path: root/manual/sysinfo.texi

@node System Management, System Configuration, Users and Groups, Top
@c %MENU% Controlling the system and getting information about it
@chapter System Management

This chapter describes facilities for controlling the system that
underlies a process (including the operating system and hardware) and
for getting information about it.  Anyone can generally use the
informational facilities, but usually only a properly privileged process
can make changes.


@menu
* Host Identification::         Determining the name of the machine.
* Hardware/Software Type ID::   Determining the hardware type of the
                                 machine and what operating system it is
                                 running.
* Filesystem Handling::         Controlling/querying mounts
@end menu

To get information on parameters of the system that are built into the
system, such as the maximum length of a filename, @ref{System
Configuration}.

@node Host Identification
@section Host Identification

This section explains how to identify the particular machine that your
program is running on.  The identification of a machine consists of its
Internet host name and Internet address; see @ref{Internet Namespace}.
The host name should always be a fully qualified domain name, like
@w{@samp{crispy-wheats-n-chicken.ai.mit.edu}}, not a simple name like
just @w{@samp{crispy-wheats-n-chicken}}.

@pindex hostname
@pindex hostid
@pindex unistd.h
Prototypes for these functions appear in @file{unistd.h}.  The shell
commands @code{hostname} and @code{hostid} work by calling them.

@comment unistd.h
@comment BSD
@deftypefun int gethostname (char *@var{name}, size_t @var{size})
This function returns the name of the host machine in the array
@var{name}.  The @var{size} argument specifies the size of this array,
in bytes.

The return value is @code{0} on success and @code{-1} on failure.  In
the GNU C library, @code{gethostname} fails if @var{size} is not large
enough; then you can try again with a larger array.  The following
@code{errno} error condition is defined for this function:

@table @code
@item ENAMETOOLONG
The @var{size} argument is less than the size of the host name plus one.
@end table

@pindex sys/param.h
On some systems, there is a symbol for the maximum possible host name
length: @code{MAXHOSTNAMELEN}.  It is defined in @file{sys/param.h}.
But you can't count on this to exist, so it is cleaner to handle
failure and try again.

@code{gethostname} stores the beginning of the host name in @var{name}
even if the host name won't entirely fit.  For some purposes, a
truncated host name is good enough.  If it is, you can ignore the
error code.
@end deftypefun

@comment unistd.h
@comment BSD
@deftypefun int sethostname (const char *@var{name}, size_t @var{length})
The @code{sethostname} function sets the name of the host machine to
@var{name}, a string with length @var{length}.  Only privileged
processes are allowed to do this.  Usually it happens just once, at
system boot time.

The return value is @code{0} on success and @code{-1} on failure.
The following @code{errno} error condition is defined for this function:

@table @code
@item EPERM
This process cannot set the host name because it is not privileged.
@end table
@end deftypefun

@comment unistd.h
@comment BSD
@deftypefun {long int} gethostid (void)
This function returns the ``host ID'' of the machine the program is
running on.  By convention, this is usually the primary Internet address
of that machine, converted to a @w{@code{long int}}.  However, on some
systems it is a meaningless but unique number which is hard-coded for
each machine.
@end deftypefun

@comment unistd.h
@comment BSD
@deftypefun int sethostid (long int @var{id})
The @code{sethostid} function sets the ``host ID'' of the host machine
to @var{id}.  Only privileged processes are allowed to do this.  Usually
it happens just once, at system boot time.

The return value is @code{0} on success and @code{-1} on failure.
The following @code{errno} error conditions are defined for this function:

@table @code
@item EPERM
This process cannot set the host name because it is not privileged.

