Users and Groups
****************

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

Users are classified into groups for access control purposes.  Each process has one or more 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 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.

User and Group IDs
==================

Each user account on a computer system is identified by a user name (or login name) and 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 User Database.

Users are classified in groups.  Each user name also belongs to one or more groups, and has one 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 group name and group ID.  See Group Database, for how to find information about a group ID or group name.

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

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 persona of the process, because they determine ``who it is'' for purposes of access control.  These IDs are also called the effective user ID and 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. 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.  See Why Change Persona.

In addition, a user can belong to multiple groups, so the persona includes 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 Access Permission.

The user ID of a process also controls permissions for sending signals using the kill function.  See Signaling Another Process.

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.

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.  See 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.  See 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.

See 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.

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;
          }

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 setuid (uid_t NEWUID)
     This function sets both the real and effective user ID of the process to NEWUID, provided that the process has appropriate privileges.

     If the process is not privileged, then NEWUID must either be equal to the real user ID or the saved user ID (if the system supports the _POSIX_SAVED_IDS feature).  In this case, setuid sets only the effective user ID and not the real user ID.

     The setuid 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 does not have the appropriate privileges; you do not have permission to change to the specified ID.

 -- 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 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.  See 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.

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 setgid (gid_t NEWGID)
     This function sets both the real and effective group ID of the process to NEWGID, provided that the process has appropriate privileges.

     If the process is not privileged, then NEWGID must either be equal to the real group ID or the saved group ID.  In this case, setgid sets only the effective group ID and not the real group ID.

     The return values and error conditions for setgid are the same as those for setuid.

 -- Function: int setregid (gid_t RGID, fid_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 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. See Enable/Disable Setuid.

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

The GNU 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 GID)
     The initgroups function effectively calls setgroups to set the process's supplementary group IDs to be the normal default for the user name USER.  The group ID GID is also included.

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 setuid.  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

     setuid (user_user_id);

and turn it on with

     setuid (game_user_id);

Throughout this process, the real user ID remains jdoe and the saved user ID remains 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 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

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 set-user-ID 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 set-user-ID 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 saved 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>

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

     /* Main program. */

     int
     main (void)
     {
       /* Save 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 original effective 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;
     }

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.

  * Be cautious about using the system and 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 (see 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.

  * 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. See 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 (see Access Permission) to check this; it uses the real user and group IDs, rather than the effective IDs.

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 (see Reading Persona).

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 it is already removed in 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 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. See Standard Environment.

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

This section describes all about 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.

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.

Looking Up One User
-------------------

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

 -- Function: struct passwd * getpwuid (uid_t UID)
     This function returns a pointer to a statically-allocated structure containing information about the user whose user ID is UID.  This structure may be overwritten on subsequent calls to getpwuid.

     A null pointer value indicates there is no user in the data base with user ID UID.

 -- Function: int getpwuid_r (uid_t UID, struct passwd *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct passwd **RESULT)
     This function is similar to getpwuid in that is returns information about the user whose user ID is UID.  But the result is not placed in a static buffer.  Instead the user supplied structure pointed to by RESULT_BUF is filled with the information.  The first BUFLEN bytes of the additional buffer pointed to by 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 0 the pointer returned in RESULT points to the record which contains the wanted data (i.e., RESULT contains the value RESULT_BUF).  In case the return value is non null there is no user in the data base with user ID UID or the buffer BUFFER is too small to contain all the needed information. In the later case the global ERRNO variable is set to ERANGE.

 -- Function: struct passwd * getpwnam (const char *NAME)
     This function returns a pointer to a statically-allocated structure containing information about the user whose user name is NAME. This structure may be overwritten on subsequent calls to getpwnam.

     A null pointer value indicates there is no user named NAME.

 -- Function: int getpwnam_r (const char *NAME, struct passwd *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct passwd **RESULT)
     This function is similar to getpwnam in that is returns information about the user whose user name is NAME.  But the result is not placed in a static buffer.  Instead the user supplied structure pointed to by RESULT_BUF is filled with the information.  The first BUFLEN bytes of the additional buffer pointed to by 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 0 the pointer returned in RESULT points to the record which contains the wanted data (i.e., RESULT contains the value RESULT_BUF).  In case the return value is non null there is no user in the data base with user name NAME or the buffer BUFFER is too small to contain all the needed information. In the later case the global ERRNO variable is set to ERANGE.

