This is libc.info, produced by makeinfo version 4.3 from libc.texinfo.

INFO-DIR-SECTION GNU libraries
START-INFO-DIR-ENTRY
* Libc: (libc).                 C library.
END-INFO-DIR-ENTRY

   This file documents the GNU C library.

   This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library
Reference Manual', for Version 2.2.x.

   Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002
Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
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Invariant Sections being "Free Software Needs Free Documentation" and
"GNU Lesser General Public License", the Front-Cover texts being (a)
(see below), and with the Back-Cover Texts being (b) (see below).  A
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   (a) The FSF's Front-Cover Text is:

   A GNU Manual

   (b) The FSF's Back-Cover Text is:

   You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.


File: libc.info,  Node: Process Persona,  Next: Why Change Persona,  Prev: User and Group IDs,  Up: Users and Groups

The Persona of a Process
========================

   At any time, each process has an "effective user ID", a "effective
group ID", and a set of "supplementary group IDs".  These IDs determine
the privileges of the process.  They are collectively called the
"persona" of the process, because they determine "who it is" for
purposes of access control.

   Your login shell starts out with a persona which consists of your
user ID, your default group ID, and your supplementary group IDs (if
you are in more than one group).  In normal circumstances, all your
other processes inherit these values.

   A process also has a "real user ID" which identifies the user who
created the process, and a "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.  *Note Why Change Persona::.

   For details on how a process's effective user ID and group IDs affect
its permission to access files, see *Note Access Permission::.

   The effective user ID of a process also controls permissions for
sending signals using the `kill' function.  *Note Signaling Another
Process::.

   Finally, there are many operations which can only be performed by a
process whose effective user ID is zero.  A process with this user ID is
a "privileged process".  Commonly the user name `root' is associated
with user ID 0, but there may be other user names with this ID.


File: libc.info,  Node: Why Change Persona,  Next: How Change Persona,  Prev: Process Persona,  Up: Users and Groups

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 `login' program.  When `login'
starts running, its user ID is `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, `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, `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 `games'.  In effect, the program must adopt the
persona of `games' so it can write the scores file.


File: libc.info,  Node: How Change Persona,  Next: Reading Persona,  Prev: Why Change Persona,  Up: Users and Groups

How an Application Can Change Persona
=====================================

   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.  *Note
Permission Bits::.

   When the setuid bit of an executable file is on, executing that file
gives the process a third user ID: the "file user ID".  This ID is set
to the owner ID of the file.  The system then changes the effective
user ID to the file user ID.  The real user ID remains as it was.
Likewise, if the setgid bit is on, the process is given a "file group
ID" equal to the group ID of the file, and its effective group ID is
changed to the file group ID.

   If a process has a file ID (user or group), then it can at any time
change its effective ID to its real ID and back to its file ID.
Programs use this feature to relinquish their special privileges except
when they actually need them.  This makes it less likely that they can
be tricked into doing something inappropriate with their privileges.

   *Portability Note:* Older systems do not have file IDs.  To
determine if a system has this feature, you can test the compiler
define `_POSIX_SAVED_IDS'.  (In the POSIX standard, file IDs are known
as saved IDs.)

   *Note File Attributes::, for a more general discussion of file modes
and accessibility.


File: libc.info,  Node: Reading Persona,  Next: Setting User ID,  Prev: How Change Persona,  Up: Users and Groups

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 `sys/types.h' and
`unistd.h'.

 - Data Type: uid_t
     This is an integer data type used to represent user IDs.  In the
     GNU library, this is an alias for `unsigned int'.

 - Data Type: gid_t
     This is an integer data type used to represent group IDs.  In the
     GNU library, this is an alias for `unsigned int'.

 - Function: uid_t getuid (void)
     The `getuid' function returns the real user ID of the process.

 - Function: gid_t getgid (void)
     The `getgid' function returns the real group ID of the process.

 - Function: uid_t geteuid (void)
     The `geteuid' function returns the effective user ID of the
     process.