@item ENOSYS
The operating system does not support setting the host ID.  On some
systems, the host ID is a meaningless but unique number hard-coded for
each machine.
@end table
@end deftypefun

@node Hardware/Software Type ID
@section Hardware/Software Type Identification

You can use the @code{uname} function to find out some information about
the type of computer your program is running on.  This function and the
associated data type are declared in the header file
@file{sys/utsname.h}.
@pindex sys/utsname.h

@comment sys/utsname.h
@comment POSIX.1
@deftp {Data Type} {struct utsname}
The @code{utsname} structure is used to hold information returned
by the @code{uname} function.  It has the following members:

@table @code
@item char sysname[]
This is the name of the operating system in use.

@item char nodename[]
This is the network name of this particular computer.  In the GNU
library, the value is the same as that returned by @code{gethostname};
see @ref{Host Identification}.

@item char release[]
This is the current release level of the operating system implementation.

@item char version[]
This is the current version level within the release of the operating
system.

@item char machine[]
This is a description of the type of hardware that is in use.

Some systems provide a mechanism to interrogate the kernel directly for
this information.  On systems without such a mechanism, the GNU C
library fills in this field based on the configuration name that was
specified when building and installing the library.

GNU uses a three-part name to describe a system configuration; the three
parts are @var{cpu}, @var{manufacturer} and @var{system-type}, and they
are separated with dashes.  Any possible combination of three names is
potentially meaningful, but most such combinations are meaningless in
practice and even the meaningful ones are not necessarily supported by
any particular GNU program.

Since the value in @code{machine} is supposed to describe just the
hardware, it consists of the first two parts of the configuration name:
@samp{@var{cpu}-@var{manufacturer}}.  For example, it might be one of these:

@quotation
@code{"sparc-sun"},
@code{"i386-@var{anything}"},
@code{"m68k-hp"},
@code{"m68k-sony"},
@code{"m68k-sun"},
@code{"mips-dec"}
@end quotation
@end table
@end deftp

@comment sys/utsname.h
@comment POSIX.1
@deftypefun int uname (struct utsname *@var{info})
The @code{uname} function fills in the structure pointed to by
@var{info} with information about the operating system and host machine.
A non-negative value indicates that the data was successfully stored.

@code{-1} as the value indicates an error.  The only error possible is
@code{EFAULT}, which we normally don't mention as it is always a
possibility.
@end deftypefun


@node Filesystem Handling
@section Controlling/querying mounts

All files are in filesystems, and before you can access any file, its
filesystem must be mounted.  Because of Unix's concept of
@emph{Everything is a file}, mounting of filesystems is central to doing
almost anything.  This section explains how to find out what filesystems
are currently mounted and what filesystems are available for mounting,
and how to change what is mounted.

The classic filesystem is the contents of a disk drive.  The concept is
considerably more abstract, though, and lots of things other than disk
drives can be mounted.  

Some block devices don't correspond to traditional devices like disk
drives.  For example, a loop device is a block device whose driver uses
a regular file in another filesystem as its medium.  So if that regular
file contains appropriate data for a filesystem, you can by mounting the
loop device essentially mount a regular file.

Some filesystems aren't based on a device of any kind.  The ``proc''
filesystem, for example, contains files whose data is made up by the
filesystem driver on the fly whenever you ask for it.  And when you
write to it, the data you write causes changes in the system.  No data
gets stored.

@c It would be good to mention NFS mounts here.

@menu
* Mount Information::           What is or could be mounted?
* Mount/Unmount/Remount::       Controlling what is mounted and how
@end menu

@node Mount Information, Mount/Unmount/Remount, , Filesystem Handling

For some programs it is desirable and necessary to access information
about whether a certain filesystem is mounted and, if it is, where, or
simply to get lists of all the available filesystems.  The GNU libc
provides some functions to retrieve this information portably.

Traditionally Unix systems have a file named @file{/etc/fstab} which
describes all possibly mounted filesystems.  The @code{mount} program
uses this file to mount at startup time of the system all the necessary
filesystems.  The information about all the filesystems actually mounted
is normally kept in a file named @file{/etc/mtab}.  Both files share
the same syntax and it is crucial that this syntax is followed all the
time.  Therefore it is best to never directly write the files.  The
functions described in this section can do this and they also provide
the functionality to convert the external textual representation to the
internal representation.

