@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.

* Platform Type::               Determining operating system and basic

                                  machine type

* Filesystem Handling::         Controlling/querying mounts

* System Parameters::           Getting and setting various system parameters

@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 system on which your

program is running.  First, let's review the various ways computer systems

are named, which is a little complicated because of the history of the

development of the Internet.

Every Unix system (also known as a host) has a host name, whether it's

connected to a network or not.  In its simplest form, as used before

computer networks were an issue, it's just a word like @samp{chicken}.

@cindex host name

But any system attached to the Internet or any network like it conforms

to a more rigorous naming convention as part of the Domain Name System

(DNS).  In the DNS, every host name is composed of two parts:

@cindex DNS

@cindex Domain Name System

@enumerate

@item

hostname

@cindex hostname

@item

domain name

@cindex domain name

@end enumerate

You will note that ``hostname'' looks a lot like ``host name'', but is

not the same thing, and that people often incorrectly refer to entire

host names as ``domain names.''

In the DNS, the full host name is properly called the FQDN (Fully Qualified

Domain Name) and consists of the hostname, then a period, then the

domain name.  The domain name itself usually has multiple components

separated by periods.  So for example, a system's hostname may be

@samp{chicken} and its domain name might be @samp{ai.mit.edu}, so

its FQDN (which is its host name) is @samp{chicken.ai.mit.edu}.

@cindex FQDN

Adding to the confusion, though, is that the DNS is not the only name space

in which a computer needs to be known.  Another name space is the

NIS (aka YP) name space.  For NIS purposes, there is another domain

name, which is called the NIS domain name or the YP domain name.  It

need not have anything to do with the DNS domain name.

@cindex YP

@cindex NIS

@cindex NIS domain name

@cindex YP domain name

Confusing things even more is the fact that in the DNS, it is possible for

multiple FQDNs to refer to the same system.  However, there is always

exactly one of them that is the true host name, and it is called the

canonical FQDN.

In some contexts, the host name is called a ``node name.''

For more information on DNS host naming, see @ref{Host Names}.

@pindex hostname

@pindex hostid

@pindex unistd.h

Prototypes for these functions appear in @file{unistd.h}.

The programs @code{hostname}, @code{hostid}, and @code{domainname} work

by calling these functions.

@deftypefun int gethostname (char *@var{name}, size_t @var{size})

@standards{BSD, unistd.h}

@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}

@c Direct syscall on unix; implemented in terms of uname on posix and of

@c hurd_get_host_config on hurd.

This function returns the host name of the system on which it is called,

in the array @var{name}.  The @var{size} argument specifies the size of

this array, in bytes.  Note that this is @emph{not} the DNS hostname.

If the system participates in the DNS, this is the FQDN (see above).

The return value is @code{0} on success and @code{-1} on failure.  In

@theglibc{}, @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

@deftypefun int sethostname (const char *@var{name}, size_t @var{length})

@standards{BSD, unistd.h}

@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}

@c Direct syscall on unix; implemented in terms of hurd_set_host_config

@c on hurd.

The @code{sethostname} function sets the host name of the system that

calls it to @var{name}, a string with length @var{length}.  Only

privileged processes are permitted to do this.

Usually @code{sethostname} gets called just once, at system boot time.

Often, the program that calls it sets it to the value it finds in the

file @code{/etc/hostname}.

@cindex /etc/hostname

Be sure to set the host name to the full host name, not just the DNS

hostname (see above).

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

@deftypefun int getdomainnname (char *@var{name}, size_t @var{length})

@standards{???, unistd.h}

@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}

@c Syscalls uname, then strlen and memcpy.

@cindex NIS domain name

@cindex YP domain name

@code{getdomainname} returns the NIS (aka YP) domain name of the system

on which it is called.  Note that this is not the more popular DNS

domain name.  Get that with @code{gethostname}.

The specifics of this function are analogous to @code{gethostname}, above.

@end deftypefun

@deftypefun int setdomainname (const char *@var{name}, size_t @var{length})

@standards{???, unistd.h}

@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}

@c Direct syscall.