 - Function: gid_t getegid (void)
     The `getegid' function returns the effective group ID of the
     process.

 - Function: int getgroups (int COUNT, gid_t *GROUPS)
     The `getgroups' function is used to inquire about the supplementary
     group IDs of the process.  Up to COUNT of these group IDs are
     stored in the array GROUPS; the return value from the function is
     the number of group IDs actually stored.  If COUNT is smaller than
     the total number of supplementary group IDs, then `getgroups'
     returns a value of `-1' and `errno' is set to `EINVAL'.

     If COUNT is zero, then `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 `getgroups' to read all the supplementary group
     IDs:

          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;
          }


File: libc.info,  Node: Setting User ID,  Next: Setting Groups,  Prev: Reading Persona,  Up: Users and Groups

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 `sys/types.h' and `unistd.h'.

 - Function: int seteuid (uid_t NEWEUID)
     This function sets the effective user ID of a process to NEWUID,
     provided that the process is allowed to change its effective user
     ID.  A privileged process (effective user ID zero) can change its
     effective user ID to any legal value.  An unprivileged process
     with a file user ID can change its effective user ID to its real
     user ID or to its file user ID.  Otherwise, a process may not
     change its effective user ID at all.

     The `seteuid' function returns a value of `0' to indicate
     successful completion, and a value of `-1' to indicate an error.
     The following `errno' error conditions are defined for this
     function:

    `EINVAL'
          The value of the NEWUID argument is invalid.

    `EPERM'
          The process may not change to the specified ID.

     Older systems (those without the `_POSIX_SAVED_IDS' feature) do not
     have this function.

 - Function: int setuid (uid_t NEWUID)
     If the calling process is privileged, this function sets both the
     real and effective user ID of the process to NEWUID.  It also
     deletes the file user ID of the process, if any.  NEWUID may be any
     legal value.  (Once this has been done, there is no way to recover
     the old effective user ID.)

     If the process is not privileged, and the system supports the
     `_POSIX_SAVED_IDS' feature, then this function behaves like
     `seteuid'.

     The return values and error conditions are the same as for
     `seteuid'.

 - Function: int setreuid (uid_t RUID, uid_t EUID)
     This function sets the real user ID of the process to RUID and the
     effective user ID to EUID.  If RUID is `-1', it means not to
     change the real user ID; likewise if EUID is `-1', it means not to
     change the effective user ID.

     The `setreuid' function exists for compatibility with 4.3 BSD Unix,
     which does not support file 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 file IDs
     are supported, you should use that feature instead of this
     function.  *Note Enable/Disable Setuid::.

     The return value is `0' on success and `-1' on failure.  The
     following `errno' error conditions are defined for this function:

    `EPERM'
          The process does not have the appropriate privileges; you do
          not have permission to change to the specified ID.


File: libc.info,  Node: Setting Groups,  Next: Enable/Disable Setuid,  Prev: Setting User ID,  Up: Users and Groups

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 `sys/types.h' and `unistd.h'.

 - Function: int setegid (gid_t NEWGID)
     This function sets the effective group ID of the process to
     NEWGID, provided that the process is allowed to change its group
     ID.  Just as with `seteuid', if the process is privileged it may
     change its effective group ID to any value; if it isn't, but it
     has a file group ID, then it may change to its real group ID or
     file group ID; otherwise it may not change its effective group ID.

     Note that a process is only privileged if its effective _user_ ID
     is zero.  The effective group ID only affects access permissions.

     The return values and error conditions for `setegid' are the same
     as those for `seteuid'.

     This function is only present if `_POSIX_SAVED_IDS' is defined.

 - Function: int setgid (gid_t NEWGID)
     This function sets both the real and effective group ID of the
     process to NEWGID, provided that the process is privileged.  It
     also deletes the file group ID, if any.

     If the process is not privileged, then `setgid' behaves like
     `setegid'.

     The return values and error conditions for `setgid' are the same
     as those for `seteuid'.