Scanning the List of All Users
------------------------------

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 pwd.h.

You can use the fgetpwent function to read user entries from a particular file.

 -- Function: struct passwd * fgetpwent (FILE *STREAM)
     This function reads the next user entry from STREAM and returns a pointer to the entry.  The structure is statically allocated and is rewritten on subsequent calls to 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.

 -- Function: int fgetpwent_r (FILE *STREAM, struct passwd *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct passwd **RESULT)
     This function is similar to fgetpwent in that it reads the next user entry from STREAM.  But the result is returned in the structure pointed to by RESULT_BUF.  The first BUFLEN bytes of the additional buffer pointed to by 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 RESULT points to the structure with the wanted data (normally this is in RESULT_BUF).  If errors occurred the return value is non-null and RESULT contains a null pointer.

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

 -- Function: void setpwent (void)
     This function initializes a stream which getpwent and getpwent_r use to read the user database.

 -- Function: struct passwd * getpwent (void)
     The getpwent function reads the next entry from the stream initialized by setpwent.  It returns a pointer to the entry.  The structure is statically allocated and is rewritten on subsequent calls to 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.

 -- Function: int getpwent_r (struct passwd *RESULT_BUF, char *BUFFER, int BUFLEN, struct passwd **RESULT)
     This function is similar to getpwent in that it returns the next entry from the stream initialized by setpwent.  But in contrast to the getpwent function this function is reentrant since the result is placed in the user supplied structure pointed to by RESULT_BUF.  Additional data, normally the strings pointed to by the elements of the result structure, are placed in the additional buffer or length BUFLEN starting at BUFFER.

     If the function returns zero RESULT points to the structure with the wanted data (normally this is in RESULT_BUF).  If errors occurred the return value is non-zero and RESULT contains a null pointer.

 -- Function: void endpwent (void)
     This function closes the internal stream used by getpwent or getpwent_r.

Writing a User Entry
--------------------

 -- Function: int putpwent (const struct passwd *P, FILE *STREAM)
     This function writes the user entry *P to the stream 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 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 putpwent is declared in pwd.h.

Group Database
==============

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

The Data Structure for a Group
------------------------------

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

 -- Data Type: struct group
     The group structure is used to hold information about an entry in the system group database.  It has at least the following members:

     char *gr_name
          The name of the group.

     gid_t gr_gid
          The group ID of the group.

     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.

Looking Up One Group
--------------------

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

 -- Function: struct group * getgrgid (gid_t GID)
     This function returns a pointer to a statically-allocated structure containing information about the group whose group ID is GID. This structure may be overwritten by subsequent calls to getgrgid.

     A null pointer indicates there is no group with ID GID.

 -- Function: int getgrgid_r (gid_t GID, struct group *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct group **RESULT)
     This function is similar to getgrgid in that is returns information about the group whose group ID is GID.  But the result is not placed in a static buffer.  Instead the user supplied structure pointed to by RESULT_BUF is filled with the information.  The first BUFLEN bytes of the additional buffer pointed to by 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 0 the pointer returned in RESULT points to the record which contains the wanted data (i.e., RESULT contains the value RESULT_BUF).  If the return value is non-zero there is no group in the data base with group ID GID or the buffer BUFFER is too small to contain all the needed information. In the later case the global ERRNO variable is set to ERANGE.

 -- Function: struct group * getgrnam (const char *NAME)
     This function returns a pointer to a statically-allocated structure containing information about the group whose group name is NAME. This structure may be overwritten by subsequent calls to getgrnam.

     A null pointer indicates there is no group named NAME.

 -- Function: int getgrnam_r (const char *NAME, struct group *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct group **RESULT)
     This function is similar to getgrnam in that is returns information about the group whose group name is NAME.  But the result is not placed in a static buffer.  Instead the user supplied structure pointed to by RESULT_BUF is filled with the information.  The first BUFLEN bytes of the additional buffer pointed to by 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 0 the pointer returned in RESULT points to the record which contains the wanted data (i.e., RESULT contains the value RESULT_BUF).  If the return value is non-zero there is no group in the data base with group name NAME or the buffer BUFFER is too small to contain all the needed information. In the later case the global ERRNO variable is set to ERANGE.