@cindex NIS domain name

@cindex YP domain name

@code{setdomainname} sets the NIS (aka YP) domain name of the system

on which it is called.  Note that this is not the more popular DNS

domain name.  Set that with @code{sethostname}.

The specifics of this function are analogous to @code{sethostname}, above.

@end deftypefun

@deftypefun {long int} gethostid (void)

@standards{BSD, unistd.h}

@safety{@prelim{}@mtsafe{@mtshostid{} @mtsenv{} @mtslocale{}}@asunsafe{@ascudlopen{} @ascuplugin{} @asucorrupt{} @ascuheap{} @asulock{}}@acunsafe{@aculock{} @acucorrupt{} @acsmem{} @acsfd{}}}

@c On HURD, calls _hurd_get_host_config and strtol.  On Linux, open

@c HOSTIDFILE, reads an int32_t and closes; if that fails, it calls

@c gethostname and gethostbyname_r to use the h_addr.

This function returns the ``host ID'' of the machine the program is

running on.  By convention, this is usually the primary Internet IP 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.

This is not widely used.  It arose in BSD 4.2, but was dropped in BSD 4.4.

It is not required by POSIX.

The proper way to query the IP address is to use @code{gethostbyname}

on the results of @code{gethostname}.  For more information on IP addresses,

@xref{Host Addresses}.

@end deftypefun

@deftypefun int sethostid (long int @var{id})

@standards{BSD, unistd.h}

@safety{@prelim{}@mtunsafe{@mtasuconst{:@mtshostid{}}}@asunsafe{}@acunsafe{@acucorrupt{} @acsfd{}}}

The @code{sethostid} function sets the ``host ID'' of the host machine

to @var{id}.  Only privileged processes are permitted to do this.  Usually

it happens just once, at system boot time.

The proper way to establish the primary IP address of a system

is to configure the IP address resolver to associate that IP address with

the system's host name as returned by @code{gethostname}.  For example,

put a record for the system in @file{/etc/hosts}.

See @code{gethostid} above for more information on host ids.

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 Platform Type

@section Platform 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

As a bonus, @code{uname} also gives some information identifying the

particular system your program is running on.  This is the same information

which you can get with functions targeted to this purpose described in

@ref{Host Identification}.

@deftp {Data Type} {struct utsname}

@standards{POSIX.1, sys/utsname.h}

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 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, @theglibc{}

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

@item char nodename[]

This is the host name of this particular computer.  In @theglibc{},

the value is the same as that returned by @code{gethostname};

see @ref{Host Identification}.

@code{gethostname} is implemented with a call to @code{uname}.

@item char domainname[]

This is the NIS or YP domain name.  It is the same value returned by

@code{getdomainname}; see @ref{Host Identification}.  This element

is a relatively recent invention and use of it is not as portable as

use of the rest of the structure.

@c getdomainname() is implemented with a call to uname().

@end table

@end deftp

@deftypefun int uname (struct utsname *@var{info})

@standards{POSIX.1, sys/utsname.h}

@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}

@c Direct syscall on unix; the posix fallback is to call gethostname and

@c then fills in the other fields with constants; on HURD, it calls

@c proc_uname and then gethostname.

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 return value indicates that the data was successfully stored.

@code{-1} as the return 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 and 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

@subsection Mount Information

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.  @Theglibc{}

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 either

@file{/var/run/mtab} or @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 to 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 _PATH_MOUNTED

@vindex FSTAB

@vindex MNTTAB

@vindex MOUNTED

The filenames given above should never be used directly.  The portable

way to handle these files is to use the macros @code{_PATH_FSTAB},

defined in @file{fstab.h}, or @code{_PATH_MNTTAB}, defined in

@file{mntent.h} and @file{paths.h}, for @file{fstab}; and the macro

@code{_PATH_MOUNTED}, also defined in @file{mntent.h} and

@file{paths.h}, for @file{mtab}.  There are also two alternate macro

names @code{FSTAB}, @code{MNTTAB}, and @code{MOUNTED} defined but

these names are deprecated and kept only for backward compatibility.

