MOUNT(2) System Calls Manual MOUNT(2)

mount, fmount, unmount — mount or dismount a filesystem

#include <sys/param.h>
#include <sys/mount.h>

int
mount(const char *type, const char *dir, int flags, void *data);

int
fmount(const char *type, int fd, int flags, void *data);

int
unmount(const char *dir, int flags);

The () function grafts a filesystem object onto the system file tree at the point dir. The argument data describes the filesystem object to be mounted. The argument type tells the kernel how to interpret data (See type below). The contents of the filesystem become available through the new mount point dir. Any files in dir at the time of a successful mount are swept under the carpet so to speak, and are unavailable until the filesystem is unmounted.

The following flags may be specified to suppress default semantics which affect filesystem access.

The filesystem should be treated as read-only; Even the super-user may not write on it.
Do not allow files to be executed from the filesystem.
Do not honor setuid or setgid bits on files when executing them.
Do not interpret special files on the filesystem.
Union with underlying filesystem instead of obscuring it.
All I/O to the filesystem should be done synchronously.
Enable data protection on the filesystem if the filesystem is configured for it.

The flag MNT_UPDATE indicates that the mount command is being applied to an already mounted filesystem. This allows the mount flags to be changed without requiring that the filesystem be unmounted and remounted. Some filesystems may not allow all flags to be changed. For example, most filesystems will not allow a change from read-write to read-only.

The flag MNT_RELOAD causes the vfs subsystem to update its data structures pertaining to the specified already mounted filesystem.

The type argument defines the type of the filesystem.

Data is a pointer to a structure that contains the type specific arguments to mount. The format for these argument structures is described in the manual page for each filesystem.

The () function call is equivalent to the mount() function call, except in the use of the second argument. It takes an open file descriptor representing mount point instead of the string literal containing full path to the mount point in the filesystem hierarchy.

The () function call disassociates the filesystem from the specified mount point dir.

The flags argument may specify MNT_FORCE to specify that the filesystem should be forcibly unmounted even if files are still active. Active special devices continue to work, but any further accesses to any other active files result in errors even if the filesystem is later remounted.

The mount() and fmount() return the value 0 if the mount was successful, otherwise -1 is returned and the variable errno is set to indicate the error.

unmount returns the value 0 if the unmount succeeded; otherwise -1 is returned and the variable errno is set to indicate the error.

mount() and fmount() will fail when one of the following occurs:

[]
The caller is not the super-user, and the device-node and the mountpoint do not have adequate ownership and permissions.
[]
A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters.
[]
Too many symbolic links were encountered in translating a pathname.
[]
A component of dir does not exist.
[]
A component of name is not a directory, or a path prefix of special is not a directory.
[]
A pathname contains a character with the high-order bit set.
[]
Another process currently holds a reference to dir.
[]
Dir points outside the process's allocated address space.

unmount may fail with one of the following errors:

[]
The caller is not the super-user, and the mount() was not done by the user.
[]
A system policy denied the operation.
[]
A component of the path is not a directory.
[]
The pathname contains a character with the high-order bit set.
[]
A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters.
[]
Too many symbolic links were encountered in translating the pathname.
[]
A component of dir does not exist.
[]
The requested directory is not in the mount table.
[]
A process is holding a reference to a file located on the filesystem.
[]
An I/O error occurred while writing cached filesystem information.
[]
Dir points outside the process's allocated address space.

mount(8), unmount(8), open(2)

Some of the error codes need translation to more obvious messages.

mount() and unmount() function calls appeared in Version 6 AT&T UNIX. fmount() function call first appeared in macOS version 10.13.

December 11, 1993 BSD 4