glibc/manual/users.texi

@node Users and Groups
@chapter Users and Groups

Every user who can log in on the system is identified by a unique number
called the @dfn{user ID}.  Each process has an effective user ID which
says which user's access permissions it has.

Users are classified into @dfn{groups} for access control purposes.  Each
process has one or more @dfn{group ID values} which say which groups the
process can use for access to files.

The effective user and group IDs of a process collectively form its
@dfn{persona}.  This determines which files the process can access.
Normally, a process inherits its persona from the parent process, but
under special circumstances a process can change its persona and thus
change its access permissions.

Each file in the system also has a user ID and a group ID.  Access
control works by comparing the user and group IDs of the file with those
of the running process.

The system keeps a database of all the registered users, and another
database of all the defined groups.  There are library functions you
can use to examine these databases.

@menu
* User and Group IDs::          Each user has a unique numeric ID;
				 likewise for groups.
* Process Persona::             The user IDs and group IDs of a process.
* Why Change Persona::          Why a program might need to change
				 its user and/or group IDs.
* How Change Persona::          Changing the user and group IDs.
* Reading Persona::             How to examine the user and group IDs.

* Setting User ID::             Functions for setting the user ID.
* Setting Groups::              Functions for setting the group IDs.

* Enable/Disable Setuid::       Turning setuid access on and off.
* Setuid Program Example::      The pertinent parts of one sample program.
* Tips for Setuid::             How to avoid granting unlimited access.

* Who Logged In::               Getting the name of the user who logged in,
				 or of the real user ID of the current process.

* User Accounting Database::    Keeping information about users and various
                                 actions in databases.

* User Database::               Functions and data structures for
                        	 accessing the user database.
* Group Database::              Functions and data structures for
                        	 accessing the group database.
* Netgroup Database::           Functions for accessing the netgroup database.
* Database Example::            Example program showing use of database
				 inquiry functions.
@end menu

@node User and Group IDs
@section User and Group IDs

@cindex login name
@cindex user name
@cindex user ID
Each user account on a computer system is identified by a @dfn{user
name} (or @dfn{login name}) and @dfn{user ID}.  Normally, each user name
has a unique user ID, but it is possible for several login names to have
the same user ID.  The user names and corresponding user IDs are stored
in a data base which you can access as described in @ref{User Database}.

@cindex group name
@cindex group ID
Users are classified in @dfn{groups}.  Each user name also belongs to
one or more groups, and has one @dfn{default group}.  Users who are
members of the same group can share resources (such as files) that are
not accessible to users who are not a member of that group.  Each group
has a @dfn{group name} and @dfn{group ID}.  @xref{Group Database},
for how to find information about a group ID or group name.

@node Process Persona
@section The Persona of a Process
@cindex persona
@cindex effective user ID
@cindex effective group ID

@c !!! bogus; not single ID.  set of effective group IDs (and, in GNU,
@c set of effective UIDs) determines privilege.  lying here and then
@c telling the truth below is confusing.
At any time, each process has a single user ID and a group ID which
determine the privileges of the process.  These are collectively called
the @dfn{persona} of the process, because they determine ``who it is''
for purposes of access control.  These IDs are also called the
@dfn{effective user ID} and @dfn{effective group ID} of the process.

Your login shell starts out with a persona which consists of your user
ID and your default group ID.
@c !!! also supplementary group IDs.
In normal circumstances, all your other processes inherit these values.

@cindex real user ID
@cindex real group ID
A process also has a @dfn{real user ID} which identifies the user who
created the process, and a @dfn{real group ID} which identifies that
user's default group.  These values do not play a role in access
control, so we do not consider them part of the persona.  But they are
also important.

Both the real and effective user ID can be changed during the lifetime
of a process.  @xref{Why Change Persona}.

@cindex supplementary group IDs
In addition, a user can belong to multiple groups, so the persona
includes @dfn{supplementary group IDs} that also contribute to access
permission.

For details on how a process's effective user IDs and group IDs affect
its permission to access files, see @ref{Access Permission}.

The user ID of a process also controls permissions for sending signals
using the @code{kill} function.  @xref{Signaling Another Process}.

@node Why Change Persona
@section Why Change the Persona of a Process?

The most obvious situation where it is necessary for a process to change
its user and/or group IDs is the @code{login} program.  When
@code{login} starts running, its user ID is @code{root}.  Its job is to
start a shell whose user and group IDs are those of the user who is
logging in.  (To accomplish this fully, @code{login} must set the real
user and group IDs as well as its persona.  But this is a special case.)

The more common case of changing persona is when an ordinary user
program needs access to a resource that wouldn't ordinarily be
accessible to the user actually running it.

For example, you may have a file that is controlled by your program but
that shouldn't be read or modified directly by other users, either
because it implements some kind of locking protocol, or because you want
to preserve the integrity or privacy of the information it contains.
This kind of restricted access can be implemented by having the program
change its effective user or group ID to match that of the resource.

Thus, imagine a game program that saves scores in a file.  The game
program itself needs to be able to update this file no matter who is
running it, but if users can write the file without going through the
game, they can give themselves any scores they like.  Some people
consider this undesirable, or even reprehensible.  It can be prevented
by creating a new user ID and login name (say, @code{games}) to own the
scores file, and make the file writable only by this user.  Then, when
the game program wants to update this file, it can change its effective
user ID to be that for @code{games}.  In effect, the program must
adopt the persona of @code{games} so it can write the scores file.

@node How Change Persona
@section How an Application Can Change Persona
@cindex @code{setuid} programs

The ability to change the persona of a process can be a source of
unintentional privacy violations, or even intentional abuse.  Because of
the potential for problems, changing persona is restricted to special
circumstances.

You can't arbitrarily set your user ID or group ID to anything you want;
only privileged processes can do that.  Instead, the normal way for a
program to change its persona is that it has been set up in advance to
change to a particular user or group.  This is the function of the setuid
and setgid bits of a file's access mode.  @xref{Permission Bits}.

When the setuid bit of an executable file is set, executing that file
automatically changes the effective user ID to the user that owns the
file.  Likewise, executing a file whose setgid bit is set changes the
effective group ID to the group of the file.  @xref{Executing a File}.
Creating a file that changes to a particular user or group ID thus
requires full access to that user or group ID.

@xref{File Attributes}, for a more general discussion of file modes and
accessibility.

A process can always change its effective user (or group) ID back to its
real ID.  Programs do this so as to turn off their special privileges
when they are not needed, which makes for more robustness.

@c !!! talk about _POSIX_SAVED_IDS

@node Reading Persona
@section Reading the Persona of a Process

Here are detailed descriptions of the functions for reading the user and
group IDs of a process, both real and effective.  To use these
facilities, you must include the header files @file{sys/types.h} and
@file{unistd.h}.
@pindex unistd.h
@pindex sys/types.h

@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} uid_t
This is an integer data type used to represent user IDs.  In the GNU
library, this is an alias for @code{unsigned int}.
@end deftp

@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} gid_t
This is an integer data type used to represent group IDs.  In the GNU
library, this is an alias for @code{unsigned int}.
@end deftp

@comment unistd.h
@comment POSIX.1
@deftypefun uid_t getuid (void)
The @code{getuid} function returns the real user ID of the process.
@end deftypefun

@comment unistd.h
@comment POSIX.1
@deftypefun gid_t getgid (void)
The @code{getgid} function returns the real group ID of the process.
@end deftypefun

@comment unistd.h
@comment POSIX.1
@deftypefun uid_t geteuid (void)
The @code{geteuid} function returns the effective user ID of the process.
@end deftypefun

@comment unistd.h
@comment POSIX.1
@deftypefun gid_t getegid (void)
The @code{getegid} function returns the effective group ID of the process.
@end deftypefun

@comment unistd.h
@comment POSIX.1
@deftypefun int getgroups (int @var{count}, gid_t *@var{groups})
The @code{getgroups} function is used to inquire about the supplementary
group IDs of the process.  Up to @var{count} of these group IDs are
stored in the array @var{groups}; the return value from the function is
the number of group IDs actually stored.  If @var{count} is smaller than
the total number of supplementary group IDs, then @code{getgroups}
returns a value of @code{-1} and @code{errno} is set to @code{EINVAL}.