 - Function: int setregid (gid_t RGID, gid_t EGID)
     This function sets the real group ID of the process to RGID and
     the effective group ID to EGID.  If RGID is `-1', it means not to
     change the real group ID; likewise if EGID is `-1', it means not
     to change the effective group ID.

     The `setregid' function is provided for compatibility with 4.3 BSD
     Unix, which does not support file 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 file IDs
     are supported, you should use that feature instead of using this
     function.  *Note Enable/Disable Setuid::.

     The return values and error conditions for `setregid' are the same
     as those for `setreuid'.

   `setuid' and `setgid' behave differently depending on whether the
effective user ID at the time is zero.  If it is not zero, they behave
like `seteuid' and `setegid'.  If it is, they change both effective and
real IDs and delete the file ID.  To avoid confusion, we recommend you
always use `seteuid' and `setegid' except when you know the effective
user ID is zero and your intent is to change the persona permanently.
This case is rare--most of the programs that need it, such as `login'
and `su', have already been written.

   Note that if your program is setuid to some user other than `root',
there is no way to drop privileges permanently.

   The system also lets privileged processes change their supplementary
group IDs.  To use `setgroups' or `initgroups', your programs should
include the header file `grp.h'.

 - Function: int setgroups (size_t COUNT, gid_t *GROUPS)
     This function sets the process's supplementary group IDs.  It can
     only be called from privileged processes.  The COUNT argument
     specifies the number of group IDs in the array GROUPS.

     This function returns `0' if successful and `-1' on error.  The
     following `errno' error conditions are defined for this function:

    `EPERM'
          The calling process is not privileged.

 - Function: int initgroups (const char *USER, gid_t GROUP)
     The `initgroups' function sets the process's supplementary group
     IDs to be the normal default for the user name USER.  The group
     GROUP is automatically included.

     This function works by scanning the group database for all the
     groups USER belongs to.  It then calls `setgroups' with the list it
     has constructed.

     The return values and error conditions are the same as for
     `setgroups'.

   If you are interested in the groups a particular user belongs to,
but do not want to change the process's supplementary group IDs, you
can use `getgrouplist'.  To use `getgrouplist', your programs should
include the header file `grp.h'.

 - Function: int getgrouplist (const char *USER, gid_t GROUP, gid_t
          *GROUPS, int *NGROUPS)
     The `getgrouplist' function scans the group database for all the
     groups USER belongs to.  Up to *NGROUPS group IDs corresponding to
     these groups are stored in the array GROUPS; the return value from
     the function is the number of group IDs actually stored.  If
     *NGROUPS is smaller than the total number of groups found, then
     `getgrouplist' returns a value of `-1' and stores the actual
     number of groups in *NGROUPS.  The group GROUP is automatically
     included in the list of groups returned by `getgrouplist'.

     Here's how to use `getgrouplist' to read all supplementary groups
     for USER:

          gid_t *
          supplementary_groups (char *user)
          {
            int ngroups = 16;
            gid_t *groups
              = (gid_t *) xmalloc (ngroups * sizeof (gid_t));
            struct passwd *pw = getpwnam (user);
          
            if (pw == NULL)
              return NULL;
          
            if (getgrouplist (pw->pw_name, pw->pw_gid, groups, &ngroups) < 0)
              {
                groups = xrealloc (ngroups * sizeof (gid_t));
                getgrouplist (pw->pw_name, pw->pw_gid, groups, &ngroups);
              }
            return groups;
          }


File: libc.info,  Node: Enable/Disable Setuid,  Next: Setuid Program Example,  Prev: Setting Groups,  Up: Users and Groups

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 `_POSIX_SAVED_IDS' feature, you can
accomplish this with `seteuid'.  When the game program starts, its real
user ID is `jdoe', its effective user ID is `games', and its saved user
ID is also `games'.  The program should record both user ID values once
at the beginning, like this:

     user_user_id = getuid ();
     game_user_id = geteuid ();

   Then it can turn off game file access with

     seteuid (user_user_id);

and turn it on with

     seteuid (game_user_id);

Throughout this process, the real user ID remains `jdoe' and the file
user ID remains `games', so the program can always set its effective
user ID to either one.