Note that the @file{fstab} and @file{mtab} files are maintained on a
system by @emph{convention}.  It is possible for the files not to exist
or not to be consistent with what is really mounted or available to
mount, if the system's administration policy allows it.  But programs
that mount and unmount filesystems typically maintain and use these
files as described herein.

@vindex _PATH_FSTAB
@vindex _PATH_MNTTAB
@vindex FSTAB
@vindex _PATH_MOUNTED
The filenames given above should never be used directly.  The portable
way to handle these file is to use the macros @code{_PATH_FSTAB},
defined in @file{fstab.h} and @code{_PATH_MNTTAB}, defined in
@file{mntent.h}, respectively.  There are also two alternate macro names
@code{FSTAB} and @code{_PATH_MOUNTED} defined but both names are
deprecated and kept only for backward compatibility.  The two former
names should always be used.

@menu
* fstab::                       The @file{fstab} file
* mtab::                        The @file{mtab} file
* Other Mount Information::     Other (non-libc) sources of mount information
@end menu

@node fstab
@subsection The @file{fstab} file

The internal representation for entries of the file is @w{@code{struct
fstab}}, defined in @file{fstab.h}.

@comment fstab.h
@comment BSD
@deftp {Data Type} {struct fstab}
This structure is used with the @code{getfsent}, @code{getfsspec}, and
@code{getfsfile} functions.

@table @code
@item char *fs_spec
This element describes the device from which the filesystem is mounted.
Normally this is the name of a special device, such as a hard disk
partition, but it could also be a more or less generic string.  For
@dfn{NFS} it would be a hostname and directory name combination.

Even though the element is not declared @code{const} it shouldn't be
modified.  The missing @code{const} has historic reasons, since this
function predates @w{ISO C}.  The same is true for the other string
elements of this structure.

@item char *fs_file
This describes the mount point on the local system.  I.e., accessing any
file in this filesystem has implicitly or explicitly this string as a
prefix.

@item char *fs_vfstype
This is the type of the filesystem.  Depending on what the underlying
kernel understands it can be any string.

@item char *fs_mntops
This is a string containing options passed to the kernel with the
@code{mount} call.  Again, this can be almost anything.  There can be
more than one option, separated from the others by a comma.  Each option
consists of a name and an optional value part, introduced by an @code{=}
character.

If the value of this element must be processed it should ideally be done
using the @code{getsubopt} function; see @ref{Suboptions}.

@item const char *fs_type
This name is poorly chosen.  This element points to a string (possibly
in the @code{fs_mntops} string) which describes the modes with which the
filesystem is mounted.  @file{fstab} defines five macros to describe the
possible values:

@vtable @code
@item FSTAB_RW
The filesystems gets mounted with read and write enabled.
@item FSTAB_RQ
The filesystems gets mounted with read and write enabled.  Write access
is restricted by quotas.
@item FSTAB_RO
The filesystem gets mounted read-only.
@item FSTAB_SW
This is not a real filesystem, it is a swap device.
@item FSTAB_XX
This entry from the @file{fstab} file is totally ignored.
@end vtable

Testing for equality with these value must happen using @code{strcmp}
since these are all strings.  Comparing the pointer will probably always
fail.

@item int fs_freq
This element describes the dump frequency in days.

@item int fs_passno
This element describes the pass number on parallel dumps.  It is closely
related to the @code{dump} utility used on Unix systems.
@end table
@end deftp


To read the entire content of the of the @file{fstab} file the GNU libc
contains a set of three functions which are designed in the usual way.

@comment fstab.h
@comment BSD
@deftypefun int setfsent (void)
This function makes sure that the internal read pointer for the
@file{fstab} file is at the beginning of the file.  This is done by
either opening the file or resetting the read pointer.

Since the file handle is internal to the libc this function is not
thread-safe.

