@node Users and Groups, System Information, Name Service Switch, Top @c %MENU% How users are identified and classified @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. * Database Example:: Example program showing the use of database inquiry functions. * Netgroup Database:: Functions for accessing the netgroup database. @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 belongs to one @dfn{default group} and may also belong to any number of @dfn{supplementary groups}. 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 @cindex supplementary group IDs @c When Hurd is more widely used, explain multiple effective user IDs @c here. -zw At any time, each process has an @dfn{effective user ID}, a @dfn{effective group ID}, and a set of @dfn{supplementary group IDs}. These IDs determine the privileges of the process. They are collectively called the @dfn{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. @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}. For details on how a process's effective user ID and group IDs affect its permission to access files, see @ref{Access Permission}. The effective user ID of a process also controls permissions for sending signals using the @code{kill} function. @xref{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 @dfn{privileged process}. Commonly the user name @code{root} is associated with user ID 0, but there may be other user names with this ID. @c !!! should mention POSIX capabilities here. @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 @cindex saved set-user-ID @cindex saved set-group-ID @cindex @code{_POSIX_SAVED_IDS} 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 on, executing that file gives the process a third user ID: the @dfn{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 @dfn{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. @strong{Portability Note:} Older systems do not have file IDs. To determine if a system has this feature, you can test the compiler define @code{_POSIX_SAVED_IDS}. (In the POSIX standard, file IDs are known as saved IDs.) @xref{File Attributes}, for a more general discussion of file modes and accessibility. @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 seteuid (uid_t @var{neweuid}) This function sets the effective user ID of a process to @var{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 @code{seteuid} 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 may not change to the specified ID. @end table Older systems (those without the @code{_POSIX_SAVED_IDS} feature) do not have this function. @end deftypefun @comment unistd.h @comment POSIX.1 @deftypefun int setuid (uid_t @var{newuid}) If the calling process is privileged, this function sets both the real and effective user ID of the process to @var{newuid}. It also deletes the file user ID of the process, if any. @var{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 @code{_POSIX_SAVED_IDS} feature, then this function behaves like @code{seteuid}. The return values and error conditions are the same as for @code{seteuid}. @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 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. @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 setegid (gid_t @var{newgid}) This function sets the effective group ID of the process to @var{newgid}, provided that the process is allowed to change its group ID. Just as with @code{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 @emph{user} ID is zero. The effective group ID only affects access permissions. The return values and error conditions for @code{setegid} are the same as those for @code{seteuid}. This function is only present if @code{_POSIX_SAVED_IDS} is defined. @end deftypefun @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 is privileged. It also deletes the file group ID, if any. If the process is not privileged, then @code{setgid} behaves like @code{setegid}. The return values and error conditions for @code{setgid} are the same as those for @code{seteuid}. @end deftypefun @comment unistd.h @comment BSD @deftypefun int setregid (gid_t @var{rgid}, gid_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 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. @xref{Enable/Disable Setuid}. The return values and error conditions for @code{setregid} are the same as those for @code{setreuid}. @end deftypefun @code{setuid} and @code{setgid} behave differently depending on whether the effective user ID at the time is zero. If it is not zero, they behave like @code{seteuid} and @code{setegid}. If it is, they change both effective and real IDs and delete the file ID. To avoid confusion, we recommend you always use @code{seteuid} and @code{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 @code{login} and @code{su}, have already been written. Note that if your program is setuid to some user other than @code{root}, there is no way to drop privileges permanently. The 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 sets the process's supplementary group IDs to be the normal default for the user name @var{user}. If @var{gid} is not -1, it includes that group also. This function works by scanning the group database for all the groups @var{user} belongs to. It then calls @code{setgroups} with the list it has constructed. The return values and error conditions are the same as for @code{setgroups}. @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 @code{_POSIX_SAVED_IDS} feature, you can accomplish this with @code{seteuid}. 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 seteuid (user_user_id); @end smallexample @noindent and turn it on with @smallexample seteuid (game_user_id); @end smallexample @noindent Throughout this process, the real user ID remains @code{jdoe} and the file 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 file user IDs, 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 setuid 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 setuid 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 file 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{Remember 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 = seteuid (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 = seteuid (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{Remember 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 file 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. It's better if you can write your program to use a special group than a special user. @item Be cautious about using the @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. 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 @code{system} in a privileged program, because it invokes a shell. @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 has already been removed from newer revisions of 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; see @ref{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 @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 to 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 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 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 it returns information about the user whose user ID is @var{uid}. However, it fills the user supplied structure pointed to by @var{result_buf} with the information instead of using a static buffer. 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 a user with ID @var{uid} is found, 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 no user is found or if an error occured, the pointer returned in @var{result} is a null pointer. The function returns zero or an error code. If the buffer @var{buffer} is too small to contain all the needed information, the error code @code{ERANGE} is returned and @var{errno} 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 return 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}. However, like @code{getpwuid_r}, it fills the user supplied buffers in @var{result_buf} and @var{buffer} with the information instead of using a static buffer. The return values are the same as for @code{getpwuid_r}. @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. The stream must correspond to a file in the same format as the standard password database file. @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. The stream must correspond to a file in the same format as the standard password 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 nonzero 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 when no more entries are 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}. Like @code{fgetpwent_r}, it uses the user-supplied buffers in @var{result_buf} and @var{buffer} to return the information requested. The return values are the same as for @code{fgetpwent_r}. @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. @c Then how are programmers to modify the password file? -zw 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 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 it returns information about the group whose group ID is @var{gid}. However, it fills the user supplied structure pointed to by @var{result_buf} with the information instead of using a static buffer. 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 a group with ID @var{gid} is found, 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 no group is found or if an error occured, the pointer returned in @var{result} is a null pointer. The function returns zero or an error code. If the buffer @var{buffer} is too small to contain all the needed information, the error code @code{ERANGE} is returned and @var{errno} 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}. Like @code{getgrgid_r}, it uses the user supplied buffers in @var{result_buf} and @var{buffer}, not a static buffer. The return values are the same as for @code{getgrgid_r} @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 overwritten 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 overwritten 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}. Like @code{fgetgrent_r}, it places the result in user-supplied buffers pointed to @var{result_buf} and @var{buffer}. If the function returns zero @var{result} contains a pointer to the data (normally equal to @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 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 @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 to group users according to other criteria (@pxref{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 @comment BSD @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 @comment BSD @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 @comment GNU @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 @comment BSD @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 @comment BSD @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