   On other systems that don't support file user IDs, you can turn
setuid access on and off by using `setreuid' to swap the real and
effective user IDs of the process, as follows:

     setreuid (geteuid (), getuid ());

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 `jdoe'
while its effective user ID is `games'.  In this state, the game can
write the scores file.  If it swaps the two uids, the real becomes
`games' and the effective becomes `jdoe'; now the program has only
`jdoe' access.  Another swap brings `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:

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


File: libc.info,  Node: Setuid Program Example,  Next: Tips for Setuid,  Prev: Enable/Disable Setuid,  Up: Users and Groups

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 `caber-toss' that manipulates
a file `scores' that should be writable only by the game program
itself.  The program assumes that its executable file will be installed
with the setuid bit set and owned by the same user as the `scores'
file.  Typically, a system administrator will set up an account like
`games' for this purpose.

   The executable file is given mode `4755', so that doing an `ls -l'
on it produces output like:

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

The setuid bit shows up in the file modes as the `s'.

   The scores file is given mode `644', and doing an `ls -l' on it
shows:

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

   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
file IDs feature if it is supported, and otherwise uses `setreuid' to
swap the effective and real user IDs.

     #include <stdio.h>
     #include <sys/types.h>
     #include <unistd.h>
     #include <stdlib.h>
     
     
     /* Remember the effective and real UIDs. */
     
     static uid_t euid, ruid;
     
     
     /* Restore the effective UID to its original value. */
     
     void
     do_setuid (void)
     {
       int status;
     
     #ifdef _POSIX_SAVED_IDS
       status = seteuid (euid);
     #else
       status = setreuid (ruid, euid);
     #endif
       if (status < 0) {
         fprintf (stderr, "Couldn't set uid.\n");
         exit (status);
         }
     }
     
     
     /* Set the effective UID to the real UID. */
     
     void
     undo_setuid (void)
     {
       int status;
     
     #ifdef _POSIX_SAVED_IDS
       status = seteuid (ruid);
     #else
       status = setreuid (euid, ruid);
     #endif
       if (status < 0) {
         fprintf (stderr, "Couldn't set uid.\n");
         exit (status);
         }
     }
     
     /* Main program. */
     
     int
     main (void)
     {
       /* Remember the real and effective user IDs.  */
       ruid = getuid ();
       euid = geteuid ();
       undo_setuid ();
     
       /* Do the game and record the score.  */
       ...
     }

   Notice how the first thing the `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 file user ID,
like this:

     /* Record the score. */
     
     int
     record_score (int score)
     {
       FILE *stream;
       char *myname;
     
       /* Open the scores file. */
       do_setuid ();
       stream = fopen (SCORES_FILE, "a");
       undo_setuid ();
     
       /* 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;
     }


File: libc.info,  Node: Tips for Setuid,  Next: Who Logged In,  Prev: Setuid Program Example,  Up: Users and Groups

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:

   * Don't have `setuid' programs with privileged user IDs such as
     `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.
     It's better if you can write your program to use a special group
     than a special user.

   * Be cautious about using the `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 `execlp' and
     `execvp' functions are a potential risk (since the program they
     execute depends on the user's `PATH' environment variable).

     If you must `exec' another program under a changed ID, specify an
     absolute file name (*note File Name Resolution::) for the
     executable, and make sure that the protections on that executable
     and _all_ containing directories are such that ordinary users
     cannot replace it with some other program.

     You should also check the arguments passed to the program to make
     sure they do not have unexpected effects.  Likewise, you should
     examine the environment variables.  Decide which arguments and
     variables are safe, and reject all others.

     You should never use `system' in a privileged program, because it
     invokes a shell.

   * 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.  *Note Enable/Disable Setuid::.

   * If the `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 `access' function (*note Access Permission::) to check
     this; it uses the real user and group IDs, rather than the
     effective IDs.


