@node Users and Groups @chapter Users and Groups Every user who can log in on the system is identified by a unique number called the @dfn{user ID}. Each process has an effective user ID which says which user's access permissions it has. Users are classified into @dfn{groups} for access control purposes. Each process has one or more @dfn{group ID values} which say which groups the process can use for access to files. The effective user and group IDs of a process collectively form its @dfn{persona}. This determines which files the process can access. Normally, a process inherits its persona from the parent process, but under special circumstances a process can change its persona and thus change its access permissions. Each file in the system also has a user ID and a group ID. Access control works by comparing the user and group IDs of the file with those of the running process. The system keeps a database of all the registered users, and another database of all the defined groups. There are library functions you can use to examine these databases. @menu * User and Group IDs:: Each user has a unique numeric ID; likewise for groups. * Process Persona:: The user IDs and group IDs of a process. * Why Change Persona:: Why a program might need to change its user and/or group IDs. * How Change Persona:: Changing the user and group IDs. * Reading Persona:: How to examine the user and group IDs. * Setting User ID:: Functions for setting the user ID. * Setting Groups:: Functions for setting the group IDs. * Enable/Disable Setuid:: Turning setuid access on and off. * Setuid Program Example:: The pertinent parts of one sample program. * Tips for Setuid:: How to avoid granting unlimited access. * Who Logged In:: Getting the name of the user who logged in, or of the real user ID of the current process. * User Database:: Functions and data structures for accessing the user database. * Group Database:: Functions and data structures for accessing the group database. * Netgroup Database:: Functions for accessing the netgroup database. * Database Example:: Example program showing use of database inquiry functions. @end menu @node User and Group IDs, Process Persona, Users and Groups, Users and Groups @section User and Group IDs @cindex login name @cindex user name @cindex user ID Each user account on a computer system is identified by a @dfn{user name} (or @dfn{login name}) and @dfn{user ID}. Normally, each user name has a unique user ID, but it is possible for several login names to have the same user ID. The user names and corresponding user IDs are stored in a data base which you can access as described in @ref{User Database}. @cindex group name @cindex group ID Users are classified in @dfn{groups}. Each user name also belongs to one or more groups, and has one @dfn{default group}. Users who are members of the same group can share resources (such as files) that are not accessible to users who are not a member of that group. Each group has a @dfn{group name} and @dfn{group ID}. @xref{Group Database}, for how to find information about a group ID or group name. @node Process Persona, Why Change Persona, User and Group IDs, Users and Groups @section The Persona of a Process @cindex persona @cindex effective user ID @cindex effective group ID @c !!! bogus; not single ID. set of effective group IDs (and, in GNU, @c set of effective UIDs) determines privilege. lying here and then @c telling the truth below is confusing. At any time, each process has a single user ID and a group ID which determine the privileges of the process. These are collectively called the @dfn{persona} of the process, because they determine ``who it is'' for purposes of access control. These IDs are also called the @dfn{effective user ID} and @dfn{effective group ID} of the process. Your login shell starts out with a persona which consists of your user ID and your default group ID. @c !!! also supplementary group IDs. In normal circumstances, all your other processes inherit these values. @cindex real user ID @cindex real group ID A process also has a @dfn{real user ID} which identifies the user who created the process, and a @dfn{real group ID} which identifies that user's default group. These values do not play a role in access control, so we do not consider them part of the persona. But they are also important. Both the real and effective user ID can be changed during the lifetime of a process. @xref{Why Change Persona}. @cindex supplementary group IDs In addition, a user can belong to multiple groups, so the persona includes @dfn{supplementary group IDs} that also contribute to access permission. For details on how a process's effective user IDs and group IDs affect its permission to access files, see @ref{Access Permission}. The user ID of a process also controls permissions for sending signals using the @code{kill} function. @xref{Signaling Another Process}. @node Why Change Persona, How Change Persona, Process Persona, Users and Groups @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, Reading Persona, Why Change Persona, Users and Groups @section How an Application Can Change Persona @cindex @code{setuid} programs The ability to