The names @code{_PATH_MNTTAB} and @code{_PATH_MOUNTED} 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

@subsubsection The @file{fstab} file

The internal representation for entries of the file is @w{@code{struct

fstab}}, defined in @file{fstab.h}.

@deftp {Data Type} {struct fstab}

@standards{BSD, fstab.h}

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 filesystem gets mounted with read and write enabled.

@item FSTAB_RQ

The filesystem 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 values 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 @theglibc{}

contains a set of three functions which are designed in the usual way.

@deftypefun int setfsent (void)

@standards{BSD, fstab.h}

@safety{@prelim{}@mtunsafe{@mtasurace{:fsent}}@asunsafe{@ascuheap{} @asucorrupt{} @asulock{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}

@c setfsent @mtasurace:fsent @ascuheap @asucorrupt @asulock @acucorrupt @aculock @acsmem @acsfd

@c  fstab_init(1) @mtasurace:fsent @ascuheap @asucorrupt @asulock @acucorrupt @aculock @acsmem @acsfd

@c   malloc dup @ascuheap @acsmem

@c   rewind dup @asucorrupt @acucorrupt [no @aculock]

@c   setmntent dup @ascuheap @asulock @acsmem @acsfd @aculock

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

@deftypefun void endfsent (void)

@standards{BSD, fstab.h}

@safety{@prelim{}@mtunsafe{@mtasurace{:fsent}}@asunsafe{@ascuheap{} @asucorrupt{} @asulock{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}

@c endfsent @mtasurace:fsent @ascuheap @asucorrupt @asulock @acucorrupt @aculock @acsmem @acsfd

@c  endmntent dup @ascuheap @asulock @aculock @acsmem @acsfd

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

@deftypefun {struct fstab *} getfsent (void)

@standards{BSD, fstab.h}

@safety{@prelim{}@mtunsafe{@mtasurace{:fsent} @mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{}}}

@c getfsent @mtasurace:fsent @mtslocale @asucorrupt @ascuheap @asulock @acucorrupt @aculock @acsmem

@c  fstab_init(0) dup @mtasurace:fsent @ascuheap @asucorrupt @asulock @acucorrupt @aculock @acsmem @acsfd

@c  fstab_fetch @mtasurace:fsent @mtslocale @asucorrupt @ascuheap @acucorrupt @aculock @acsmem

@c   getmntent_r dup @mtslocale @asucorrupt @ascuheap @acucorrupt @aculock @acsmem

@c  fstab_convert @mtasurace:fsent

@c   hasmntopt dup ok

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

@deftypefun {struct fstab *} getfsspec (const char *@var{name})

@standards{BSD, fstab.h}

@safety{@prelim{}@mtunsafe{@mtasurace{:fsent} @mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{}}}

@c getffsspec @mtasurace:fsent @mtslocale @asucorrupt @ascuheap @asulock @acucorrupt @aculock @acsmem

@c  fstab_init(1) dup @mtasurace:fsent @ascuheap @asucorrupt @asulock @acucorrupt @aculock @acsmem @acsfd

@c  fstab_fetch dup @mtasurace:fsent @mtslocale @asucorrupt @ascuheap @acucorrupt @aculock @acsmem

@c  strcmp dup ok

@c  fstab_convert dup @mtasurace:fsent

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

@deftypefun {struct fstab *} getfsfile (const char *@var{name})

@standards{BSD, fstab.h}

@safety{@prelim{}@mtunsafe{@mtasurace{:fsent} @mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{}}}

@c getffsfile @mtasurace:fsent @mtslocale @asucorrupt @ascuheap @asulock @acucorrupt @aculock @acsmem

@c  fstab_init(1) dup @mtasurace:fsent @ascuheap @asucorrupt @asulock @acucorrupt @aculock @acsmem @acsfd

@c  fstab_fetch dup @mtasurace:fsent @mtslocale @asucorrupt @ascuheap @acucorrupt @aculock @acsmem

@c  strcmp dup ok

@c  fstab_convert dup @mtasurace:fsent

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

@subsubsection The @file{mtab} file

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

@deftp {Data Type} {struct mntent}

@standards{BSD, mntent.h}

This structure is used with the @code{getmntent}, @code{getmntent_r},

@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 sometimes 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

Expands 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.  Besides this @theglibc{}

contains functions to alter the file and test for specific options.