File: libc.info,  Node: Who Logged In,  Next: User Accounting Database,  Prev: Tips for Setuid,  Up: Users and Groups

Identifying Who Logged In
=========================

   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 `getuid'
and friends (*note Reading Persona::).  How this information is
collected by the system and how to control/add/remove information from
the background storage is described in *Note User Accounting Database::.

   The `getlogin' function is declared in `unistd.h', while `cuserid'
and `L_cuserid' are declared in `stdio.h'.

 - Function: char * getlogin (void)
     The `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 `cuserid'.

 - Function: char * cuserid (char *STRING)
     The `cuserid' function returns a pointer to a string containing a
     user name associated with the effective ID of the process.  If
     STRING is not a null pointer, it should be an array that can hold
     at least `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 `getlogin'.

     The use of this function is deprecated since it is marked to be
     withdrawn in XPG4.2 and has already been removed from newer
     revisions of POSIX.1.

 - Macro: int L_cuserid
     An integer constant that indicates how long an array you might
     need to store a user name.

   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; see *Note Process Persona::.)  The user
cannot do anything to fool these functions.

   For most purposes, it is more useful to use the environment variable
`LOGNAME' to find out who the user is.  This is more flexible precisely
because the user can set `LOGNAME' arbitrarily.  *Note Standard
Environment::.


File: libc.info,  Node: User Accounting Database,  Next: User Database,  Prev: Who Logged In,  Up: Users and Groups

The 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 `/etc/utmp',
`/var/adm/utmp' or `/var/run/utmp'.  However, these files should
*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.


File: libc.info,  Node: Manipulating the Database,  Next: XPG Functions,  Up: User Accounting Database

Manipulating the User Accounting Database
-----------------------------------------

   These functions and the corresponding data structures are declared in
the header file `utmp.h'.

 - Data Type: struct exit_status
     The `exit_status' data structure is used to hold information about
     the exit status of processes marked as `DEAD_PROCESS' in the user
     accounting database.

    `short int e_termination'
          The exit status of the process.

    `short int e_exit'
          The exit status of the process.

 - Data Type: struct utmp
     The `utmp' data structure is used to hold information about entries
     in the user accounting database.  On the GNU system it has the
     following members:

    `short int ut_type'
          Specifies the type of login; one of `EMPTY', `RUN_LVL',
          `BOOT_TIME', `OLD_TIME', `NEW_TIME', `INIT_PROCESS',
          `LOGIN_PROCESS', `USER_PROCESS', `DEAD_PROCESS' or
          `ACCOUNTING'.

    `pid_t ut_pid'
          The process ID number of the login process.

    `char ut_line[]'
          The device name of the tty (without `/dev/').

    `char ut_id[]'
          The inittab ID of the process.

    `char ut_user[]'
          The user's login name.

    `char ut_host[]'
          The name of the host from which the user logged in.

    `struct exit_status ut_exit'
          The exit status of a process marked as `DEAD_PROCESS'.

    `long ut_session'
          The Session ID, used for windowing.

    `struct timeval ut_tv'
          Time the entry was made.  For entries of type `OLD_TIME' this
          is the time when the system clock changed, and for entries of
          type `NEW_TIME' this is the time the system clock was set to.

    `int32_t ut_addr_v6[4]'
          The Internet address of a remote host.

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

   The following macros are defined for use as values for the `ut_type'
member of the `utmp' structure.  The values are integer constants.

`EMPTY'
     This macro is used to indicate that the entry contains no valid
     user accounting information.

`RUN_LVL'
     This macro is used to identify the systems runlevel.

`BOOT_TIME'
     This macro is used to identify the time of system boot.

`OLD_TIME'
     This macro is used to identify the time when the system clock
     changed.

`NEW_TIME'
     This macro is used to identify the time after the system changed.

`INIT_PROCESS'
     This macro is used to identify a process spawned by the init
     process.

`LOGIN_PROCESS'
     This macro is used to identify the session leader of a logged in
     user.

