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31.3.2 Mount, Unmount, Remount

This section describes the functions for mounting, unmounting, and remounting filesystems.

Only the superuser can mount, unmount, or remount a filesystem.

These functions do not access the fstab and mtab files. You should maintain and use these separately. See Mount Information.

The symbols in this section are declared in sys/mount.h.

Function: int mount (const char *special_file, const char *dir, const char *fstype, unsigned long int options, const void *data)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

mount mounts or remounts a filesystem. The two operations are quite different and are merged rather unnaturally into this one function. The MS_REMOUNT option, explained below, determines whether mount mounts or remounts.

For a mount, the filesystem on the block device represented by the device special file named special_file gets mounted over the mount point dir. This means that the directory dir (along with any files in it) is no longer visible; in its place (and still with the name dir) is the root directory of the filesystem on the device.

As an exception, if the filesystem type (see below) is one which is not based on a device (e.g. “proc”), mount instantiates a filesystem and mounts it over dir and ignores special_file.

For a remount, dir specifies the mount point where the filesystem to be remounted is (and remains) mounted and special_file is ignored. Remounting a filesystem means changing the options that control operations on the filesystem while it is mounted. It does not mean unmounting and mounting again.

For a mount, you must identify the type of the filesystem with fstype. This type tells the kernel how to access the filesystem and can be thought of as the name of a filesystem driver. The acceptable values are system dependent. On a system with a Linux kernel and the proc filesystem, the list of possible values is in the file filesystems in the proc filesystem (e.g. type cat /proc/filesystems to see the list). With a Linux kernel, the types of filesystems that mount can mount, and their type names, depends on what filesystem drivers are configured into the kernel or loaded as loadable kernel modules. An example of a common value for fstype is ext2.

For a remount, mount ignores fstype.

options specifies a variety of options that apply until the filesystem is unmounted or remounted. The precise meaning of an option depends on the filesystem and with some filesystems, an option may have no effect at all. Furthermore, for some filesystems, some of these options (but never MS_RDONLY) can be overridden for individual file accesses via ioctl.

options is a bit string with bit fields defined using the following mask and masked value macros:


This multibit field contains a magic number. If it does not have the value MS_MGC_VAL, mount assumes all the following bits are zero and the data argument is a null string, regardless of their actual values.


This bit on means to remount the filesystem. Off means to mount it.


This bit on specifies that no writing to the filesystem shall be allowed while it is mounted. This cannot be overridden by ioctl. This option is available on nearly all filesystems.


This bit on specifies that Setuid and Setgid permissions on files in the filesystem shall be ignored while it is mounted.


This bit on specifies that no files in the filesystem shall be executed while the filesystem is mounted.


This bit on specifies that no device special files in the filesystem shall be accessible while the filesystem is mounted.


This bit on specifies that all writes to the filesystem while it is mounted shall be synchronous; i.e., data shall be synced before each write completes rather than held in the buffer cache.


This bit on specifies that mandatory locks on files shall be permitted while the filesystem is mounted.


This bit on specifies that access times of files shall not be updated when the files are accessed while the filesystem is mounted.


This bit on specifies that access times of directories shall not be updated when the directories are accessed while the filesystem in mounted.

Any bits not covered by the above masks should be set off; otherwise, results are undefined.

The meaning of data depends on the filesystem type and is controlled entirely by the filesystem driver in the kernel.


#include <sys/mount.h>

mount("/dev/hdb", "/cdrom", MS_MGC_VAL | MS_RDONLY | MS_NOSUID, "");

mount("/dev/hda2", "/mnt", MS_MGC_VAL | MS_REMOUNT, "");

Appropriate arguments for mount are conventionally recorded in the fstab table. See Mount Information.

The return value is zero if the mount or remount is successful. Otherwise, it is -1 and errno is set appropriately. The values of errno are filesystem dependent, but here is a general list:


The process is not superuser.


The file system type fstype is not known to the kernel.


The file dev is not a block device special file.

  • The device is already mounted.
  • The mount point is busy. (E.g. it is some process’ working directory or has a filesystem mounted on it already).
  • The request is to remount read-only, but there are files open for writing.
  • A remount was attempted, but there is no filesystem mounted over the specified mount point.
  • The supposed filesystem has an invalid superblock.
  • The filesystem is inherently read-only (possibly due to a switch on the device) and the process attempted to mount it read/write (by setting the MS_RDONLY bit off).
  • special_file or dir is not accessible due to file permissions.
  • special_file is not accessible because it is in a filesystem that is mounted with the MS_NODEV option.

The table of dummy devices is full. mount needs to create a dummy device (aka “unnamed” device) if the filesystem being mounted is not one that uses a device.

Function: int umount2 (const char *file, int flags)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

umount2 unmounts a filesystem.

You can identify the filesystem to unmount either by the device special file that contains the filesystem or by the mount point. The effect is the same. Specify either as the string file.

flags contains the one-bit field identified by the following mask macro:


This bit on means to force the unmounting even if the filesystem is busy, by making it unbusy first. If the bit is off and the filesystem is busy, umount2 fails with errno = EBUSY. Depending on the filesystem, this may override all, some, or no busy conditions.

All other bits in flags should be set to zero; otherwise, the result is undefined.


#include <sys/mount.h>

umount2("/mnt", MNT_FORCE);

umount2("/dev/hdd1", 0);

After the filesystem is unmounted, the directory that was the mount point is visible, as are any files in it.

As part of unmounting, umount2 syncs the filesystem.

If the unmounting is successful, the return value is zero. Otherwise, it is -1 and errno is set accordingly:


The process is not superuser.


The filesystem cannot be unmounted because it is busy. E.g. it contains a directory that is some process’s working directory or a file that some process has open. With some filesystems in some cases, you can avoid this failure with the MNT_FORCE option.


file validly refers to a file, but that file is neither a mount point nor a device special file of a currently mounted filesystem.

This function is not available on all systems.

Function: int umount (const char *file)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

umount does the same thing as umount2 with flags set to zeroes. It is more widely available than umount2 but since it lacks the possibility to forcefully unmount a filesystem is deprecated when umount2 is also available.

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