This function returns a non-zero value if the operation was successful
and the @code{getfs*} functions can be used to read the entries of the
file.
@end deftypefun

@comment fstab.h
@comment BSD
@deftypefun void endfsent (void)
This function makes sure that all resources acquired by a prior call to
@code{setfsent} (explicitly or implicitly by calling @code{getfsent}) are
freed.
@end deftypefun

@comment fstab.h
@comment BSD
@deftypefun {struct fstab *} getfsent (void)
This function returns the next entry of the @file{fstab} file.  If this
is the first call to any of the functions handling @file{fstab} since
program start or the last call of @code{endfsent}, the file will be
opened.

The function returns a pointer to a variable of type @code{struct
fstab}.  This variable is shared by all threads and therefore this
function is not thread-safe.  If an error occurred @code{getfsent}
returns a @code{NULL} pointer.
@end deftypefun

@comment fstab.h
@comment BSD
@deftypefun {struct fstab *} getfsspec (const char *@var{name})
This function returns the next entry of the @file{fstab} file which has
a string equal to @var{name} pointed to by the @code{fs_spec} element.
Since there is normally exactly one entry for each special device it
makes no sense to call this function more than once for the same
argument.  If this is the first call to any of the functions handling
@file{fstab} since program start or the last call of @code{endfsent},
the file will be opened.

The function returns a pointer to a variable of type @code{struct
fstab}.  This variable is shared by all threads and therefore this
function is not thread-safe.  If an error occurred @code{getfsent}
returns a @code{NULL} pointer.
@end deftypefun

@comment fstab.h
@comment BSD
@deftypefun {struct fstab *} getfsfile (const char *@var{name})
This function returns the next entry of the @file{fstab} file which has
a string equal to @var{name} pointed to by the @code{fs_file} element.
Since there is normally exactly one entry for each mount point it
makes no sense to call this function more than once for the same
argument.  If this is the first call to any of the functions handling
@file{fstab} since program start or the last call of @code{endfsent},
the file will be opened.

The function returns a pointer to a variable of type @code{struct
fstab}.  This variable is shared by all threads and therefore this
function is not thread-safe.  If an error occurred @code{getfsent}
returns a @code{NULL} pointer.
@end deftypefun


@node mtab
@subsection The @file{mtab} file
The following functions and data structure access the @file{mtab} file.

@comment mntent.h
@comment BSD
@deftp {Data Type} {struct mntent}
This structure is used with the @code{getmntent}, @code{getmntent_t},
@code{addmntent}, and @code{hasmntopt} functions.

@table @code
@item char *mnt_fsname
This element contains a pointer to a string describing the name of the
special device from which the filesystem is mounted.  It corresponds to
the @code{fs_spec} element in @code{struct fstab}.

@item char *mnt_dir
This element points to a string describing the mount point of the
filesystem.  It corresponds to the @code{fs_file} element in
@code{struct fstab}.

@item char *mnt_type
@code{mnt_type} describes the filesystem type and is therefore
equivalent to @code{fs_vfstype} in @code{struct fstab}.  @file{mntent.h}
defines a few symbolic names for some of the values this string can have.
But since the kernel can support arbitrary filesystems it does not
make much sense to give them symbolic names.  If one knows the symbol
name one also knows the filesystem name.  Nevertheless here follows the
list of the symbols provided in @file{mntent.h}.

@vtable @code
@item MNTTYPE_IGNORE
This symbol expands to @code{"ignore"}.  The value is sometime used in
@file{fstab} files to make sure entries are not used without removing them.
@item MNTTYPE_NFS
Expands to @code{"nfs"}.  Using this macro sometimes could make sense
since it names the default NFS implementation, in case both version 2
and 3 are supported.
@item MNTTYPE_SWAP
This symbol expands to @code{"swap"}.  It names the special @file{fstab}
entry which names one of the possibly multiple swap partitions.
@end vtable

@item char *mnt_opts
The element contains a string describing the options used while mounting
the filesystem.  As for the equivalent element @code{fs_mntops} of
@code{struct fstab} it is best to use the function @code{getsubopt}
(@pxref{Suboptions}) to access the parts of this string.