`USER_PROCESS'
     This macro is used to identify a user process.

`DEAD_PROCESS'
     This macro is used to identify a terminated process.

`ACCOUNTING'
     ???

   The size of the `ut_line', `ut_id', `ut_user' and `ut_host' arrays
can be found using the `sizeof' operator.

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

 - Function: void setutent (void)
     This function opens the user accounting database to begin scanning
     it.  You can then call `getutent', `getutid' or `getutline' to
     read entries and `pututline' to write entries.

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

 - Function: struct utmp * getutent (void)
     The `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
     `getutent'.  You must copy the contents of the structure if you
     wish to save the information or you can use the `getutent_r'
     function which stores the data in a user-provided buffer.

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

 - Function: void endutent (void)
     This function closes the user accounting database.

 - Function: struct utmp * getutid (const struct utmp *ID)
     This function searches forward from the current point in the
     database for an entry that matches ID.  If the `ut_type' member of
     the ID structure is one of `RUN_LVL', `BOOT_TIME', `OLD_TIME' or
     `NEW_TIME' the entries match if the `ut_type' members are
     identical.  If the `ut_type' member of the ID structure is
     `INIT_PROCESS', `LOGIN_PROCESS', `USER_PROCESS' or `DEAD_PROCESS',
     the entries match if the `ut_type' member of the entry read from
     the database is one of these four, and the `ut_id' members match.
     However if the `ut_id' member of either the ID structure or the
     entry read from the database is empty it checks if the `ut_line'
     members match instead.  If a matching entry is found, `getutid'
     returns a pointer to the entry, which is statically allocated, and
     may be overwritten by a subsequent call to `getutent', `getutid'
     or `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 `getutid' function may cache the last read entry.  Therefore,
     if you are using `getutid' to search for multiple occurrences, it
     is necessary to zero out the static data after each call.
     Otherwise `getutid' could just return a pointer to the same entry
     over and over again.

 - Function: struct utmp * getutline (const struct utmp *LINE)
     This function searches forward from the current point in the
     database until it finds an entry whose `ut_type' value is
     `LOGIN_PROCESS' or `USER_PROCESS', and whose `ut_line' member
     matches the `ut_line' member of the 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 `getutent', `getutid' or `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 `getutline' function may cache the last read entry.  Therefore
     if you are using `getutline' to search for multiple occurrences, it
     is necessary to zero out the static data after each call.
     Otherwise `getutline' could just return a pointer to the same
     entry over and over again.

 - Function: struct utmp * pututline (const struct utmp *UTMP)
     The `pututline' function inserts the entry `*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 `getutid' to search for the position to insert the entry,
     however this will not modify the static structure returned by
     `getutent', `getutid' and `getutline'.  If this search fails, the
     entry is appended to the database.

     The `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 `errno' error conditions
     are defined for this function:

    `EPERM'
          The process does not have the appropriate privileges; you
          cannot modify the user accounting database.

   All the `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 returned for the request is overwritten by 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.

 - Function: int getutent_r (struct utmp *BUFFER, struct utmp **RESULT)
     The `getutent_r' is equivalent to the `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 BUFFER.

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

     This function is a GNU extension.

 - Function: int getutid_r (const struct utmp *ID, struct utmp *BUFFER,
          struct utmp **RESULT)
     This function retrieves just like `getutid' the next entry matching
     the information stored in ID.  But the result is stored in the
     buffer pointed to by the parameter BUFFER.

     If successful the function returns `0' and the pointer variable
     pointed to by the parameter RESULT contains a pointer to the
     buffer with the result (probably the same as RESULT.  If not
     successful the function return `-1'.

     This function is a GNU extension.

 - Function: int getutline_r (const struct utmp *LINE, struct utmp
          *BUFFER, struct utmp **RESULT)
     This function retrieves just like `getutline' the next entry
     matching the information stored in LINE.  But the result is stored
     in the buffer pointed to by the parameter BUFFER.