@deftypefun {FILE *} setmntent (const char *@var{file}, const char *@var{mode})

@standards{BSD, mntent.h}

@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@acsmem{} @acsfd{} @aculock{}}}

@c setmntent @ascuheap @asulock @acsmem @acsfd @aculock

@c  strlen dup ok

@c  mempcpy dup ok

@c  memcpy dup ok

@c  fopen dup @ascuheap @asulock @acsmem @acsfd @aculock

@c  fsetlocking dup ok [no @mtasurace:stream @asulock: exclusive stream]

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

handle for future use.  Otherwise the return value is @code{NULL}

and @code{errno} is set accordingly.

@end deftypefun

@deftypefun int endmntent (FILE *@var{stream})

@standards{BSD, mntent.h}

@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@aculock{} @acsmem{} @acsfd{}}}

@c endmntent @ascuheap @asulock @aculock @acsmem @acsfd

@c  fclose dup @ascuheap @asulock @aculock @acsmem @acsfd

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

@deftypefun {struct mntent *} getmntent (FILE *@var{stream})

@standards{BSD, mntent.h}

@safety{@prelim{}@mtunsafe{@mtasurace{:mntentbuf} @mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{} @asuinit{}}@acunsafe{@acuinit{} @acucorrupt{} @aculock{} @acsmem{}}}

@c getmntent @mtasurace:mntentbuf @mtslocale @asucorrupt @ascuheap @asuinit @acuinit @acucorrupt @aculock @acsmem

@c  libc_once @ascuheap @asuinit @acuinit @acsmem

@c   allocate @ascuheap @acsmem

@c    malloc dup @ascuheap @acsmem

@c  getmntent_r dup @mtslocale @asucorrupt @ascuheap @acucorrupt @aculock @acsmem

The @code{getmntent} function takes as the parameter a file handle

previously returned by a 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 names 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{'\011'} to

encode a tab character, @code{'\012'} to encode a newline 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

@deftypefun {struct mntent *} getmntent_r (FILE *@var{stream}, struct mntent *@var{result}, char *@var{buffer}, int @var{bufsize})

@standards{BSD, mntent.h}

@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{}}}

@c getmntent_r @mtslocale @asucorrupt @ascuheap @acucorrupt @aculock @acsmem

@c  flockfile dup @aculock

@c  fgets_unlocked dup @asucorrupt @acucorrupt [locked, so no @mtsrace:stream]

@c  funlockfile dup @aculock

@c  strchr dup ok

@c  strspn dup ok

@c  strsep dup ok

@c  decode_name ok

@c  sscanf dup @mtslocale @ascuheap @acsmem

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

@deftypefun int addmntent (FILE *@var{stream}, const struct mntent *@var{mnt})

@standards{BSD, mntent.h}

@safety{@prelim{}@mtsafe{@mtsrace{:stream} @mtslocale{}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}}

@c addmntent @mtasurace:stream @mtslocale @asucorrupt @acucorrupt

@c  fseek dup @asucorrupt @acucorrupt [no @aculock]

@c  encode_name ok

@c  fprintf dup @mtslocale @asucorrupt @acucorrupt [no @ascuheap @acsmem, no @aculock]

@c  fflush dup @asucorrupt @acucorrupt [no @aculock]

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 described 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

@deftypefun {char *} hasmntopt (const struct mntent *@var{mnt}, const char *@var{opt})

@standards{BSD, mntent.h}

@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}

@c hasmntopt ok

@c  strlen dup ok

@c  strstr dup ok

@c  strchr dup ok

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

@subsubsection 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

@subsection Mount, Unmount, Remount

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}.

@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})

@standards{SVID, sys/mount.h}

@standards{BSD, sys/mount.h}

@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}

@c Direct syscall.

@code{mount} mounts or remounts a filesystem.  The two operations are

quite different and are merged rather unnaturally 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 with

@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}.