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MOUNT(2) System Calls Manual MOUNT(2)

mount, unmount — mount or dismount a file system

library “libc”

#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);

The () 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.

The file system should be treated as read-only; even the super-user may not write on it.
Union with underlying file system instead of obscuring it.
Cause the df(1) program, and perhaps others, to, by default, exclude this file system from its output.
Do not allow files to be executed from the file system.
Do not honor setuid or setgid bits on files when executing them.
Do not interpret special files on the file system.
Do not allow programs to dump core files on the file system.
Never update access time in the file system.
Update access time on write and change. This helps programs that verify that the file has been read after written to work.
Never update modification time of device files.
Recognize the permission of symbolic link when reading or traversing.
All I/O to the file system should be done synchronously. This will slow I/O performance considerably, but enhances overall file system reliability.
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.
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 () 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.

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.

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

[]
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.
[]
dir points outside the process's allocated address space.
[]
Too many symbolic links were encountered in translating a pathname.
[]
A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters.
[]
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.
[]
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:

[]
Fspec is already mounted.
[]
Fspec points outside the process's allocated address space.
[]
The super block for the file system had a bad magic number or an out of range block size.
[]
An I/O error occurred while reading the super block or cylinder group information.
[]
No space remains in the mount table.
[]
A component of ufs_args fspec does not exist.
[]
Not enough memory was available to read the cylinder group information for the file system.
[]
Fspec is not a block device.
[]
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 file system mount:

[]
Some part of the information described by nfs_args points outside the process's allocated address space.
[]
timed out trying to contact the server.

The following errors can occur for a mfs file system mount:

[]
points outside the process's allocated address space.
[]
The super block for the file system had a bad magic number or an out of range block size.
[]
A paging error occurred while reading the super block or cylinder group information.
[]
No space remains in the mount table.
[]
Not enough memory was available to read the cylinder group information for the file system.

unmount() may fail with one of the following errors:

[]
A process is holding a reference to a file located on the file system.
[]
dir points outside the process's allocated address space.
[]
The requested directory is not in the mount table.
[]
An I/O error occurred while writing cached file system information.
[]
Too many symbolic links were encountered in translating the pathname.
[]
A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters.
[]
A component of the path is not a directory.
[]
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.

df(1), getvfsstat(2), nfssvc(2), getmntinfo(3), symlink(7), mount(8), sysctl(8), umount(8)

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.

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

Far more file systems are supported than those those listed.

September 2, 2019 NetBSD-9.2