     If successful the function returns `0' and the pointer variable
     pointed to by the parameter RESULT contains a pointer to the
     buffer with the result (probably the same as RESULT.  If not
     successful the function return `-1'.

     This function is a GNU extension.

   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 `/etc/wtmp' or `/var/log/wtmp').

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

 - Function: int utmpname (const char *FILE)
     The `utmpname' function changes the name of the database to be
     examined to FILE, and closes any previously opened database.  By
     default `getutent', `getutid', `getutline' and `pututline' read
     from and write to the user accounting database.

     The following macros are defined for use as the FILE argument:

      - Macro: char * _PATH_UTMP
          This macro is used to specify the user accounting database.

      - Macro: char * _PATH_WTMP
          This macro is used to specify the user accounting log file.

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

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

 - Function: void updwtmp (const char *WTMP_FILE, const struct utmp
          *UTMP)
     The `updwtmp' function appends the entry *UTMP to the database
     specified by WTMP_FILE.  For possible values for the WTMP_FILE
     argument see the `utmpname' function.

   *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 `struct utmp'.  When programming for
the GNU system, it is 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 *Note XPG
Functions::, or take a look at the BSD compatible functions in *Note
Logging In and Out::.


File: libc.info,  Node: XPG Functions,  Next: Logging In and Out,  Prev: Manipulating the Database,  Up: User Accounting Database

XPG User Accounting Database Functions
--------------------------------------

   These functions, described in the X/Open Portability Guide, are
declared in the header file `utmpx.h'.

 - Data Type: struct utmpx
     The `utmpx' data structure contains at least the following members:

    `short int ut_type'
          Specifies the type of login; one of `EMPTY', `RUN_LVL',
          `BOOT_TIME', `OLD_TIME', `NEW_TIME', `INIT_PROCESS',
          `LOGIN_PROCESS', `USER_PROCESS' or `DEAD_PROCESS'.

    `pid_t ut_pid'
          The process ID number of the login process.

    `char ut_line[]'
          The device name of the tty (without `/dev/').

    `char ut_id[]'
          The inittab ID of the process.

    `char ut_user[]'
          The user's login name.

    `struct timeval ut_tv'
          Time the entry was made.  For entries of type `OLD_TIME' this
          is the time when the system clock changed, and for entries of
          type `NEW_TIME' this is the time the system clock was set to.
     On the GNU system, `struct utmpx' is identical to `struct utmp'
     except for the fact that including `utmpx.h' does not make visible
     the declaration of `struct exit_status'.

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

`EMPTY'
     This macro is used to indicate that the entry contains no valid
     user accounting information.

`RUN_LVL'
     This macro is used to identify the systems runlevel.

`BOOT_TIME'
     This macro is used to identify the time of system boot.

`OLD_TIME'
     This macro is used to identify the time when the system clock
     changed.

`NEW_TIME'
     This macro is used to identify the time after the system changed.

`INIT_PROCESS'
     This macro is used to identify a process spawned by the init
     process.

`LOGIN_PROCESS'
     This macro is used to identify the session leader of a logged in
     user.

`USER_PROCESS'
     This macro is used to identify a user process.

`DEAD_PROCESS'
     This macro is used to identify a terminated process.

   The size of the `ut_line', `ut_id' and `ut_user' arrays can be found
using the `sizeof' operator.

 - Function: void setutxent (void)
     This function is similar to `setutent'.  On the GNU system it is
     simply an alias for `setutent'.

 - Function: struct utmpx * getutxent (void)
     The `getutxent' function is similar to `getutent', but returns a
     pointer to a `struct utmpx' instead of `struct utmp'.  On the GNU
     system it simply is an alias for `getutent'.

 - Function: void endutxent (void)
     This function is similar to `endutent'.  On the GNU system it is
     simply an alias for `endutent'.

 - Function: struct utmpx * getutxid (const struct utmpx *ID)
     This function is similar to `getutid', but uses `struct utmpx'
     instead of `struct utmp'.  On the GNU system it is simply an alias
     for `getutid'.