The @file{mntent.h} file defines a number of macros with string values
which correspond to some of the options understood by the kernel.  There
might be many more options which are possible so it doesn't make much sense
to rely on these macros but to be consistent here is the list:

@vtable @code
@item MNTOPT_DEFAULTS
Expands to @code{"defaults"}.  This option should be used alone since it
indicates all values for the customizable values are chosen to be the
default.
@item MNTOPT_RO
Expands to @code{"ro"}.  See the @code{FSTAB_RO} value, it means the
filesystem is mounted read-only.
@item MNTOPT_RW
Expand to @code{"rw"}.  See the @code{FSTAB_RW} value, it means the
filesystem is mounted with read and write permissions.
@item MNTOPT_SUID
Expands to @code{"suid"}.  This means that the SUID bit (@pxref{How
Change Persona}) is respected when a program from the filesystem is
started.
@item MNTOPT_NOSUID
Expands to @code{"nosuid"}.  This is the opposite of @code{MNTOPT_SUID},
the SUID bit for all files from the filesystem is ignored.
@item MNTOPT_NOAUTO
Expands to @code{"noauto"}.  At startup time the @code{mount} program
will ignore this entry if it is started with the @code{-a} option to
mount all filesystems mentioned in the @file{fstab} file.
@end vtable

As for the @code{FSTAB_*} entries introduced above it is important to
use @code{strcmp} to check for equality.

@item mnt_freq
This elements corresponds to @code{fs_freq} and also specifies the
frequency in days in which dumps are made.

@item mnt_passno
This element is equivalent to @code{fs_passno} with the same meaning
which is uninteresting for all programs beside @code{dump}.
@end table
@end deftp

For accessing the @file{mtab} file there is again a set of three
functions to access all entries in a row.  Unlike the functions to
handle @file{fstab} these functions do not access a fixed file and there
is even a thread safe variant of the get function.  Beside this the GNU
libc contains functions to alter the file and test for specific options.

@comment mntent.h
@comment BSD
@deftypefun {FILE *} setmntent (const char *@var{file}, const char *@var{mode})
The @code{setmntent} function prepares the file named @var{FILE} which
must be in the format of a @file{fstab} and @file{mtab} file for the
upcoming processing through the other functions of the family.  The
@var{mode} parameter can be chosen in the way the @var{opentype}
parameter for @code{fopen} (@pxref{Opening Streams}) can be chosen.  If
the file is opened for writing the file is also allowed to be empty.

If the file was successfully opened @code{setmntent} returns a file
descriptor for future use.  Otherwise the return value is @code{NULL}
and @code{errno} is set accordingly.
@end deftypefun

@comment mntent.h
@comment BSD
@deftypefun int endmntent (FILE *@var{stream})
This function takes for the @var{stream} parameter a file handle which
previously was returned from the @code{setmntent} call.
@code{endmntent} closes the stream and frees all resources.

The return value is @math{1} unless an error occurred in which case it
is @math{0}.
@end deftypefun

@comment mntent.h
@comment BSD
@deftypefun {struct mntent *} getmntent (FILE *@var{stream})
The @code{getmntent} function takes as the parameter a file handle
previously returned by successful call to @code{setmntent}.  It returns
a pointer to a static variable of type @code{struct mntent} which is
filled with the information from the next entry from the file currently
read.

The file format used prescribes the use of spaces or tab characters to
separate the fields.  This makes it harder to use name containing one of
these characters (e.g., mount points using spaces).  Therefore these
characters are encoded in the files and the @code{getmntent} function
takes care of the decoding while reading the entries back in.
@code{'\040'} is used to encode a space character, @code{'\012'} to
encode a tab character and @code{'\\'} to encode a backslash.