change the persona of a process can be a source of unintentional privacy violations, or even intentional abuse. Because of the potential for problems, changing persona is restricted to special circumstances. You can't arbitrarily set your user ID or group ID to anything you want; only privileged processes can do that. Instead, the normal way for a program to change its persona is that it has been set up in advance to change to a particular user or group. This is the function of the setuid and setgid bits of a file's access mode. @xref{Permission Bits}. When the setuid bit of an executable file is set, executing that file automatically changes the effective user ID to the user that owns the file. Likewise, executing a file whose setgid bit is set changes the effective group ID to the group of the file. @xref{Executing a File}. Creating a file that changes to a particular user or group ID thus requires full access to that user or group ID. @xref{File Attributes}, for a more general discussion of file modes and accessibility. A process can always change its effective user (or group) ID back to its real ID. Programs do this so as to turn off their special privileges when they are not needed, which makes for more robustness. @c !!! talk about _POSIX_SAVED_IDS @node Reading Persona, Setting User ID, How Change Persona, Users and Groups @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, Setting Groups, Reading Persona, Users and Groups @section Setting the User ID This section describes the functions for altering the user ID (real and/or effective) of a process. To use these facilities, you must include the header files @file{sys/types.h} and @file{unistd.h}. @pindex unistd.h @pindex sys/types.h @comment unistd.h @comment POSIX.1 @deftypefun int setuid (uid_t @var{newuid}) This function sets both the real and effective user ID of the process to @var{newuid}, provided that the process has appropriate privileges. @c !!! also sets saved-id If the process is not privileged, then @var{newuid} must either be equal to the real user ID or the saved user ID (if the system supports the @code{_POSIX_SAVED_IDS} feature). In this case, @code{setuid} sets only the effective user ID and not the real user ID. @c !!! xref to discussion of _POSIX_SAVED_IDS The @code{setuid} function returns a value of @code{0} to indicate successful completion, and a value of @code{-1} to indicate an error. The following @code{errno} error conditions are defined for this function: @table @code @item EINVAL The value of the @var{newuid} argument is invalid. @item EPERM The process does not have the appropriate privileges; you do not have permission to change to the specified ID. @end table @end deftypefun @comment unistd.h @comment BSD @deftypefun int setreuid (uid_t @var{ruid}, uid_t @var{euid}) This function sets the real user ID of the process to @var{ruid} and the effective user ID to @var{euid}. If @var{ruid} is @code{-1}, it means not to change the real user ID; likewise if @var{euid} is @code{-1}, it means not to change the effective user ID. The @code{setreuid} function exists for compatibility with 4.3 BSD Unix, which does not support saved IDs. You can use this function to swap the effective and real user IDs of the process. (Privileged processes are not limited to this particular usage.) If saved IDs are supported, you should use that feature instead of this function. @xref{Enable/Disable Setuid}. The return value is @code{0} on success and @code{-1} on failure. The following @code{errno} error conditions are defined for this function: @table @code @item EPERM The process does not have the appropriate privileges; you do not have permission to change to the specified ID. @end table @end deftypefun @node Setting Groups, Enable/Disable Setuid, Setting User ID, Users and Groups @section Setting the Group IDs This section describes the functions for altering the group IDs (real and effective) of a process. To use these facilities, you must include the header files @file{sys/types.h} and @file{unistd.h}. @pindex unistd.h @pindex sys/types.h @comment unistd.h @comment POSIX.1 @deftypefun int setgid (gid_t @var{newgid}) This function sets both the real and effective group ID of the process to @var{newgid}, provided that the process has appropriate privileges. @c !!! also sets saved-id If the process is not privileged, then @var{newgid} must either be equal to the real group ID or the saved group ID. In this case, @code{setgid} sets only the effective group ID and not the real group ID. The return values and error conditions for @code{setgid} are the same as those for @code{setuid}. @end deftypefun @comment unistd.h @comment BSD @deftypefun int setregid (gid_t @var{rgid}, fid_t @var{egid}) This function sets the real group ID of the process to @var{rgid} and the effective group ID to @var{egid}. If @var{rgid} is @code{-1}, it means not to change the real group ID; likewise if @var{egid} is @code{-1}, it means not to change the effective group ID. The @code{setregid} function is provided for compatibility with 4.3 BSD Unix, which does not support saved IDs. You can use this function to swap the effective and real group IDs