mount(2) | System Calls Manual | mount(2) |
mount - mount filesystem
Standard C library (libc, -lc)
#include <sys/mount.h>
int mount(const char *source, const char *target, const char *filesystemtype, unsigned long mountflags, const void *_Nullable data);
mount() 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_ADMIN capability) is required to mount filesystems.
Values for the filesystemtype argument 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 data argument 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. This argument may be specified as NULL, if there are no options.
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:
Each of these operations is detailed later in this page. Further flags may be specified in mountflags to modify the behavior of mount(), as described below.
The 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.
From Linux 2.4 onward, some of the above flags are settable on a per-mount basis, while others apply to the superblock of the mounted filesystem, meaning that all mounts of the same filesystem share those flags. (Previously, all of the flags were per-superblock.)
The per-mount-point flags are as follows:
The following flags are per-superblock: MS_DIRSYNC, MS_LAZYTIME, MS_MANDLOCK, MS_SILENT, and MS_SYNCHRONOUS. The initial settings of these flags are determined on the first mount of the filesystem, and will be shared by all subsequent mounts of the same filesystem. Subsequently, the settings of the flags can be changed via a remount operation (see below). Such changes will be visible via all mounts associated with the filesystem.
Since Linux 2.6.16, MS_RDONLY can be set or cleared on a per-mount-point basis as well as on the underlying filesystem superblock. The mounted filesystem will be writable only if neither the filesystem nor the mountpoint are flagged as read-only.
An existing mount may be remounted by specifying MS_REMOUNT in mountflags. This allows you to change the mountflags and data of an existing mount without having to unmount and remount the filesystem. target should be the same value specified in the initial mount() call.
The source and filesystemtype arguments are ignored.
The mountflags and data arguments should match the values used in the original mount() call, except for those parameters that are being deliberately changed.
The following mountflags can be changed: MS_LAZYTIME, MS_MANDLOCK, MS_NOATIME, MS_NODEV, MS_NODIRATIME, MS_NOEXEC, MS_NOSUID, MS_RELATIME, MS_RDONLY, MS_STRICTATIME (whose effect is to clear the MS_NOATIME and MS_RELATIME flags), and MS_SYNCHRONOUS. Attempts to change the setting of the MS_DIRSYNC and MS_SILENT flags during a remount are silently ignored. Note that changes to per-superblock flags are visible via all mounts of the associated filesystem (because the per-superblock flags are shared by all mounts).
Since Linux 3.17, if none of MS_NOATIME, MS_NODIRATIME, MS_RELATIME, or MS_STRICTATIME is 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, the MS_REMOUNT flag can be used with MS_BIND to modify only the per-mount-point flags. This is particularly useful for setting or clearing the "read-only" flag on a mount without changing the underlying filesystem. Specifying mountflags as:
MS_REMOUNT | MS_BIND | MS_RDONLY
will make access through this mountpoint read-only, without affecting other mounts.
If mountflags includes 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 filesystemtype and data arguments are ignored.
The remaining bits (other than MS_REC, described below) in the mountflags argument are also ignored. (The bind mount has the same mount options as the underlying mount.) 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_REC flag is also specified, then a recursive bind mount operation is performed: all submounts under the source subtree (other than unbindable mounts) are also bind mounted at the corresponding location in the target subtree.
If mountflags includes 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 other flags that can be specified while changing the propagation type are MS_REC (described below) and MS_SILENT (which is ignored).
The source, filesystemtype, and data arguments are ignored.
The meanings of the propagation type flags are as follows:
By default, changing the propagation type affects only the target mount. If the MS_REC flag is also specified in mountflags, then the propagation type of all mounts under target is also changed.
For further details regarding mount propagation types (including the default propagation type assigned to new mounts), see mount_namespaces(7).
If mountflags contains the flag MS_MOVE (available since Linux 2.4.18), then move a subtree: source specifies an existing mount and target specifies the new location to which that mount is to be relocated. The move is atomic: at no point is the subtree unmounted.
The remaining bits in the mountflags argument are ignored, as are the filesystemtype and data arguments.
If none of MS_REMOUNT, MS_BIND, MS_MOVE, MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE is specified in mountflags, then mount() performs its default action: creating a new mount. source specifies the source for the new mount, and target specifies the directory at which to create the mount point.
The filesystemtype and data arguments are employed, and further bits may be specified in mountflags to modify the behavior of the call.
On success, zero is returned. On error, -1 is returned, and errno is set to indicate the error.
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.
The definitions of MS_DIRSYNC, MS_MOVE, MS_PRIVATE, MS_REC, MS_RELATIME, MS_SHARED, MS_SLAVE, MS_STRICTATIME, and MS_UNBINDABLE were added to glibc headers in glibc 2.12.
This function is Linux-specific and should not be used in programs intended to be portable.
Since 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 mountflags argument 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_VAL was required before Linux 2.4, but since Linux 2.4 is no longer required and is ignored if specified.
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-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.
Starting with Linux 2.4.19, Linux provides mount namespaces. A mount namespace is the set of filesystem mounts that are visible to a process. Mount 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_NEWNS flag, in which case its new namespace is initialized to be a copy of the namespace of the process that called clone(2); or it calls unshare(2) with the CLONE_NEWNS flag, 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.
For further details on mount namespaces, see mount_namespaces(7).
Each mount has a parent mount. The overall parental relationship of all mounts defines the single directory hierarchy seen by the processes within a mount namespace.
The parent of a new mount is defined when the mount is created. In the usual case, the parent of a new mount is the mount of the filesystem containing the directory or file at which the new mount is attached. In the case where a new mount is stacked on top of an existing mount, the parent of the new mount is the previous mount that was stacked at that location.
The parental relationship between mounts can be discovered via the /proc/[pid]/mountinfo file (see below).
The Linux-specific /proc/[pid]/mounts file exposes the list of mounts in the mount namespace of the process with the specified ID. The /proc/[pid]/mountinfo file exposes even more information about mounts, including the propagation type and mount ID information that makes it possible to discover the parental relationship between mounts. See proc(5) and mount_namespaces(7) for details of this file.
mountpoint(1), chroot(2), ioctl_iflags(2), mount_setattr(2), pivot_root(2), umount(2), mount_namespaces(7), path_resolution(7), findmnt(8), lsblk(8), mount(8), umount(8)
2023-02-05 | Linux man-pages 6.03 |