If @var{count} is zero, then @code{getgroups} just returns the total
number of supplementary group IDs.  On systems that do not support
supplementary groups, this will always be zero.

Here's how to use @code{getgroups} to read all the supplementary group
IDs:

@smallexample
@group
gid_t *
read_all_groups (void)
@{
  int ngroups = getgroups (0, NULL);
  gid_t *groups
    = (gid_t *) xmalloc (ngroups * sizeof (gid_t));
  int val = getgroups (ngroups, groups);
  if (val < 0)
    @{
      free (groups);
      return NULL;
    @}
  return groups;
@}
@end group
@end smallexample
@end deftypefun

@node Setting User ID
@section Setting the User ID

This section describes the functions for altering the user ID (real
and/or effective) of a process.  To use these facilities, you must
include the header files @file{sys/types.h} and @file{unistd.h}.
@pindex unistd.h
@pindex sys/types.h

@comment unistd.h
@comment POSIX.1
@deftypefun int setuid (uid_t @var{newuid})
This function sets both the real and effective user ID of the process
to @var{newuid}, provided that the process has appropriate privileges.
@c !!! also sets saved-id

If the process is not privileged, then @var{newuid} must either be equal
to the real user ID or the saved user ID (if the system supports the
@code{_POSIX_SAVED_IDS} feature).  In this case, @code{setuid} sets only
the effective user ID and not the real user ID.
@c !!! xref to discussion of _POSIX_SAVED_IDS

The @code{setuid} function returns a value of @code{0} to indicate
successful completion, and a value of @code{-1} to indicate an error.
The following @code{errno} error conditions are defined for this
function:

@table @code
@item EINVAL
The value of the @var{newuid} argument is invalid.

@item EPERM
The process does not have the appropriate privileges; you do not
have permission to change to the specified ID.
@end table
@end deftypefun

@comment unistd.h
@comment BSD
@deftypefun int setreuid (uid_t @var{ruid}, uid_t @var{euid})
This function sets the real user ID of the process to @var{ruid} and the
effective user ID to @var{euid}.  If @var{ruid} is @code{-1}, it means
not to change the real user ID; likewise if @var{euid} is @code{-1}, it
means not to change the effective user ID.

The @code{setreuid} function exists for compatibility with 4.3 BSD Unix,
which does not support saved IDs.  You can use this function to swap the
effective and real user IDs of the process.  (Privileged processes are
not limited to this particular usage.)  If saved IDs are supported, you
should use that feature instead of this function.  @xref{Enable/Disable
Setuid}.

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
The process does not have the appropriate privileges; you do not
have permission to change to the specified ID.
@end table
@end deftypefun

@node Setting Groups
@section Setting the Group IDs

This section describes the functions for altering the group IDs (real
and effective) of a process.  To use these facilities, you must include
the header files @file{sys/types.h} and @file{unistd.h}.
@pindex unistd.h
@pindex sys/types.h

@comment unistd.h
@comment POSIX.1
@deftypefun int setgid (gid_t @var{newgid})
This function sets both the real and effective group ID of the process
to @var{newgid}, provided that the process has appropriate privileges.
@c !!! also sets saved-id

If the process is not privileged, then @var{newgid} must either be equal
to the real group ID or the saved group ID.  In this case, @code{setgid}
sets only the effective group ID and not the real group ID.

The return values and error conditions for @code{setgid} are the same
as those for @code{setuid}.
@end deftypefun

@comment unistd.h
@comment BSD
@deftypefun int setregid (gid_t @var{rgid}, fid_t @var{egid})
This function sets the real group ID of the process to @var{rgid} and
the effective group ID to @var{egid}.  If @var{rgid} is @code{-1}, it
means not to change the real group ID; likewise if @var{egid} is
@code{-1}, it means not to change the effective group ID.

The @code{setregid} function is provided for compatibility with 4.3 BSD
Unix, which does not support saved IDs.  You can use this function to
swap the effective and real group IDs of the process.  (Privileged
processes are not limited to this usage.)  If saved IDs are supported,
you should use that feature instead of using this function.
@xref{Enable/Disable Setuid}.

The return values and error conditions for @code{setregid} are the same
as those for @code{setreuid}.
@end deftypefun

The GNU system also lets privileged processes change their supplementary
group IDs.  To use @code{setgroups} or @code{initgroups}, your programs
should include the header file @file{grp.h}.
@pindex grp.h

@comment grp.h
@comment BSD
@deftypefun int setgroups (size_t @var{count}, gid_t *@var{groups})
This function sets the process's supplementary group IDs.  It can only
be called from privileged processes.  The @var{count} argument specifies
the number of group IDs in the array @var{groups}.

This function returns @code{0} if successful and @code{-1} on error.
The following @code{errno} error conditions are defined for this
function:

@table @code
@item EPERM
The calling process is not privileged.
@end table
@end deftypefun

@comment grp.h
@comment BSD
@deftypefun int initgroups (const char *@var{user}, gid_t @var{gid})
The @code{initgroups} function effectively calls @code{setgroups} to
set the process's supplementary group IDs to be the normal default for
the user name @var{user}.  The group ID @var{gid} is also included.
@c !!! explain that this works by reading the group file looking for
@c groups USER is a member of.
@end deftypefun

@node Enable/Disable Setuid
@section Enabling and Disabling Setuid Access

A typical setuid program does not need its special access all of the
time.  It's a good idea to turn off this access when it isn't needed,
so it can't possibly give unintended access.

If the system supports the saved user ID feature, you can accomplish
this with @code{setuid}.  When the game program starts, its real user ID
is @code{jdoe}, its effective user ID is @code{games}, and its saved
user ID is also @code{games}.  The program should record both user ID
values once at the beginning, like this:

@smallexample
user_user_id = getuid ();
game_user_id = geteuid ();
@end smallexample

Then it can turn off game file access with

@smallexample
setuid (user_user_id);
@end smallexample

@noindent
and turn it on with

@smallexample
setuid (game_user_id);
@end smallexample

@noindent
Throughout this process, the real user ID remains @code{jdoe} and the
saved user ID remains @code{games}, so the program can always set its
effective user ID to either one.

On other systems that don't support the saved user ID feature, you can
turn setuid access on and off by using @code{setreuid} to swap the real
and effective user IDs of the process, as follows:

@smallexample
setreuid (geteuid (), getuid ());
@end smallexample

@noindent
This special case is always allowed---it cannot fail.

Why does this have the effect of toggling the setuid access?  Suppose a
game program has just started, and its real user ID is @code{jdoe} while
its effective user ID is @code{games}.  In this state, the game can
write the scores file.  If it swaps the two uids, the real becomes
@code{games} and the effective becomes @code{jdoe}; now the program has
only @code{jdoe} access.  Another swap brings @code{games} back to
the effective user ID and restores access to the scores file.

In order to handle both kinds of systems, test for the saved user ID
feature with a preprocessor conditional, like this:

@smallexample
#ifdef _POSIX_SAVED_IDS
  setuid (user_user_id);
#else
  setreuid (geteuid (), getuid ());
#endif
@end smallexample

@node Setuid Program Example
@section Setuid Program Example

Here's an example showing how to set up a program that changes its
effective user ID.

This is part of a game program called @code{caber-toss} that
manipulates a file @file{scores} that should be writable only by the game
program itself.  The program assumes that its executable
file will be installed with the set-user-ID bit set and owned by the
same user as the @file{scores} file.  Typically, a system
administrator will set up an account like @code{games} for this purpose.