of the process. (Privileged processes are not limited to this usage.) If saved IDs are supported, you should use that feature instead of using this function. @xref{Enable/Disable Setuid}. The return values and error conditions for @code{setregid} are the same as those for @code{setreuid}. @end deftypefun The GNU system also lets privileged processes change their supplementary group IDs. To use @code{setgroups} or @code{initgroups}, your programs should include the header file @file{grp.h}. @pindex grp.h @comment grp.h @comment BSD @deftypefun int setgroups (size_t @var{count}, gid_t *@var{groups}) This function sets the process's supplementary group IDs. It can only be called from privileged processes. The @var{count} argument specifies the number of group IDs in the array @var{groups}. This function returns @code{0} if successful and @code{-1} on error. The following @code{errno} error conditions are defined for this function: @table @code @item EPERM The calling process is not privileged. @end table @end deftypefun @comment grp.h @comment BSD @deftypefun int initgroups (const char *@var{user}, gid_t @var{gid}) The @code{initgroups} function effectively calls @code{setgroups} to set the process's supplementary group IDs to be the normal default for the user name @var{user}. The group ID @var{gid} is also included. @c !!! explain that this works by reading the group file looking for @c groups USER is a member of. @end deftypefun @node Enable/Disable Setuid, Setuid Program Example, Setting Groups, Users and Groups @section Enabling and Disabling Setuid Access A typical setuid program does not need its special access all of the time. It's a good idea to turn off this access when it isn't needed, so it can't possibly give unintended access. If the system supports the saved user ID feature, you can accomplish this with @code{setuid}. When the game program starts, its real user ID is @code{jdoe}, its effective user ID is @code{games}, and its saved user ID is also @code{games}. The program should record both user ID values once at the beginning, like this: @smallexample user_user_id = getuid (); game_user_id = geteuid (); @end smallexample Then it can turn off game file access with @smallexample setuid (user_user_id); @end smallexample @noindent and turn it on with @smallexample setuid (game_user_id); @end smallexample @noindent Throughout this process, the real user ID remains @code{jdoe} and the saved user ID remains @code{games}, so the program can always set its effective user ID to either one. On other systems that don't support the saved user ID feature, you can turn setuid access on and off by using @code{setreuid} to swap the real and effective user IDs of the process, as follows: @smallexample setreuid (geteuid (), getuid ()); @end smallexample @noindent This special case is always allowed---it cannot fail. Why does this have the effect of toggling the setuid access? Suppose a game program has just started, and its real user ID is @code{jdoe} while its effective user ID is @code{games}. In this state, the game can write the scores file. If it swaps the two uids, the real becomes @code{games} and the effective becomes @code{jdoe}; now the program has only @code{jdoe} access. Another swap brings @code{games} back to the effective user ID and restores access to the scores file. In order to handle both kinds of systems, test for the saved user ID feature with a preprocessor conditional, like this: @smallexample #ifdef _POSIX_SAVED_IDS setuid (user_user_id); #else setreuid (geteuid (), getuid ()); #endif @end smallexample @node Setuid Program Example, Tips for Setuid, Enable/Disable Setuid, Users and Groups @section Setuid Program Example Here's an example showing how to set up a program that changes its effective user ID. This is part of a game program called @code{caber-toss} that manipulates a file @file{scores} that should be writable only by the game program itself. The program assumes that its executable file will be installed with the set-user-ID bit set and owned by the same user as the @file{scores} file. Typically, a system administrator will set up an account like @code{games} for this purpose. The executable file is given mode @code{4755}, so that doing an @samp{ls -l} on it produces output like: @smallexample -rwsr-xr-x 1 games 184422 Jul 30 15:17 caber-toss @end smallexample @noindent The set-user-ID bit shows up in the file modes as the @samp{s}. The scores file is given mode @code{644}, and doing an @samp{ls -l} on it shows: @smallexample -rw-r--r-- 1 games 0 Jul 31 15:33 scores @end smallexample Here are the parts of the program that show how to set up the changed user ID. This program is conditionalized so that it makes use of the saved IDs feature if it is supported, and otherwise uses @code{setreuid} to swap the effective and real user IDs. @smallexample #include #include #include #include /* @r{Save the effective and real UIDs.} */ static uid_t euid, ruid; /* @r{Restore the effective UID to its original value.} */ void do_setuid (void) @{ int status; #ifdef _POSIX_SAVED_IDS status = setuid (euid); #else status = setreuid (ruid, euid); #endif if (status < 0) @{ fprintf (stderr, "Couldn't set uid.