If there was an error or the end of the file is reached the return value
is @code{NULL}.

This function is not thread-safe since all calls to this function return
a pointer to the same static variable.  @code{getmntent_r} should be
used in situations where multiple threads access the file.
@end deftypefun

@comment mntent.h
@comment BSD
@deftypefun {struct mntent *} getmntent_r (FILE *@var{stream}, struct mentent *@var{result}, char *@var{buffer}, int @var{bufsize})
The @code{getmntent_r} function is the reentrant variant of
@code{getmntent}.  It also returns the next entry from the file and
returns a pointer.  The actual variable the values are stored in is not
static, though.  Instead the function stores the values in the variable
pointed to by the @var{result} parameter.  Additional information (e.g.,
the strings pointed to by the elements of the result) are kept in the
buffer of size @var{bufsize} pointed to by @var{buffer}.

Escaped characters (space, tab, backslash) are converted back in the
same way as it happens for @code{getmentent}.

The function returns a @code{NULL} pointer in error cases.  Errors could be:
@itemize @bullet
@item
error while reading the file,
@item
end of file reached,
@item
@var{bufsize} is too small for reading a complete new entry.
@end itemize
@end deftypefun

@comment mntent.h
@comment BSD
@deftypefun int addmntent (FILE *@var{stream}, const struct mntent *@var{mnt})
The @code{addmntent} function allows adding a new entry to the file
previously opened with @code{setmntent}.  The new entries are always
appended.  I.e., even if the position of the file descriptor is not at
the end of the file this function does not overwrite an existing entry
following the current position.

The implication of this is that to remove an entry from a file one has
to create a new file while leaving out the entry to be removed and after
closing the file remove the old one and rename the new file to the
chosen name.

This function takes care of spaces and tab characters in the names to be
written to the file.  It converts them and the backslash character into
the format describe in the @code{getmntent} description above.

This function returns @math{0} in case the operation was successful.
Otherwise the return value is @math{1} and @code{errno} is set
appropriately.
@end deftypefun

@comment mntent.h
@comment BSD
@deftypefun {char *} hasmntopt (const struct mntent *@var{mnt}, const char *@var{opt})
This function can be used to check whether the string pointed to by the
@code{mnt_opts} element of the variable pointed to by @var{mnt} contains
the option @var{opt}.  If this is true a pointer to the beginning of the
option in the @code{mnt_opts} element is returned.  If no such option
exists the function returns @code{NULL}.

This function is useful to test whether a specific option is present but
when all options have to be processed one is better off with using the
@code{getsubopt} function to iterate over all options in the string.
@end deftypefun

@node Other Mount Information
@subsection Other (Non-libc) Sources of Mount Information

On a system with a Linux kernel and the @code{proc} filesystem, you can
get information on currently mounted filesystems from the file
@file{mounts} in the @code{proc} filesystem.  Its format is similar to
that of the @file{mtab} file, but represents what is truly mounted
without relying on facilities outside the kernel to keep @file{mtab} up
to date.


@node Mount/Unmount/Remount, , Mount Information, Filesystem Handling

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 @file{fstab} and @file{mtab} files.  You
should maintain and use these separately.  @xref{Mount Information}.

The symbols in this section are declared in @file{sys/mount.h}.

@comment sys/mount.h
@deftypefun {int} mount (const char *@var{special_file}, const char *@var{dir}, const char *@var{fstype}, unsigned long int @var{options}, const void *@var{data})

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

For a mount, the filesystem on the block device represented by the
device special file named @var{special_file} gets mounted over the mount
point @var{dir}.  This means that the directory @var{dir} (along with any
files in it) is no longer visible; in its place (and still with the name
@var{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''), @code{mount} instantiates a
filesystem and mounts it over @var{dir} and ignores @var{special_file}.

