mount mounts or remounts a filesystem. The two operations are
quite different and are merged rather unnaturally into this one function.
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 as
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
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
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
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
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
option is available on nearly all filesystems.
- This bit on specifies that no writing to the files in the filesystem
shall be allowed while it is mounted. This can be overridden for a
particular file access by a properly privileged call to
This option is a relatively new invention and is not available on many
- This bit on specifies that the only file writing that shall be allowed
while the filesystem is mounted is appending. Some filesystems allow
this to be overridden for a particular process by a properly privileged
ioctl. This is a relatively new invention and is not
available on many 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.
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,
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 write.
- 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
- 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.