\n"); exit (status); @} @} @group /* @r{Set the effective UID to the real UID.} */ void undo_setuid (void) @{ int status; #ifdef _POSIX_SAVED_IDS status = setuid (ruid); #else status = setreuid (euid, ruid); #endif if (status < 0) @{ fprintf (stderr, "Couldn't set uid.\n"); exit (status); @} @} @end group /* @r{Main program.} */ int main (void) @{ /* @r{Save the real and effective user IDs.} */ ruid = getuid (); euid = geteuid (); undo_setuid (); /* @r{Do the game and record the score.} */ @dots{} @} @end smallexample Notice how the first thing the @code{main} function does is to set the effective user ID back to the real user ID. This is so that any other file accesses that are performed while the user is playing the game use the real user ID for determining permissions. Only when the program needs to open the scores file does it switch back to the original effective user ID, like this: @smallexample /* @r{Record the score.} */ int record_score (int score) @{ FILE *stream; char *myname; /* @r{Open the scores file.} */ do_setuid (); stream = fopen (SCORES_FILE, "a"); undo_setuid (); @group /* @r{Write the score to the file.} */ if (stream) @{ myname = cuserid (NULL); if (score < 0) fprintf (stream, "%10s: Couldn't lift the caber.\n", myname); else fprintf (stream, "%10s: %d feet.\n", myname, score); fclose (stream); return 0; @} else return -1; @} @end group @end smallexample @node Tips for Setuid, Who Logged In, Setuid Program Example, Users and Groups @section Tips for Writing Setuid Programs It is easy for setuid programs to give the user access that isn't intended---in fact, if you want to avoid this, you need to be careful. Here are some guidelines for preventing unintended access and minimizing its consequences when it does occur: @itemize @bullet @item Don't have @code{setuid} programs with privileged user IDs such as @code{root} unless it is absolutely necessary. If the resource is specific to your particular program, it's better to define a new, nonprivileged user ID or group ID just to manage that resource. @item Be cautious about using the @code{system} and @code{exec} functions in combination with changing the effective user ID. Don't let users of your program execute arbitrary programs under a changed user ID. Executing a shell is especially bad news. Less obviously, the @code{execlp} and @code{execvp} functions are a potential risk (since the program they execute depends on the user's @code{PATH} environment variable). If you must @code{exec} another program under a changed ID, specify an absolute file name (@pxref{File Name Resolution}) for the executable, and make sure that the protections on that executable and @emph{all} containing directories are such that ordinary users cannot replace it with some other program. @item Only use the user ID controlling the resource in the part of the program that actually uses that resource. When you're finished with it, restore the effective user ID back to the actual user's user ID. @xref{Enable/Disable Setuid}. @item If the @code{setuid} part of your program needs to access other files besides the controlled resource, it should verify that the real user would ordinarily have permission to access those files. You can use the @code{access} function (@pxref{Access Permission}) to check this; it uses the real user and group IDs, rather than the effective IDs. @end itemize @node Who Logged In, User Database, Tips for Setuid, Users and Groups @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}). The @code{getlogin} function is declared in @file{unistd.h}, while @code{cuserid} and @code{L_cuserid} are declared in @file{stdio.h}. @pindex stdio.h @pindex unistd.h @comment unistd.h @comment POSIX.1 @deftypefun {char *} getlogin (void) The @code{getlogin} function returns a pointer to a string containing the name of the user logged in on the controlling terminal of the process, or a null pointer if this information cannot be determined. The string is statically allocated and might be overwritten on subsequent calls to this function or to @code{cuserid}. @end deftypefun @comment stdio.h @comment POSIX.1 @deftypefun {char *} cuserid (char *@var{string}) The @code{cuserid} function returns a pointer to a string containing a user name associated with the effective ID of the process. If @var{string} is not a null pointer, it should be an array that can hold at least @code{L_cuserid} characters; the string is returned in this array. Otherwise, a pointer to a string in a static area is returned. This string is statically allocated and might be overwritten on subsequent calls to this function or to @code{getlogin}. The use of this function is deprecated since it is marked to be withdrawn in XPG4.2 and it is already removed in POSIX.1. @end deftypefun @comment stdio.h @comment POSIX.1 @deftypevr Macro int L_cuserid An integer constant that indicates how long an array you might need to store a user name. @end deftypevr These functions let your program identify positively the user who is running or the user who logged in this session. (These can differ when setuid programs are involved; @xref{Process Persona}.) The user cannot