For a remount, @var{dir} specifies the mount point where the filesystem
to be remounted is (and remains) mounted and @var{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
@var{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 @code{proc} filesystem, the list of possible values is in the
file @file{filesystems} in the @code{proc} filesystem (e.g. type
@kbd{cat /proc/filesystems} to see the list).  With a Linux kernel, the
types of filesystems that @code{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
@var{fstype} is @code{ext2}.

For a remount, @code{mount} ignores @var{fstype}.

@c This is traditionally called "rwflag" for historical reasons.
@c No point in confusing people today, though.
@var{options} specifies a variety of options that apply until the
filesystem in 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 @code{MS_RDONLY}) can be overridden for individual
file accesses via @code{ioctl}.

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

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

@item MS_REMOUNT
This bit on means to remount the filesystem.  Off means to mount it.
@c There is a mask MS_RMT_MASK in mount.h that says only two of the options
@c can be reset by remount.  But the Linux kernel has it's own version of
@c MS_RMT_MASK that says they all can be reset.  As far as I can tell,
@c libc just passes the arguments straight through to the kernel.

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

@item S_IMMUTABLE
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 @code{ioctl}.
This option is a relatively new invention and is not available on many
filesystems.

@item S_APPEND
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
call to @code{ioctl}.  This is a relatively new invention and is not
available on many filesystems.

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

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

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

@item MS_SYNCHRONOUS
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.

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

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

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

@c there is also S_QUOTA Linux fs.h (mount.h still uses its former name
@c S_WRITE), but I can't see what it does.  Turns on quotas, I guess.

@end table

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

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

Example:

@smallexample
@group
#include <sys/mount.h>

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

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

@end group
@end smallexample

Appropriate arguments for @code{mount} are conventionally recorded in
the @file{fstab} table.  @xref{Mount Information}.

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

@table @code
@item EPERM
The process is not superuser.
@item ENODEV
The file system type @var{fstype} is not known to the kernel.
@item ENOTBLK
The file @var{dev} is not a block device special file.
@item EBUSY

@itemize

@item 
The device is already mounted.

@item
The mount point is busy.  (E.g. it is some process' working directory or
has a filesystem mounted on it already).

@item
The request is to remount read-only, but there are files open for write.
@end itemize

@item EINVAL
@itemize

@item
A remount was attempted, but there is no filesystem mounted over the
specified mount point.

@item 
The supposed filesystem has an invalid superblock.

@end itemize

@item EACCESS
@itemize

@item
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
@code{MS_RDONLY} bit off).

@item
@var{special_file} or @var{dir} is not accessible due to file permissions.

@item
@var{special_file} is not accessible because it is in a filesystem that is
mounted with the @code{MS_NODEV} option.

@end itemize

@item EM_FILE
The table of dummy devices is full.  @code{mount} needs to create a
dummy device (aka ``unnamed'' device) if the filesystem being mounted is
not one that uses a device.

@end table

@end deftypefun


@comment sys/mount.h
@deftypefun {int} umount2 (const char *@var{file}, int @var{flags})

@code{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 @var{file}.

@var{flags} contains the one-bit field identified by the following 
mask macro:

@table @code

@item MNT_FORCE
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, @code{umount2} fails with @code{errno} = @code{EBUSY}.  Depending
on the filesystem, this may override all, some, or no busy conditions.

@end table

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

Example:

@smallexample
@group
#include <sys/mount.h>

umount2("/mnt", MNT_FORCE);

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

@end group
@end smallexample

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

As part of unmounting, @code{umount2} syncs the filesystem.

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

@table @code
@item EPERM
The process is not superuser.
@item EBUSY
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 @code{MNT_FORCE} option.

@item EINVAL
@var{file} validly refers to a file, but that file is neither a mount 
point nor a device special file of a currently mounted filesystem.

@end table
@end deftypefun

@comment sys/mount.h
@deftypefun {int} umount (const char *@var{file})

@code{umount} does the same thing as @code{umount2} with @var{flags}
set to zeroes.  It exists for compatibility.


@end deftypefun