The executable file is given mode @code{4755}, so that doing an
@samp{ls -l} on it produces output like:

@smallexample
-rwsr-xr-x   1 games    184422 Jul 30 15:17 caber-toss
@end smallexample

@noindent
The set-user-ID bit shows up in the file modes as the @samp{s}.

The scores file is given mode @code{644}, and doing an @samp{ls -l} on
it shows:

@smallexample
-rw-r--r--  1 games           0 Jul 31 15:33 scores
@end smallexample

Here are the parts of the program that show how to set up the changed
user ID.  This program is conditionalized so that it makes use of the
saved IDs feature if it is supported, and otherwise uses @code{setreuid}
to swap the effective and real user IDs.

@smallexample
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>


/* @r{Save the effective and real UIDs.} */

static uid_t euid, ruid;


/* @r{Restore the effective UID to its original value.} */

void
do_setuid (void)
@{
  int status;

#ifdef _POSIX_SAVED_IDS
  status = setuid (euid);
#else
  status = setreuid (ruid, euid);
#endif
  if (status < 0) @{
    fprintf (stderr, "Couldn't set uid.\n");
    exit (status);
    @}
@}


@group
/* @r{Set the effective UID to the real UID.} */

void
undo_setuid (void)
@{
  int status;

#ifdef _POSIX_SAVED_IDS
  status = setuid (ruid);
#else
  status = setreuid (euid, ruid);
#endif
  if (status < 0) @{
    fprintf (stderr, "Couldn't set uid.\n");
    exit (status);
    @}
@}
@end group

/* @r{Main program.} */

int
main (void)
@{
  /* @r{Save the real and effective user IDs.}  */
  ruid = getuid ();
  euid = geteuid ();
  undo_setuid ();

  /* @r{Do the game and record the score.}  */
  @dots{}
@}
@end smallexample

Notice how the first thing the @code{main} function does is to set the
effective user ID back to the real user ID.  This is so that any other
file accesses that are performed while the user is playing the game use
the real user ID for determining permissions.  Only when the program
needs to open the scores file does it switch back to the original
effective user ID, like this:

@smallexample
/* @r{Record the score.} */

int
record_score (int score)
@{
  FILE *stream;
  char *myname;

  /* @r{Open the scores file.} */
  do_setuid ();
  stream = fopen (SCORES_FILE, "a");
  undo_setuid ();

@group
  /* @r{Write the score to the file.} */
  if (stream)
    @{
      myname = cuserid (NULL);
      if (score < 0)
        fprintf (stream, "%10s: Couldn't lift the caber.\n", myname);
      else
        fprintf (stream, "%10s: %d feet.\n", myname, score);
      fclose (stream);
      return 0;
    @}
  else
    return -1;
@}
@end group
@end smallexample

@node Tips for Setuid
@section Tips for Writing Setuid Programs

It is easy for setuid programs to give the user access that isn't
intended---in fact, if you want to avoid this, you need to be careful.
Here are some guidelines for preventing unintended access and
minimizing its consequences when it does occur:

@itemize @bullet
@item
Don't have @code{setuid} programs with privileged user IDs such as
@code{root} unless it is absolutely necessary.  If the resource is
specific to your particular program, it's better to define a new,
nonprivileged user ID or group ID just to manage that resource.

@item
Be cautious about using the @code{system} and @code{exec} functions in
combination with changing the effective user ID.  Don't let users of
your program execute arbitrary programs under a changed user ID.
Executing a shell is especially bad news.  Less obviously, the
@code{execlp} and @code{execvp} functions are a potential risk (since
the program they execute depends on the user's @code{PATH} environment
variable).

If you must @code{exec} another program under a changed ID, specify an
absolute file name (@pxref{File Name Resolution}) for the executable,
and make sure that the protections on that executable and @emph{all}
containing directories are such that ordinary users cannot replace it
with some other program.

@item
Only use the user ID controlling the resource in the part of the program
that actually uses that resource.  When you're finished with it, restore
the effective user ID back to the actual user's user ID.
@xref{Enable/Disable Setuid}.

@item
If the @code{setuid} part of your program needs to access other files
besides the controlled resource, it should verify that the real user
would ordinarily have permission to access those files.  You can use the
@code{access} function (@pxref{Access Permission}) to check this; it
uses the real user and group IDs, rather than the effective IDs.
@end itemize

@node Who Logged In
@section Identifying Who Logged In
@cindex login name, determining
@cindex user ID, determining

You can use the functions listed in this section to determine the login
name of the user who is running a process, and the name of the user who
logged in the current session.  See also the function @code{getuid} and
friends (@pxref{Reading Persona}).  How this information is collected by
the system and how to control/add/remove information from the background
storage is described in @ref{User Accounting Database}.

The @code{getlogin} function is declared in @file{unistd.h}, while
@code{cuserid} and @code{L_cuserid} are declared in @file{stdio.h}.
@pindex stdio.h
@pindex unistd.h

@comment unistd.h
@comment POSIX.1
@deftypefun {char *} getlogin (void)
The @code{getlogin} function returns a pointer to a string containing the
name of the user logged in on the controlling terminal of the process,
or a null pointer if this information cannot be determined.  The string
is statically allocated and might be overwritten on subsequent calls to
this function or to @code{cuserid}.
@end deftypefun

@comment stdio.h
@comment POSIX.1
@deftypefun {char *} cuserid (char *@var{string})
The @code{cuserid} function returns a pointer to a string containing a
user name associated with the effective ID of the process.  If
@var{string} is not a null pointer, it should be an array that can hold
at least @code{L_cuserid} characters; the string is returned in this
array.  Otherwise, a pointer to a string in a static area is returned.
This string is statically allocated and might be overwritten on
subsequent calls to this function or to @code{getlogin}.

The use of this function is deprecated since it is marked to be
withdrawn in XPG4.2 and it is already removed in POSIX.1.
@end deftypefun

@comment stdio.h
@comment POSIX.1
@deftypevr Macro int L_cuserid
An integer constant that indicates how long an array you might need to
store a user name.
@end deftypevr

These functions let your program identify positively the user who is
running or the user who logged in this session.  (These can differ when
setuid programs are involved; @xref{Process Persona}.)  The user cannot
do anything to fool these functions.

For most purposes, it is more useful to use the environment variable
@code{LOGNAME} to find out who the user is.  This is more flexible
precisely because the user can set @code{LOGNAME} arbitrarily.
@xref{Standard Environment}.


@node User Accounting Database
@section The User Accounting Database
@cindex user accounting database

Most Unix-like operating systems keep track of logged in users by
maintaining a user accounting database.  This user accounting database
stores for each terminal, who has logged on, at what time, the process
ID of the user's login shell, etc., etc., but also stores information
about the run level of the system, the time of the last system reboot,
and possibly more.

The user accounting database typically lives in @file{/etc/utmp},
@file{/var/adm/utmp} or @file{/var/run/utmp}.  However, these files
should @strong{never} be accessed directly.  For reading information
from and writing information to the user accounting database, the
functions described in this section should be used.


@menu
* Manipulating the Database::   Scanning and modifying the user
                                 accounting database.
* XPG Functions::               A standardized way for doing the same thing.
* Logging In and Out::          Functions from BSD that modify the user
                                 accounting database.
@end menu

@node Manipulating the Database
@subsection Manipulating the User Accounting Database

These functions and the corresponding data structures are declared in
the header file @file{utmp.h}.
@pindex utmp.h

@comment utmp.h
@comment SVID
@deftp {Data Type} {struct exit_status}
The @code{exit_status} data structure is used to hold information about
the exit status of processes marked as @code{DEAD_PROCESS} in the user
accounting database.

@table @code
@item short int e_termination
The exit status of the process.

@item short int e_exit
The exit status of the process.
@end table
@end deftp

@deftp {Data Type} {struct utmp}
The @code{utmp} data structure is used to hold information about entries
in the user accounting database.  On the GNU system it has the following
members:

@table @code
@item short int ut_type
Specifies the type of login; one of @code{EMPTY}, @code{RUN_LVL},
@code{BOOT_TIME}, @code{OLD_TIME}, @code{NEW_TIME}, @code{INIT_PROCESS},
@code{LOGIN_PROCESS}, @code{USER_PROCESS}, @code{DEAD_PROCESS} or
@code{ACCOUNTING}.