do anything to fool these functions. For most purposes, it is more useful to use the environment variable @code{LOGNAME} to find out who the user is. This is more flexible precisely because the user can set @code{LOGNAME} arbitrarily. @xref{Standard Environment}. @node User Database, Group Database, Who Logged In, Users and Groups @section User Database @cindex user database @cindex password database @pindex /etc/passwd This section describes all about how to search and scan the database of registered users. The database itself is kept in the file @file{/etc/passwd} on most systems, but on some systems a special network server gives access to it. @menu * User Data Structure:: What each user record contains. * Lookup User:: How to look for a particular user. * Scanning All Users:: Scanning the list of all users, one by one. * Writing a User Entry:: How a program can rewrite a user's record. @end menu @node User Data Structure, Lookup User, User Database, User Database @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, Scanning All Users, User Data Structure, User Database @subsection Looking Up One User @cindex converting user ID to user name @cindex converting user name to user ID You can search the system user database for information about a specific user using @code{getpwuid} or @code{getpwnam}. These functions are declared in @file{pwd.h}. @comment pwd.h @comment POSIX.1 @deftypefun {struct passwd *} getpwuid (uid_t @var{uid}) This function returns a pointer to a statically-allocated structure containing information about the user whose user ID is @var{uid}. This structure may be overwritten on subsequent calls to @code{getpwuid}. A null pointer value indicates there is no user in the data base with user ID @var{uid}. @end deftypefun @comment pwd.h @comment POSIX.1c @deftypefun int getpwuid_r (uid_t @var{uid}, struct passwd *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct passwd **@var{result}) This function is similar to @code{getpwuid} in that is returns information about the user whose user ID is @var{uid}. But the result is not placed in a static buffer. Instead the user supplied structure pointed to by @var{result_buf} is filled with the information. The first @var{buflen} bytes of the additional buffer pointed to by @var{buffer} are used to contain additional information, normally strings which are pointed to by the elements of the result structure. If the return value is @code{0} the pointer returned in @var{result} points to the record which contains the wanted data (i.e., @var{result} contains the value @var{result_buf}). In case the return value is non null there is no user in the data base with user ID @var{uid} or the buffer @var{buffer} is too small to contain all the needed information. In the later case the global @var{errno} variable is set to @code{ERANGE}. @end deftypefun @comment pwd.h @comment POSIX.1 @deftypefun {struct passwd *} getpwnam (const char *@var{name}) This function returns a pointer to a statically-allocated structure containing information about the user whose user name is @var{name}. This structure may be overwritten on subsequent calls to @code{getpwnam}. A null pointer value indicates there is no user named @var{name}. @end deftypefun @comment pwd.h @comment POSIX.1c @deftypefun int getpwnam_r (const char *@var{name}, struct passwd *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct passwd **@var{result}) This function is similar to @code{getpwnam} in that is returns information about the user whose user name is @var{name}. But the result is not placed in a static buffer. Instead the user supplied structure pointed to by @var{result_buf} is filled with the information. The first @var{buflen} bytes of the additional buffer pointed to by @var{buffer} are used to contain additional information, normally strings which are pointed to by the elements of the result structure. If the return value is @code{0} the pointer returned in @var{result} points to the record which contains the wanted data (i.e., @var{result} contains the value @var{result_buf}). In case the return value is non null there is no user in the data base with user name @var{name} or the buffer @var{buffer} is too small to contain all the needed information. In the later case the global @var{errno} variable is set to @code{ERANGE}. @end deftypefun @node Scanning All Users, Writing a User Entry, Lookup User, User Database @subsection Scanning the List of All Users @cindex scanning the user list This section explains how a program can read the list of all users in the system, one user at a time. The functions described here are declared in @file{pwd.h}. You can use the @code{fgetpwent} function to read user entries from a particular file. @comment pwd.h @comment SVID @deftypefun {struct passwd *} fgetpwent (FILE *@var{stream}) This function reads the next user entry from @var{stream} and returns a pointer to the entry. The structure is statically allocated and is rewritten on subsequent calls to @code{fgetpwent}. You must copy the contents of the structure if you wish to save the information. This stream must correspond to a file in the same format as the standard password