 - Function: struct utmpx * getutxline (const struct utmpx *LINE)
     This function is similar to `getutid', but uses `struct utmpx'
     instead of `struct utmp'.  On the GNU system it is simply an alias
     for `getutline'.

 - Function: struct utmpx * pututxline (const struct utmpx *UTMP)
     The `pututxline' function is functionally identical to
     `pututline', but uses `struct utmpx' instead of `struct utmp'.  On
     the GNU system, `pututxline' is simply an alias for `pututline'.

 - Function: int utmpxname (const char *FILE)
     The `utmpxname' function is functionally identical to `utmpname'.
     On the GNU system, `utmpxname' is simply an alias for `utmpname'.

   You can translate between a traditional `struct utmp' and an XPG
`struct utmpx' with the following functions.  On the GNU system, these
functions are merely copies, since the two structures are identical.

 - Function: int getutmp (const struct utmpx *utmpx, struct utmp *utmp)
     `getutmp' copies the information, insofar as the structures are
     compatible, from UTMPX to UTMP.

 - Function: int getutmpx (const struct utmp *utmp, struct utmpx *utmpx)
     `getutmpx' copies the information, insofar as the structures are
     compatible, from UTMP to UTMPX.


File: libc.info,  Node: Logging In and Out,  Prev: XPG Functions,  Up: User Accounting Database

Logging In and Out
------------------

   These functions, derived from BSD, are available in the separate
`libutil' library, and declared in `utmp.h'.

   Note that the `ut_user' member of `struct utmp' is called `ut_name'
in BSD.  Therefore, `ut_name' is defined as an alias for `ut_user' in
`utmp.h'.

 - Function: int login_tty (int FILEDES)
     This function makes FILEDES the controlling terminal of the
     current process, redirects standard input, standard output and
     standard error output to this terminal, and closes FILEDES.

     This function returns `0' on successful completion, and `-1' on
     error.

 - Function: void login (const struct utmp *ENTRY)
     The `login' functions inserts an entry into the user accounting
     database.  The `ut_line' member is set to the name of the terminal
     on standard input.  If standard input is not a terminal `login'
     uses standard output or standard error output to determine the
     name of the terminal.  If `struct utmp' has a `ut_type' member,
     `login' sets it to `USER_PROCESS', and if there is an `ut_pid'
     member, it will be set to the process ID of the current process.
     The remaining entries are copied from ENTRY.

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

 - Function: int logout (const char *UT_LINE)
     This function modifies the user accounting database to indicate
     that the user on UT_LINE has logged out.

     The `logout' function returns `1' if the entry was successfully
     written to the database, or `0' on error.

 - Function: void logwtmp (const char *UT_LINE, const char *UT_NAME,
          const char *UT_HOST)
     The `logwtmp' function appends an entry to the user accounting log
     file, for the current time and the information provided in the
     UT_LINE, UT_NAME and UT_HOST arguments.

   *Portability Note:* The BSD `struct utmp' only has the `ut_line',
`ut_name', `ut_host' and `ut_time' members.  Older systems do not even
have the `ut_host' member.


File: libc.info,  Node: User Database,  Next: Group Database,  Prev: User Accounting Database,  Up: Users and Groups

User Database
=============

   This section describes how to search and scan the database of
registered users.  The database itself is kept in the 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.


File: libc.info,  Node: User Data Structure,  Next: Lookup User,  Up: User Database

The Data Structure that Describes a User
----------------------------------------

   The functions and data structures for accessing the system user
database are declared in the header file `pwd.h'.

 - Data Type: struct passwd
     The `passwd' data structure is used to hold information about
     entries in the system user data base.  It has at least the
     following members:

    `char *pw_name'
          The user's login name.

    `char *pw_passwd.'
          The encrypted password string.

    `uid_t pw_uid'
          The user ID number.

    `gid_t pw_gid'
          The user's default group ID number.

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

    `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.

    `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.