@item pid_t ut_pid
The process ID number of the login process.

@item char ut_line[]
The device name of the tty (without @file{/dev/}).

@item char ut_id[]
The inittab ID of the process.

@item char ut_user[]
The user's login name.

@item char ut_host[]
The name of the host from which the user logged in.

@item struct exit_status ut_exit
The exit status of a process marked as @code{DEAD_PROCESS}.

@item long ut_session
The Session ID, used for windowing.

@item struct timeval ut_tv
Time the entry was made.  For entries of type @code{OLD_TIME} this is
the time when the system clock changed, and for entries of type
@code{NEW_TIME} this is the time the system clock was set to.

@item int32_t ut_addr_v6[4]
The Internet address of a remote host.
@end table
@end deftp

The @code{ut_type}, @code{ut_pid}, @code{ut_id}, @code{ut_tv}, and
@code{ut_host} fields are not available on all systems.  Portable
applications therefore should be prepared for these situations.  To help
doing this the @file{utmp.h} header provides macros
@code{_HAVE_UT_TYPE}, @code{_HAVE_UT_PID}, @code{_HAVE_UT_ID},
@code{_HAVE_UT_TV}, and @code{_HAVE_UT_HOST} if the respective field is
available.  The programmer can handle the situations by using
@code{#ifdef} in the program code.

The following macros are defined for use as values for the
@code{ut_type} member of the @code{utmp} structure.  The values are
integer constants.

@table @code
@comment utmp.h
@comment SVID
@vindex EMPTY
@item EMPTY
This macro is used to indicate that the entry contains no valid user
accounting information.

@comment utmp.h
@comment SVID
@vindex RUN_LVL
@item RUN_LVL
This macro is used to identify the systems runlevel.

@comment utmp.h
@comment SVID
@vindex BOOT_TIME
@item BOOT_TIME
This macro is used to identify the time of system boot.

@comment utmp.h
@comment SVID
@vindex OLD_TIME
@item OLD_TIME
This macro is used to identify the time when the system clock changed.

@comment utmp.h
@comment SVID
@vindex NEW_TIME
@item NEW_TIME
This macro is used to identify the time after the system changed.

@comment utmp.h
@comment SVID
@vindex INIT_PROCESS
@item INIT_PROCESS
This macro is used to identify a process spawned by the init process.

@comment utmp.h
@comment SVID
@vindex LOGIN_PROCESS
@item LOGIN_PROCESS
This macro is used to identify the session leader of a logged in user.

@comment utmp.h
@comment SVID
@vindex USER_PROCESS
@item USER_PROCESS
This macro is used to identify a user process.

@comment utmp.h
@comment SVID
@vindex DEAD_PROCESS
@item DEAD_PROCESS
This macro is used to identify a terminated process.

@comment utmp.h
@comment SVID
@vindex ACCOUNTING
@item ACCOUNTING
???
@end table

The size of the @code{ut_line}, @code{ut_id}, @code{ut_user} and
@code{ut_host} arrays can be found using the @code{sizeof} operator.

Many older systems have, instead of an @code{ut_tv} member, an
@code{ut_time} member, usually of type @code{time_t}, for representing
the time associated with the entry.  Therefore, for backwards
compatibility only, @file{utmp.h} defines @code{ut_time} as an alias for
@code{ut_tv.tv_sec}.

@comment utmp.h
@comment SVID
@deftypefun void setutent (void)
This function opens the user accounting database to begin scanning it.
You can then call @code{getutent}, @code{getutid} or @code{getutline} to
read entries and @code{pututline} to write entries.

If the database is already open, it resets the input to the beginning of
the database.
@end deftypefun

@comment utmp.h
@comment SVID
@deftypefun {struct utmp *} getutent (void)
The @code{getutent} function reads the next entry from the user
accounting database.  It returns a pointer to the entry, which is
statically allocated and may be overwritten by subsequent calls to
@code{getutent}.  You must copy the contents of the structure if you
wish to save the information or you can use the @code{getutent_r}
function which stores the data in a user-provided buffer.

A null pointer is returned in case no further entry is available.
@end deftypefun

@comment utmp.h
@comment SVID
@deftypefun void endutent (void)
This function closes the user accounting database.
@end deftypefun

@comment utmp.h
@comment SVID
@deftypefun {struct utmp *} getutid (const struct utmp *@var{id})
This function searches forward from the current point in the database
for an entry that matches @var{id}.  If the @code{ut_type} member of the
@var{id} structure is one of @code{RUN_LVL}, @code{BOOT_TIME},
@code{OLD_TIME} or @code{NEW_TIME} the entries match if the
@code{ut_type} members are identical.  If the @code{ut_type} member of
the @var{id} structure is @code{INIT_PROCESS}, @code{LOGIN_PROCESS},
@code{USER_PROCESS} or @code{DEAD_PROCESS}, the entries match if the the
@code{ut_type} member of the entry read from the database is one of
these four, and the @code{ut_id} members match.  However if the
@code{ut_id} member of either the @var{id} structure or the entry read
from the database is empty it checks if the @code{ut_line} members match
instead.  If a matching entry is found, @code{getutid} returns a pointer
to the entry, which is statically allocated, and may be overwritten by a
subsequent call to @code{getutent}, @code{getutid} or @code{getutline}.
You must copy the contents of the structure if you wish to save the
information.

A null pointer is returned in case the end of the database is reached
without a match.

The @code{getutid} function may cache the last read entry.  Therefore,
if you are using @code{getutid} to search for multiple occurrences, it
is necessary to zero out the static data after each call.  Otherwise
@code{getutid} could just return a pointer to the same entry over and
over again.
@end deftypefun

@comment utmp.h
@comment SVID
@deftypefun {struct utmp *} getutline (const struct utmp *@var{line})
This function searches forward from the current point in the database
until it finds an entry whose @code{ut_type} value is
@code{LOGIN_PROCESS} or @code{USER_PROCESS}, and whose @code{ut_line}
member matches the @code{ut_line} member of the @var{line} structure.
If it finds such an entry, it returns a pointer to the entry which is
statically allocated, and may be overwritten by a subsequent call to
@code{getutent}, @code{getutid} or @code{getutline}.  You must copy the
contents of the structure if you wish to save the information.

A null pointer is returned in case the end of the database is reached
without a match.

The @code{getutline} function may cache the last read entry.  Therefore
if you are using @code{getutline} to search for multiple occurrences, it
is necessary to zero out the static data after each call.  Otherwise
@code{getutline} could just return a pointer to the same entry over and
over again.
@end deftypefun

@comment utmp.h
@comment SVID
@deftypefun {struct utmp *} pututline (const struct utmp *@var{utmp})
The @code{pututline} function inserts the entry @code{*@var{utmp}} at
the appropriate place in the user accounting database.  If it finds that
it is not already at the correct place in the database, it uses
@code{getutid} to search for the position to insert the entry, however
this will not modify the static structure returned by @code{getutent},
@code{getutid} and @code{getutline}.  If this search fails, the entry
is appended to the database.

The @code{pututline} function returns a pointer to a copy of the entry
inserted in the user accounting database, or a null pointer if the entry
could not be added.  The following @code{errno} error conditions are
defined for this function:

@table @code
@item EPERM
The process does not have the appropriate privileges; you cannot modify
the user accounting database.
@end table
@end deftypefun

All the @code{get*} functions mentioned before store the information
they return in a static buffer.  This can be a problem in multi-threaded
programs since the data return for the request is overwritten be the
return value data in another thread.  Therefore the GNU C Library
provides as extensions three more functions which return the data in a
user-provided buffer.