Scanning the List of All Groups
-------------------------------

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 grp.h.

You can use the fgetgrent function to read group entries from a particular file.

 -- Function: struct group * fgetgrent (FILE *STREAM)
     The fgetgrent function reads the next entry from STREAM. It returns a pointer to the entry.  The structure is statically allocated and is rewritten on subsequent calls to 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.

 -- Function: int fgetgrent_r (FILE *STREAM, struct group *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct group **RESULT)
     This function is similar to fgetgrent in that it reads the next user entry from STREAM.  But the result is returned in the structure pointed to by RESULT_BUF.  The first BUFLEN bytes of the additional buffer pointed to by 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 RESULT points to the structure with the wanted data (normally this is in RESULT_BUF).  If errors occurred the return value is non-zero and RESULT contains a null pointer.

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

 -- Function: void setgrent (void)
     This function initializes a stream for reading from the group data base. You use this stream by calling getgrent or getgrent_r.

 -- Function: struct group * getgrent (void)
     The getgrent function reads the next entry from the stream initialized by setgrent.  It returns a pointer to the entry.  The structure is statically allocated and is rewritten on subsequent calls to getgrent.  You must copy the contents of the structure if you wish to save the information.

 -- Function: int getgrent_r (struct group *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct group **RESULT)
     This function is similar to getgrent in that it returns the next entry from the stream initialized by setgrent.  But in contrast to the getgrent function this function is reentrant since the result is placed in the user supplied structure pointed to by RESULT_BUF.  Additional data, normally the strings pointed to by the elements of the result structure, are placed in the additional buffer or length BUFLEN starting at BUFFER.

     If the function returns zero RESULT points to the structure with the wanted data (normally this is in RESULT_BUF).  If errors occurred the return value is non-zero and RESULT contains a null pointer.

 -- Function: void endgrent (void)
     This function closes the internal stream used by getgrent or getgrent_r.

Netgroup Database
=================

Netgroup Data
-------------

Sometimes it is useful group users according to other criterias like the ones used in the see 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 nis or nisplus service see 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:

     GROUPNAME ( GROUPNAME | (HOSTNAME,USERNAME,domainname) )+

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 - shall be used.

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 netdb.h.

 -- Function: int setnetgrent (const char *NETGROUP)
     A call to this function initializes the internal state of the library to allow following calls of the getnetgrent iterate over all entries in the netgroup with name NETGROUP.

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

It is important to remember that there is only one single state for iterating the netgroups.  Even if the programmer uses the 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 innetgr function and parts of the implementation of the compat service part of the NSS implementation.

 -- Function: int getnetgrent (char **HOSTP, char **USERP, char **DOMAINP)
     This function returns the next unprocessed entry of the currently selected netgroup.  The string pointers, which addresses are passed in the arguments HOSTP, USERP, and 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 NULL. The returned string pointers are only valid unless no of the netgroup related functions are called.

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

 -- Function: int getnetgrent_r (char **HOSTP, char **USERP, char **DOMAINP, char *BUFFER, int BUFLEN)
     This function is similar to getnetgrent with only one exception: the strings the three string pointers HOSTP, USERP, and DOMAINP point to, are placed in the buffer of BUFLEN bytes starting at BUFFER.  This means the returned values are valid even after other netgroup related functions are called.

     The return value is 1 if the next entry was successfully read and the buffer contains enough room to place the strings in it.  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.

 -- Function: 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 getnetgrent are invalid afterwards.

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.

 -- Function: int innetgr (const char *NETGROUP, const char *HOST, const char *USER, const char *DOMAIN)
     This function tests whether the triple specified by the parameters HOSTP, USERP, and DOMAINP is part of the netgroup NETGROUP.  Using this function has the advantage that

      1. no other netgroup function can use the global netgroup state since internal locking is used and

      2. the function is implemented more efficiently than successive calls to the other set/get/endnetgrent functions.

     Any of the pointers HOSTP, USERP, and DOMAINP can be NULL which means any value is excepted in this position.  This is also true for the name - which should not match any other string otherwise.

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

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.

          [file db.c.texi not included]

Here is some output from this program:

     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