Updated: 2022/Sep/29
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MOUNT(2) System Calls Manual MOUNT(2)
NAME
mount, unmount - mount or dismount a file system
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/param.h>
#include <sys/mount.h>
int
mount(const char *type, const char *dir, int flags, void *data,
size_t data_len);
int
unmount(const char *dir, int flags);
DESCRIPTION
The mount() function grafts a file system object onto the system file
tree at the point dir. The argument data describes the file system
object to be mounted, and is data_len bytes long. The argument type
tells the kernel how to interpret data (See type below). The contents of
the file system 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 file system is unmounted.
The following flags may be specified to suppress default semantics which
affect file system access.
MNT_RDONLY The file system should be treated as read-only; even the
super-user may not write on it.
MNT_UNION Union with underlying file system instead of obscuring
it.
MNT_HIDDEN Cause the df(1) program, and perhaps others, to, by
default, exclude this file system from its output.
MNT_NOEXEC Do not allow files to be executed from the file system.
MNT_NOSUID Do not honor setuid or setgid bits on files when
executing them.
MNT_NODEV Do not interpret special files on the file system.
MNT_NOCOREDUMP Do not allow programs to dump core files on the file
system.
MNT_NOATIME Never update access time in the file system.
MNT_RELATIME Update access time on write and change. This helps
programs that verify that the file has been read after
written to work.
MNT_NODEVMTIME Never update modification time of device files.
MNT_SYMPERM Recognize the permission of symbolic link when reading
or traversing.
MNT_SYNCHRONOUS All I/O to the file system should be done synchronously.
This will slow I/O performance considerably, but
enhances overall file system reliability.
MNT_ASYNC All I/O to the file system should be done
asynchronously. This vastly improves I/O throughput,
but at a cost of making the file system likely to be
completely unrecoverable should the system crash while
unwritten data is pending in kernel buffers.
MNT_LOG Use a file system journal. MNT_LOG causes a journal (or
log) to be created in the file system, creating a record
of meta-data writes to be performed, allowing the actual
writes to be deferred. This improves performance in
most cases.
MNT_EXTATTR Enable extended attributes, if the file system supports
them and does not enable them by default. Currently
this is only the case for UFS1.
The MNT_UPDATE, MNT_RELOAD, and MNT_GETARGS flags indicate that the mount
command is being applied to an already mounted file system. The
MNT_UPDATE flag allows the mount flags to be changed without requiring
that the file system be unmounted and remounted. A conversion from read-
write to read-only will fail if any files are currently open for writing
on the file system, unless the MNT_FORCE flag is also applied. Some file
systems may not allow all flags to be changed. For example, some file
systems will not allow a change from read-write to read-only. The
MNT_RELOAD flag causes kernel file system data to be reloaded from the
file system device. It is only permitted on file systems mounted read-
only. Its purpose is to notify the system that the file system data has
been modified by some external process. The MNT_GETARGS flag does not
alter any of the mounted file system's properties, but returns the file
system-specific arguments for the currently mounted file system.
The type argument defines the type of the file system. The types of file
systems known to the system are defined in <sys/mount.h>, and those
supported by the current running kernel obtained using sysctl(8) to
obtain the node vfs.generic.fstypes. data is a pointer to a structure
that contains the type specific arguments to mount. Some of the
currently supported types of file systems and their type specific data
are:
MOUNT_FFS
struct ufs_args {
char *fspec; /* block special file to mount */
};
MOUNT_NFS
struct nfs_args {
int version; /* args structure version */
struct sockaddr *addr; /* file server address */
int addrlen; /* length of address */
int sotype; /* Socket type */
int proto; /* and Protocol */
u_char *fh; /* File handle to be mounted */
int fhsize; /* Size, in bytes, of fh */
int flags; /* flags */
int wsize; /* write size in bytes */
int rsize; /* read size in bytes */
int readdirsize; /* readdir size in bytes */
int timeo; /* initial timeout in .1 secs */
int retrans; /* times to retry send */
int maxgrouplist; /* Max. size of group list */
int readahead; /* # of blocks to readahead */
int leaseterm; /* Term (sec) of lease */
int deadthresh; /* Retrans threshold */
char *hostname; /* server's name */
};
MOUNT_MFS
struct mfs_args {
char *fspec; /* name to export for statfs */
struct export_args30 pad; /* unused */
caddr_t base; /* base of file system in mem */
u_long size; /* size of file system */
};
The unmount() function call disassociates the file system from the
specified mount point dir.