@comment utmp.h
@comment GNU
@deftypefun int getutent_r (struct utmp *@var{buffer}, struct utmp **@var{result})
The @code{getutent_r} is equivalent to the @code{getutent} function.  It
returns the next entry from the database.  But instead of storing the
information in a static buffer it stores it in the buffer pointed to by
the parameter @var{buffer}.

If the call was successful, the function returns @code{0} and the
pointer variable pointed to by the parameter @var{result} contains a
pointer to the buffer which contains the result (this is most probably
the same value as @var{buffer}).  If something went wrong during the
execution of @code{getutent_r} the function returns @code{-1}.

This function is a GNU extension.
@end deftypefun

@comment utmp.h
@comment GNU
@deftypefun int getutid_r (const struct utmp *@var{id}, struct utmp *@var{buffer}, struct utmp **@var{result})
This function retrieves just like @code{getutid} the next entry matching
the information stored in @var{id}.  But the result is stored in the
buffer pointed to by the parameter @var{buffer}.

If successful the function returns @code{0} and the pointer variable
pointed to by the parameter @var{result} contains a pointer to the
buffer with the result (probably the same as @var{result}.  If not
successful the function return @code{-1}.

This function is a GNU extension.
@end deftypefun

@comment utmp.h
@comment GNU
@deftypefun int getutline_r (const struct utmp *@var{line}, struct utmp *@var{buffer}, struct utmp **@var{result})
This function retrieves just like @code{getutline} the next entry
matching the information stored in @var{line}.  But the result is stored
in the buffer pointed to by the parameter @var{buffer}.

If successful the function returns @code{0} and the pointer variable
pointed to by the parameter @var{result} contains a pointer to the
buffer with the result (probably the same as @var{result}.  If not
successful the function return @code{-1}.

This function is a GNU extension.
@end deftypefun


In addition to the user accounting database, most systems keep a number
of similar databases.  For example most systems keep a log file with all
previous logins (usually in @file{/etc/wtmp} or @file{/var/log/wtmp}).

For specifying which database to examine, the following function should
be used.

@comment utmp.h
@comment SVID
@deftypefun int utmpname (const char *@var{file})
The @code{utmpname} function changes the name of the database to be
examined to @var{file}, and closes any previously opened database.  By
default @code{getutent}, @code{getutid}, @code{getutline} and
@code{pututline} read from and write to the user accounting database.

The following macros are defined for use as the @var{file} argument:

@deftypevr Macro {char *} _PATH_UTMP
This macro is used to specify the user accounting database.
@end deftypevr

@deftypevr Macro {char *} _PATH_WTMP
This macro is used to specify the user accounting log file.
@end deftypevr

The @code{utmpname} function returns a value of @code{0} if the new name
was successfully stored, and a value of @code{-1} to indicate an error.
Note that @code{utmpname} does not try open the database, and that
therefore the return value does not say anything about whether the
database can be successfully opened.
@end deftypefun

Specially for maintaining log-like databases the GNU C Library provides
the following function:

@comment utmp.h
@comment SVID
@deftypefun void updwtmp (const char *@var{wtmp_file}, const struct utmp *@var{utmp})
The @code{updwtmp} function appends the entry *@var{utmp} to the
database specified by @var{wtmp_file}.  For possible values for the
@var{wtmp_file} argument see the @code{utmpname} function.
@end deftypefun

@strong{Portability Note:} Although many operating systems provide a
subset of these functions, they are not standardized.  There are often
subtle differences in the return types, and there are considerable
differences between the various definitions of @code{struct utmp}.  When
programming for the GNU system, it is probably probably best to stick
with the functions described in this section.  If however, you want your
program to be portable, consider using the XPG functions described in
@ref{XPG Functions}, or take a look at the BSD compatible functions in
@ref{Logging In and Out}.


@node XPG Functions
@subsection XPG User Accounting Database Functions

These functions, described in the X/Open Portability Guide, are declared
in the header file @file{utmpx.h}.
@pindex utmpx.h

@deftp {Data Type} {struct utmpx}
The @code{utmpx} data structure contains at least the following members:

@table @code
@item short int ut_type
Specifies the type of login; one of @code{EMPTY}, @code{RUN_LVL},
@code{BOOT_TIME}, @code{OLD_TIME}, @code{NEW_TIME}, @code{INIT_PROCESS},
@code{LOGIN_PROCESS}, @code{USER_PROCESS} or @code{DEAD_PROCESS}.

@item pid_t ut_pid
The process ID number of the login process.

@item char ut_line[]
The device name of the tty (without @file{/dev/}).

@item char ut_id[]
The inittab ID of the process.

@item char ut_user[]
The user's login name.

@item struct timeval ut_tv
Time the entry was made.  For entries of type @code{OLD_TIME} this is
the time when the system clock changed, and for entries of type
@code{NEW_TIME} this is the time the system clock was set to.
@end table
On the GNU system, @code{struct utmpx} is identical to @code{struct
utmp} except for the fact that including @file{utmpx.h} does not make
visible the declaration of @code{struct exit_status}.
@end deftp

The following macros are defined for use as values for the
@code{ut_type} member of the @code{utmpx} structure.  The values are
integer constants and are, on the GNU system, identical to the
definitions in @file{utmp.h}.

@table @code
@comment utmpx.h
@comment XPG4.2
@vindex EMPTY
@item EMPTY
This macro is used to indicate that the entry contains no valid user
accounting information.

@comment utmpx.h
@comment XPG4.2
@vindex RUN_LVL
@item RUN_LVL
This macro is used to identify the systems runlevel.

@comment utmpx.h
@comment XPG4.2
@vindex BOOT_TIME
@item BOOT_TIME
This macro is used to identify the time of system boot.

@comment utmpx.h
@comment XPG4.2
@vindex OLD_TIME
@item OLD_TIME
This macro is used to identify the time when the system clock changed.

@comment utmpx.h
@comment XPG4.2
@vindex NEW_TIME
@item NEW_TIME
This macro is used to identify the time after the system changed.

@comment utmpx.h
@comment XPG4.2
@vindex INIT_PROCESS
@item INIT_PROCESS
This macro is used to identify a process spawned by the init process.

@comment utmpx.h
@comment XPG4.2
@vindex LOGIN_PROCESS
@item LOGIN_PROCESS
This macro is used to identify the session leader of a logged in user.

@comment utmpx.h
@comment XPG4.2
@vindex USER_PROCESS
@item USER_PROCESS
This macro is used to identify a user process.

@comment utmpx.h
@comment XPG4.2
@vindex DEAD_PROCESS
@item DEAD_PROCESS
This macro is used to identify a terminated process.
@end table

The size of the @code{ut_line}, @code{ut_id} and @code{ut_user} arrays
can be found using the @code{sizeof} operator.

@comment utmpx.h
@comment XPG4.2
@deftypefun void setutxent (void)
This function is similar to @code{setutent}.  On the GNU system it is
simply an alias for @code{setutent}.
@end deftypefun

@comment utmpx.h
@comment XPG4.2
@deftypefun {struct utmpx *} getutxent (void)
The @code{getutxent} function is similar to @code{getutent}, but returns
a pointer to a @code{struct utmpx} instead of @code{struct utmp}.  On
the GNU system it simply is an alias for @code{getutent}.
@end deftypefun

@comment utmpx.h
@comment XPG4.2
@deftypefun void endutxent (void)
This function is similar to @code{endutent}.  On the GNU system it is
simply an alias for @code{endutent}.
@end deftypefun

@comment utmpx.h
@comment XPG4.2
@deftypefun {struct utmpx *} getutxid (const struct utmpx *@var{id})
This function is similar to @code{getutid}, but uses @code{struct utmpx}
instead of @code{struct utmp}.  On the GNU system it is simply an alias
for @code{getutid}.
@end deftypefun

@comment utmpx.h
@comment XPG4.2
@deftypefun {struct utmpx *} getutxline (const struct utmpx *@var{line})
This function is similar to @code{getutid}, but uses @code{struct utmpx}
instead of @code{struct utmp}.  On the GNU system it is simply an alias
for @code{getutline}.
@end deftypefun

@comment utmpx.h
@comment XPG4.2
@deftypefun {struct utmpx *} pututxline (const struct utmpx *@var{utmp})
The @code{pututxline} function provides functionality identical to
@code{pututline}, but uses @code{struct utmpx} instead of @code{struct
utmp}.  On the GNU system @code{pututxline} is simply an alias for
@code{pututline}.
@end deftypefun


@node Logging In and Out
@subsection Logging In and Out

These functions, derived from BSD, are available in the separate
@file{libutil} library, and declared in @file{utmp.h}.
@pindex utmp.h

Note that the @code{ut_user} member of @code{struct utmp} is called
@code{ut_name} in BSD.  Therefore, @code{ut_name} is defined as an alias
for @code{ut_user} in @file{utmp.h}.

