLAVE, MS_STRICTATIMEand MS_UNBINDABLEwere added to glibc headers in version 2.12.
CONFORMING TOThis function is Linux-specific and should not be used in programs intended to be portable.
NOTESSince Linux 2.4 a single filesystem can be mounted at multiple mount points, and multiple mounts can be stacked on the same mount point.
The mountflagsargument may have the magic number 0xC0ED (MS_MGC_VAL) in the top 16 bits. (All of the other flags discussed in DESCRIPTION occupy the low order 16 bits of mountflags.) Specifying MS_MGC_VALwas required in kernel versions prior to 2.4, but since Linux 2.4 is no longer required and is ignored if specified.
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 namespacesStarting 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 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 ly 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.
SEE ALSOmountpoint(1), umount(2), mount_namespaces(7), path_resolution(7), findmnt(8), lsblk(8), mount(8), umount(8)
- RETURN VALUE
- CONFORMING TO
- SEE ALSO
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