The flags argument may specify MNT_FORCE to specify that the file system
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 file system is later remounted.
RETURN VALUES
mount() returns the value 0 if the mount was successful, the number of
bytes written to data for MNT_GETARGS, 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.
ERRORS
mount() will fail when one of the following occurs:
[EBUSY] Another process currently holds a reference to dir, or
for an update from read-write to read-only there are
files on the file system open for writes.
[EFAULT] dir points outside the process's allocated address
space.
[ELOOP] Too many symbolic links were encountered in
translating a pathname.
[ENAMETOOLONG] A component of a pathname exceeded {NAME_MAX}
characters, or an entire path name exceeded {PATH_MAX}
characters.
[ENOENT] A component of dir does not exist.
[ENOTDIR] A component of name is not a directory, or a path
prefix of special is not a directory.
[EPERM] The caller is not the super-user, and ordinary user
mounts are not permitted or this particular request
violates the rules.
The following errors can occur for a ufs file system mount:
[EBUSY] Fspec is already mounted.
[EFAULT] Fspec points outside the process's allocated address
space.
[EINVAL] The super block for the file system had a bad magic
number or an out of range block size.
[EIO] An I/O error occurred while reading the super block or
cylinder group information.
[EMFILE] No space remains in the mount table.
[ENODEV] A component of ufs_args fspec does not exist.
[ENOMEM] Not enough memory was available to read the cylinder
group information for the file system.
[ENOTBLK] Fspec is not a block device.
[ENXIO] The major device number of fspec is out of range (this
indicates no device driver exists for the associated
hardware).
The following errors can occur for a nfs file system mount:
[EFAULT] Some part of the information described by nfs_args
points outside the process's allocated address space.
[ETIMEDOUT] Nfs timed out trying to contact the server.
The following errors can occur for a mfs file system mount:
[EFAULT] Name points outside the process's allocated address
space.
[EINVAL] The super block for the file system had a bad magic
number or an out of range block size.
[EIO] A paging error occurred while reading the super block
or cylinder group information.
[EMFILE] No space remains in the mount table.
[ENOMEM] Not enough memory was available to read the cylinder
group information for the file system.
unmount() may fail with one of the following errors:
[EBUSY] A process is holding a reference to a file located on
the file system.
[EFAULT] dir points outside the process's allocated address
space.
[EINVAL] The requested directory is not in the mount table.
[EIO] An I/O error occurred while writing cached file system
information.
[ELOOP] Too many symbolic links were encountered in
translating the pathname.
[ENAMETOOLONG] A component of a pathname exceeded {NAME_MAX}
characters, or an entire path name exceeded {PATH_MAX}
characters.
[ENOTDIR] A component of the path is not a directory.
[EPERM] The caller is not the super-user.
A ufs or mfs mount can also fail if the maximum number of file systems
are currently mounted.
SEE ALSO
df(1), getvfsstat(2), nfssvc(2), getmntinfo(3), symlink(7), mount(8),
sysctl(8), umount(8)
HISTORY
The mount() and umount() (now unmount()) functions appeared in Version 1
AT&T UNIX.
Prior to NetBSD 4.0 the mount call was used to export NFS file systems.
This is now done through nfssvc().
The data_len argument was added for NetBSD 5.0.
BUGS
Some of the error codes need translation to more obvious messages.
Far more file systems are supported than those those listed.
NetBSD 10.99 September 2, 2019 NetBSD 10.99