@comment utmp.h
@comment BSD
@deftypefun int login_tty (int @var{filedes})
This function makes @var{filedes} the controlling terminal of the
current process, redirects standard input, standard output and
standard error output to this terminal, and closes @var{filedes}.

This function returns @code{0} on successful completion, and @code{-1}
on error.
@end deftypefun

@comment utmp.h
@comment BSD
@deftypefun void login (const struct utmp *@var{entry})
The @code{login} functions inserts an entry into the user accounting
database.  The @code{ut_line} member is set to the name of the terminal
on standard input.  If standard input is not a terminal @code{login}
uses standard output or standard error output to determine the name of
the terminal.  If @code{struct utmp} has a @code{ut_type} member,
@code{login} sets it to @code{USER_PROCESS}, and if there is an
@code{ut_pid} member, it will be set to the process ID of the current
process.  The remaining entries are copied from @var{entry}.

A copy of the entry is written to the user accounting log file.
@end deftypefun

@comment utmp.h
@comment BSD
@deftypefun int logout (const char *@var{ut_line})
This function modifies the user accounting database to indicate that the
user on @var{ut_line} has logged out.

The @code{logout} function returns @code{1} if the entry was successfully
written to the database, or @code{0} on error.
@end deftypefun

@comment utmp.h
@comment BSD
@deftypefun void logwtmp (const char *@var{ut_line}, const char *@var{ut_name}, const char *@var{ut_host})
The @code{logwtmp} function appends an entry to the user accounting log
file, for the current time and the information provided in the
@var{ut_line}, @var{ut_name} and @var{ut_host} arguments.
@end deftypefun

@strong{Portability Note:} The BSD @code{struct utmp} only has the
@code{ut_line}, @code{ut_name}, @code{ut_host} and @code{ut_time}
members.  Older systems do not even have the @code{ut_host} member.


@node User Database
@section User Database
@cindex user database
@cindex password database
@pindex /etc/passwd

This section describes all about how to search and scan the database of
registered users.  The database itself is kept in the file
@file{/etc/passwd} on most systems, but on some systems a special
network server gives access to it.

@menu
* User Data Structure::         What each user record contains.
* Lookup User::                 How to look for a particular user.
* Scanning All Users::          Scanning the list of all users, one by one.
* Writing a User Entry::        How a program can rewrite a user's record.
@end menu

@node User Data Structure
@subsection The Data Structure that Describes a User

The functions and data structures for accessing the system user database
are declared in the header file @file{pwd.h}.
@pindex pwd.h

@comment pwd.h
@comment POSIX.1
@deftp {Data Type} {struct passwd}
The @code{passwd} data structure is used to hold information about
entries in the system user data base.  It has at least the following members:

@table @code
@item char *pw_name
The user's login name.

@item char *pw_passwd.
The encrypted password string.

@item uid_t pw_uid
The user ID number.

@item gid_t pw_gid
The user's default group ID number.

@item char *pw_gecos
A string typically containing the user's real name, and possibly other
information such as a phone number.

@item char *pw_dir
The user's home directory, or initial working directory.  This might be
a null pointer, in which case the interpretation is system-dependent.

@item char *pw_shell
The user's default shell, or the initial program run when the user logs in.
This might be a null pointer, indicating that the system default should
be used.
@end table
@end deftp

@node Lookup User
@subsection Looking Up One User
@cindex converting user ID to user name
@cindex converting user name to user ID

You can search the system user database for information about a
specific user using @code{getpwuid} or @code{getpwnam}.  These
functions are declared in @file{pwd.h}.

@comment pwd.h
@comment POSIX.1
@deftypefun {struct passwd *} getpwuid (uid_t @var{uid})
This function returns a pointer to a statically-allocated structure
containing information about the user whose user ID is @var{uid}.  This
structure may be overwritten on subsequent calls to @code{getpwuid}.

A null pointer value indicates there is no user in the data base with
user ID @var{uid}.
@end deftypefun

@comment pwd.h
@comment POSIX.1c
@deftypefun int getpwuid_r (uid_t @var{uid}, struct passwd *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct passwd **@var{result})
This function is similar to @code{getpwuid} in that is returns
information about the user whose user ID is @var{uid}.  But the result
is not placed in a static buffer.  Instead the user supplied structure
pointed to by @var{result_buf} is filled with the information.  The
first @var{buflen} bytes of the additional buffer pointed to by
@var{buffer} are used to contain additional information, normally
strings which are pointed to by the elements of the result structure.

If the return value is @code{0} the pointer returned in @var{result}
points to the record which contains the wanted data (i.e., @var{result}
contains the value @var{result_buf}).  In case the return value is non
null there is no user in the data base with user ID @var{uid} or the
buffer @var{buffer} is too small to contain all the needed information.
In the later case the global @var{errno} variable is set to
@code{ERANGE}.
@end deftypefun


@comment pwd.h
@comment POSIX.1
@deftypefun {struct passwd *} getpwnam (const char *@var{name})
This function returns a pointer to a statically-allocated structure
containing information about the user whose user name is @var{name}.
This structure may be overwritten on subsequent calls to
@code{getpwnam}.

A null pointer value indicates there is no user named @var{name}.
@end deftypefun

@comment pwd.h
@comment POSIX.1c
@deftypefun int getpwnam_r (const char *@var{name}, struct passwd *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct passwd **@var{result})
This function is similar to @code{getpwnam} in that is returns
information about the user whose user name is @var{name}.  But the result
is not placed in a static buffer.  Instead the user supplied structure
pointed to by @var{result_buf} is filled with the information.  The
first @var{buflen} bytes of the additional buffer pointed to by
@var{buffer} are used to contain additional information, normally
strings which are pointed to by the elements of the result structure.

If the return value is @code{0} the pointer returned in @var{result}
points to the record which contains the wanted data (i.e., @var{result}
contains the value @var{result_buf}).  In case the return value is non
null there is no user in the data base with user name @var{name} or the
buffer @var{buffer} is too small to contain all the needed information.
In the later case the global @var{errno} variable is set to
@code{ERANGE}.
@end deftypefun


@node Scanning All Users
@subsection Scanning the List of All Users
@cindex scanning the user list

This section explains how a program can read the list of all users in
the system, one user at a time.  The functions described here are
declared in @file{pwd.h}.

You can use the @code{fgetpwent} function to read user entries from a
particular file.

@comment pwd.h
@comment SVID
@deftypefun {struct passwd *} fgetpwent (FILE *@var{stream})
This function reads the next user entry from @var{stream} and returns a
pointer to the entry.  The structure is statically allocated and is
rewritten on subsequent calls to @code{fgetpwent}.  You must copy the
contents of the structure if you wish to save the information.

This stream must correspond to a file in the same format as the standard
password database file.  This function comes from System V.
@end deftypefun

@comment pwd.h
@comment GNU
@deftypefun int fgetpwent_r (FILE *@var{stream}, struct passwd *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct passwd **@var{result})
This function is similar to @code{fgetpwent} in that it reads the next
user entry from @var{stream}.  But the result is returned in the
structure pointed to by @var{result_buf}.  The
first @var{buflen} bytes of the additional buffer pointed to by
@var{buffer} are used to contain additional information, normally
strings which are pointed to by the elements of the result structure.

This stream must correspond to a file in the same format as the standard
password database file.

If the function returns null @var{result} points to the structure with
the wanted data (normally this is in @var{result_buf}).  If errors
occurred the return value is non-null and @var{result} contains a null
pointer.
@end deftypefun

The way to scan all the entries in the user database is with
@code{setpwent}, @code{getpwent}, and @code{endpwent}.

@comment pwd.h
@comment SVID, BSD
@deftypefun void setpwent (void)
This function initializes a stream which @code{getpwent} and
@code{getpwent_r} use to read the user database.
@end deftypefun

@comment pwd.h
@comment POSIX.1
@deftypefun {struct passwd *} getpwent (void)
The @code{getpwent} function reads the next entry from the stream
initialized by @code{setpwent}.  It returns a pointer to the entry.  The
structure is statically allocated and is rewritten on subsequent calls
to @code{getpwent}.  You must copy the contents of the structure if you
wish to save the information.

A null pointer is returned in case no further entry is available.
@end deftypefun

@comment pwd.h
@comment GNU
@deftypefun int getpwent_r (struct passwd *@var{result_buf}, char *@var{buffer}, int @var{buflen}, struct passwd **@var{result})
This function is similar to @code{getpwent} in that it returns the next
entry from the stream initialized by @code{setpwent}.  But in contrast
to the @code{getpwent} function this function is reentrant since the
result is placed in the user supplied structure pointed to by
@var{result_buf}.  Additional data, normally the strings pointed to by
the elements of the result structure, are placed in the additional
buffer or length @var{buflen} starting at @var{buffer}.

If the function returns zero @var{result} points to the structure with
the wanted data (normally this is in @var{result_buf}).  If errors
occurred the return value is non-zero and @var{result} contains a null
pointer.
@end deftypefun

@comment pwd.h
@comment SVID, BSD
@deftypefun void endpwent (void)
This function closes the internal stream used by @code{getpwent} or
@code{getpwent_r}.
@end deftypefun

@node Writing a User Entry
@subsection Writing a User Entry

@comment pwd.h
@comment SVID
@deftypefun int putpwent (const struct passwd *@var{p}, FILE *@var{stream})
This function writes the user entry @code{*@var{p}} to the stream
@var{stream}, in the format used for the standard user database
file.  The return value is zero on success and nonzero on failure.

This function exists for compatibility with SVID.  We recommend that you
avoid using it, because it makes sense only on the assumption that the
@code{struct passwd} structure has no members except the standard ones;
on a system which merges the traditional Unix data base with other
extended information about users, adding an entry using this function
would inevitably leave out much of the important information.

The function @code{putpwent} is declared in @file{pwd.h}.
@end deftypefun

@node Group Database
@section Group Database
@cindex group database
@pindex /etc/group

This section describes all about how to search and scan the database of
registered groups.  The database itself is kept in the file
@file{/etc/group} on most systems, but on some systems a special network
service provides access to it.

@menu
* Group Data Structure::        What each group record contains.
* Lookup Group::                How to look for a particular group.
* Scanning All Groups::         Scanning the list of all groups.
@end menu

@node Group Data Structure
@subsection The Data Structure for a Group

The functions and data structures for accessing the system group
database are declared in the header file @file{grp.h}.
@pindex grp.h

@comment grp.h
@comment POSIX.1
@deftp {Data Type} {struct group}
The @code{group} structure is used to hold information about an entry in
the system group database.  It has at least the following members:

@table @code
@item char *gr_name
The name of the group.

@item gid_t gr_gid
The group ID of the group.

@item char **gr_mem
A vector of pointers to the names of users in the group.  Each user name
is a null-terminated string, and the vector itself is terminated by a
null pointer.
@end table
@end deftp

@node Lookup Group
@subsection Looking Up One Group
@cindex converting group name to group ID
@cindex converting group ID to group name

You can search the group database for information about a specific
group using @code{getgrgid} or @code{getgrnam}.  These functions are
declared in @file{grp.h}.

@comment grp.h
@comment POSIX.1
@deftypefun {struct group *} getgrgid (gid_t @var{gid})
This function returns a pointer to a statically-allocated structure
containing information about the group whose group ID is @var{gid}.
This structure may be overwritten by subsequent calls to
@code{getgrgid}.

A null pointer indicates there is no group with ID @var{gid}.
@end deftypefun

@comment grp.h
@comment POSIX.1c
@deftypefun int getgrgid_r (gid_t @var{gid}, struct group *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct group **@var{result})
This function is similar to @code{getgrgid} in that is returns
information about the group whose group ID is @var{gid}.  But the result
is not placed in a static buffer.  Instead the user supplied structure
pointed to by @var{result_buf} is filled with the information.  The
first @var{buflen} bytes of the additional buffer pointed to by
@var{buffer} are used to contain additional information, normally
strings which are pointed to by the elements of the result structure.

If the return value is @code{0} the pointer returned in @var{result}
points to the record which contains the wanted data (i.e., @var{result}
contains the value @var{result_buf}).  If the return value is non-zero
there is no group in the data base with group ID @var{gid} or the
buffer @var{buffer} is too small to contain all the needed information.
In the later case the global @var{errno} variable is set to
@code{ERANGE}.
@end deftypefun

@comment grp.h
@comment SVID, BSD
@deftypefun {struct group *} getgrnam (const char *@var{name})
This function returns a pointer to a statically-allocated structure
containing information about the group whose group name is @var{name}.
This structure may be overwritten by subsequent calls to
@code{getgrnam}.

A null pointer indicates there is no group named @var{name}.
@end deftypefun

@comment grp.h
@comment POSIX.1c
@deftypefun int getgrnam_r (const char *@var{name}, struct group *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct group **@var{result})
This function is similar to @code{getgrnam} in that is returns
information about the group whose group name is @var{name}.  But the result
is not placed in a static buffer.  Instead the user supplied structure
pointed to by @var{result_buf} is filled with the information.  The
first @var{buflen} bytes of the additional buffer pointed to by
@var{buffer} are used to contain additional information, normally
strings which are pointed to by the elements of the result structure.

If the return value is @code{0} the pointer returned in @var{result}
points to the record which contains the wanted data (i.e., @var{result}
contains the value @var{result_buf}).  If the return value is non-zero
there is no group in the data base with group name @var{name} or the
buffer @var{buffer} is too small to contain all the needed information.
In the later case the global @var{errno} variable is set to
@code{ERANGE}.
@end deftypefun

@node Scanning All Groups
@subsection Scanning the List of All Groups
@cindex scanning the group list

This section explains how a program can read the list of all groups in
the system, one group at a time.  The functions described here are
declared in @file{grp.h}.

You can use the @code{fgetgrent} function to read group entries from a
particular file.

@comment grp.h
@comment SVID
@deftypefun {struct group *} fgetgrent (FILE *@var{stream})
The @code{fgetgrent} function reads the next entry from @var{stream}.
It returns a pointer to the entry.  The structure is statically
allocated and is rewritten on subsequent calls to @code{fgetgrent}.  You
must copy the contents of the structure if you wish to save the
information.

The stream must correspond to a file in the same format as the standard
group database file.
@end deftypefun

@comment grp.h
@comment GNU
@deftypefun int fgetgrent_r (FILE *@var{stream}, struct group *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct group **@var{result})
This function is similar to @code{fgetgrent} in that it reads the next
user entry from @var{stream}.  But the result is returned in the
structure pointed to by @var{result_buf}.  The
first @var{buflen} bytes of the additional buffer pointed to by
@var{buffer} are used to contain additional information, normally
strings which are pointed to by the elements of the result structure.

This stream must correspond to a file in the same format as the standard
group database file.

If the function returns zero @var{result} points to the structure with
the wanted data (normally this is in @var{result_buf}).  If errors
occurred the return value is non-zero and @var{result} contains a null
pointer.
@end deftypefun

The way to scan all the entries in the group database is with
@code{setgrent}, @code{getgrent}, and @code{endgrent}.

@comment grp.h
@comment SVID, BSD
@deftypefun void setgrent (void)
This function initializes a stream for reading from the group data base.
You use this stream by calling @code{getgrent} or @code{getgrent_r}.
@end deftypefun

@comment grp.h
@comment SVID, BSD
@deftypefun {struct group *} getgrent (void)
The @code{getgrent} function reads the next entry from the stream
initialized by @code{setgrent}.  It returns a pointer to the entry.  The
structure is statically allocated and is rewritten on subsequent calls
to @code{getgrent}.  You must copy the contents of the structure if you
wish to save the information.
@end deftypefun

@comment grp.h
@comment GNU
@deftypefun int getgrent_r (struct group *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct group **@var{result})
This function is similar to @code{getgrent} in that it returns the next
entry from the stream initialized by @code{setgrent}.  But in contrast
to the @code{getgrent} function this function is reentrant since the
result is placed in the user supplied structure pointed to by
@var{result_buf}.  Additional data, normally the strings pointed to by
the elements of the result structure, are placed in the additional
buffer or length @var{buflen} starting at @var{buffer}.

If the function returns zero @var{result} points to the structure with
the wanted data (normally this is in @var{result_buf}).  If errors
occurred the return value is non-zero and @var{result} contains a null
pointer.
@end deftypefun

@comment grp.h
@comment SVID, BSD
@deftypefun void endgrent (void)
This function closes the internal stream used by @code{getgrent} or
@code{getgrent_r}.
@end deftypefun

@node Netgroup Database
@section Netgroup Database

@menu
* Netgroup Data::                  Data in the Netgroup database and where
                                   it comes from.
* Lookup Netgroup::                How to look for a particular netgroup.
* Netgroup Membership::            How to test for netgroup membership.
@end menu

@node Netgroup Data
@subsection Netgroup Data

@cindex Netgroup
Sometimes it is useful group users according to other criteria like the
ones used in the @xref{Group Database}.  E.g., it is useful to associate
a certain group of users with a certain machine.  On the other hand
grouping of host names is not supported so far.

In Sun Microsystems SunOS appeared a new kind of database, the netgroup
database.  It allows to group hosts, users, and domain freely, giving
them individual names.  More concrete: a netgroup is a list of triples
consisting of a host name, a user name, and a domain name, where any of
the entries can be a wildcard entry, matching all inputs.  A last
possibility is that names of other netgroups can also be given in the
list specifying a netgroup.  So one can construct arbitrary hierarchies
without loops.

Sun's implementation allows netgroups only for the @code{nis} or
@code{nisplus} service @pxref{Services in the NSS configuration}.  The
implementation in the GNU C library has no such restriction.  An entry
in either of the input services must have the following form:

@smallexample
@var{groupname} ( @var{groupname} | @code{(}@var{hostname}@code{,}@var{username}@code{,}@code{domainname}@code{)} )+
@end smallexample

Any of the fields in the triple can be empty which means anything
matches.  While describing the functions we will see that the opposite
case is useful as well.  I.e., there may be entries which will not
match any input.  For entries like a name consisting of the single
character @code{-} shall be used.

@node Lookup Netgroup
@subsection Looking up one Netgroup

The lookup functions for netgroups are a bit different to all other
system database handling functions.  Since a single netgroup can contain
many entries a two-step process is needed.  First a single netgroup is
selected and then one can iterate over all entries in this netgroup.
These functions are declared in @file{netdb.h}.

@comment netdb.h
@deftypefun int setnetgrent (const char *@var{netgroup})
A call to this function initializes the internal state of the library to
allow following calls of the @code{getnetgrent} iterate over all entries
in the netgroup with name @var{netgroup}.

When the call is successful (i.e., when a netgroup with this name exist)
the return value is @code{1}.  When the return value is @code{0} no
netgroup of this name is known or some other error occurred.
@end deftypefun

It is important to remember that there is only one single state for
iterating the netgroups.  Even if the programmer uses the
@code{getnetgrent_r} function the result is not really reentrant since
always only one single netgroup at a time can be processed.  If the
program needs to process more than one netgroup simultaneously she
must protect this by using external locking.  This problem was
introduced in the original netgroups implementation in SunOS and since
we must stay compatible it is not possible to change this.

Some other functions also use the netgroups state.  Currently these are
the @code{innetgr} function and parts of the implementation of the
@code{compat} service part of the NSS implementation.

@comment netdb.h
@deftypefun int getnetgrent (char **@var{hostp}, char **@var{userp}, char **@var{domainp})
This function returns the next unprocessed entry of the currently
selected netgroup.  The string pointers, which addresses are passed in
the arguments @var{hostp}, @var{userp}, and @var{domainp}, will contain
after a successful call pointers to appropriate strings.  If the string
in the next entry is empty the pointer has the value @code{NULL}.
The returned string pointers are only valid unless no of the netgroup
related functions are called.

The return value is @code{1} if the next entry was successfully read.  A
value of @code{0} means no further entries exist or internal errors occurred.
@end deftypefun

@comment netdb.h
@deftypefun int getnetgrent_r (char **@var{hostp}, char **@var{userp}, char **@var{domainp}, char *@var{buffer}, int @var{buflen})
This function is similar to @code{getnetgrent} with only one exception:
the strings the three string pointers @var{hostp}, @var{userp}, and
@var{domainp} point to, are placed in the buffer of @var{buflen} bytes
starting at @var{buffer}.  This means the returned values are valid
even after other netgroup related functions are called.

The return value is @code{1} if the next entry was successfully read and
the buffer contains enough room to place the strings in it.  @code{0} is
returned in case no more entries are found, the buffer is too small, or
internal errors occurred.

This function is a GNU extension.  The original implementation in the
SunOS libc does not provide this function.
@end deftypefun

@comment netdb.h
@deftypefun void endnetgrent (void)
This function free all buffers which were allocated to process the last
selected netgroup.  As a result all string pointers returned by calls
to @code{getnetgrent} are invalid afterwards.
@end deftypefun

@node Netgroup Membership
@subsection Testing for Netgroup Membership

It is often not necessary to scan the whole netgroup since often the
only interesting question is whether a given entry is part of the
selected netgroup.

@comment netdb.h
@deftypefun int innetgr (const char *@var{netgroup}, const char *@var{host}, const char *@var{user}, const char *@var{domain})
This function tests whether the triple specified by the parameters
@var{hostp}, @var{userp}, and @var{domainp} is part of the netgroup
@var{netgroup}.  Using this function has the advantage that

@enumerate
@item
no other netgroup function can use the global netgroup state since
internal locking is used and
@item
the function is implemented more efficiently than successive calls
to the other @code{set}/@code{get}/@code{endnetgrent} functions.
@end enumerate

Any of the pointers @var{hostp}, @var{userp}, and @var{domainp} can be
@code{NULL} which means any value is excepted in this position.  This is
also true for the name @code{-} which should not match any other string
otherwise.

The return value is @code{1} if an entry matching the given triple is
found in the netgroup.  The return value is @code{0} if the netgroup
itself is not found, the netgroup does not contain the triple or
internal errors occurred.
@end deftypefun

@node Database Example
@section User and Group Database Example

Here is an example program showing the use of the system database inquiry
functions.  The program prints some information about the user running
the program.

@smallexample
@include db.c.texi
@end smallexample

Here is some output from this program:

@smallexample
I am Throckmorton Snurd.
My login name is snurd.
My uid is 31093.
My home directory is /home/fsg/snurd.
My default shell is /bin/sh.
My default group is guest (12).
The members of this group are:
  friedman
  tami
@end smallexample