database file. This function comes from System V. @end deftypefun @comment pwd.h @comment GNU @deftypefun int fgetpwent_r (FILE *@var{stream}, struct passwd *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct passwd **@var{result}) This function is similar to @code{fgetpwent} in that it reads the next user entry from @var{stream}. But the result is returned in the structure pointed to by @var{result_buf}. The first @var{buflen} bytes of the additional buffer pointed to by @var{buffer} are used to contain additional information, normally strings which are pointed to by the elements of the result structure. This stream must correspond to a file in the same format as the standard password database file. If the function returns null @var{result} points to the structure with the wanted data (normally this is in @var{result_buf}). If errors occurred the return value is non-null and @var{result} contains a null pointer. @end deftypefun The way to scan all the entries in the user database is with @code{setpwent}, @code{getpwent}, and @code{endpwent}. @comment pwd.h @comment SVID, BSD @deftypefun void setpwent (void) This function initializes a stream which @code{getpwent} and @code{getpwent_r} use to read the user database. @end deftypefun @comment pwd.h @comment POSIX.1 @deftypefun {struct passwd *} getpwent (void) The @code{getpwent} function reads the next entry from the stream initialized by @code{setpwent}. It returns a pointer to the entry. The structure is statically allocated and is rewritten on subsequent calls to @code{getpwent}. You must copy the contents of the structure if you wish to save the information. A null pointer is returned in case no further entry is available. @end deftypefun @comment pwd.h @comment GNU @deftypefun int getpwent_r (struct passwd *@var{result_buf}, char *@var{buffer}, int @var{buflen}, struct passwd **@var{result}) This function is similar to @code{getpwent} in that it returns the next entry from the stream initialized by @code{setpwent}. But in contrast to the @code{getpwent} function this function is reentrant since the result is placed in the user supplied structure pointed to by @var{result_buf}. Additional data, normally the strings pointed to by the elements of the result structure, are placed in the additional buffer or length @var{buflen} starting at @var{buffer}. If the function returns zero @var{result} points to the structure with the wanted data (normally this is in @var{result_buf}). If errors occurred the return value is non-zero and @var{result} contains a null pointer. @end deftypefun @comment pwd.h @comment SVID, BSD @deftypefun void endpwent (void) This function closes the internal stream used by @code{getpwent} or @code{getpwent_r}. @end deftypefun @node Writing a User Entry, , Scanning All Users, User Database @subsection Writing a User Entry @comment pwd.h @comment SVID @deftypefun int putpwent (const struct passwd *@var{p}, FILE *@var{stream}) This function writes the user entry @code{*@var{p}} to the stream @var{stream}, in the format used for the standard user database file. The return value is zero on success and nonzero on failure. This function exists for compatibility with SVID. We recommend that you avoid using it, because it makes sense only on the assumption that the @code{struct passwd} structure has no members except the standard ones; on a system which merges the traditional Unix data base with other extended information about users, adding an entry using this function would inevitably leave out much of the important information. The function @code{putpwent} is declared in @file{pwd.h}. @end deftypefun @node Group Database, Netgroup Database, User Database, Users and Groups @section Group Database @cindex group database @pindex /etc/group This section describes all about how to search and scan the database of registered groups. The database itself is kept in the file @file{/etc/group} on most systems, but on some systems a special network service provides access to it. @menu * Group Data Structure:: What each group record contains. * Lookup Group:: How to look for a particular group. * Scanning All Groups:: Scanning the list of all groups. @end menu @node Group Data Structure, Lookup Group, Group Database, Group Database @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, Scanning All Groups, Group Data Structure, Group Database @subsection Looking Up One Group @cindex converting group name to group ID @cindex converting group ID to group name You can search the group database for information about a specific group using @code{getgrgid} or @code{getgrnam}. These functions are declared in @file{grp.h}. @comment grp.h @comment POSIX.1 @deftypefun {struct group *} getgrgid (gid_t @var{gid}) This function returns a pointer to a statically-allocated structure containing information about the group whose group ID is @var{gid}. This structure may be overwritten by subsequent calls to @code{getgrgid}. A null pointer indicates there is no group with ID @var{gid}. @end deftypefun @comment grp.h @comment POSIX.1c @deftypefun int getgrgid_r (gid_t @var{gid}, struct group *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct group **@var{result}) This function is similar to @code{getgrgid} in that is returns information about the group whose group ID is @var{gid}. But the result is not placed in a static buffer. Instead the user supplied structure pointed to by @var{result_buf} is filled with the information. The first @var{buflen} bytes of the additional buffer pointed to by @var{buffer} are used to contain additional information, normally strings which are pointed to by the elements of the result structure. If the return value is @code{0} the pointer returned in @var{result} points to the record which contains the wanted data (i.e., @var{result} contains the value @var{result_buf}). If the return value is non-zero there is no group in the data base with group ID @var{gid} or the buffer @var{buffer} is too small to contain all the needed information. In the later case the global @var{errno} variable is set to @code{ERANGE}. @end deftypefun @comment grp.h @comment SVID, BSD @deftypefun {struct group *} getgrnam (const char *@var{name}) This function returns a pointer to a statically-allocated structure containing information about the group whose group name is @var{name}. This structure may be overwritten by subsequent calls to @code{getgrnam}. A null pointer indicates there is no group named @var{name}. @end deftypefun @comment grp.h @comment POSIX.1c @deftypefun int getgrnam_r (const char *@var{name}, struct group *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct group **@var{result}) This function is similar to @code{getgrnam} in that is returns information about the group whose group name is @var{name}. But the result is not placed in a static buffer. Instead the user supplied structure pointed to by @var{result_buf} is filled with the information. The first @var{buflen} bytes of the additional buffer pointed to by @var{buffer} are used to contain additional information, normally strings which are pointed to by the elements of the result structure. If the return value is @code{0} the pointer returned in @var{result} points to the record which contains the wanted data (i.e., @var{result} contains the value @var{result_buf}). If the return value is non-zero there is no group in the data base with group name @var{name} or the buffer @var{buffer} is too small to contain all the needed information. In the later case the global @var{errno} variable is set to @code{ERANGE}. @end deftypefun @node Scanning All Groups, , Lookup Group, Group Database @subsection Scanning the List of All Groups @cindex scanning the group list This section explains how a program can read the list of all groups in the system, one group at a time. The functions described here are declared in @file{grp.h}. You can use the @code{fgetgrent} function to read group entries from a particular file. @comment grp.h @comment SVID @deftypefun {struct group *} fgetgrent (FILE *@var{stream}) The @code{fgetgrent} function reads the next entry from @var{stream}. It returns a pointer to the entry. The structure is statically allocated and is rewritten on subsequent calls to @code{fgetgrent}. You must copy the contents of the structure if you wish to save the information. The stream must correspond to a file in the same format as the standard group database file. @end deftypefun @comment grp.h @comment GNU @deftypefun int fgetgrent_r (FILE *@var{stream}, struct group *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct group **@var{result}) This function is similar to @code{fgetgrent} in that it reads the next user entry from @var{stream}. But the result is returned in the structure pointed to by @var{result_buf}. The first @var{buflen} bytes of the additional buffer pointed to by @var{buffer} are used to contain additional information, normally strings which are pointed to by the elements of the result structure. This stream must correspond to a file in the same format as the standard group database file. If the function returns zero @var{result} points to the structure with the wanted data (normally this is in @var{result_buf}). If errors occurred the return value is non-zero and @var{result} contains a null pointer. @end deftypefun The way to scan all the entries in the group database is with @code{setgrent}, @code{getgrent}, and @code{endgrent}. @comment grp.h @comment SVID, BSD @deftypefun void setgrent (void) This function initializes a stream for reading from the group data base. You use this stream by calling @code{getgrent} or @code{getgrent_r}. @end deftypefun @comment grp.h @comment SVID, BSD @deftypefun {struct group *} getgrent (void) The @code{getgrent} function reads the next entry from the stream initialized by @code{setgrent}. It returns a pointer to the entry. The structure is statically allocated and is rewritten on subsequent calls to @code{getgrent}. You must copy the contents of the structure if you wish to save the information. @end deftypefun @comment grp.h @comment GNU @deftypefun int getgrent_r (struct group *@var{result_buf}, char *@var{buffer}, size_t @var{buflen}, struct group **@var{result}) This function is similar to @code{getgrent} in that it returns the next entry from the stream initialized by @code{setgrent}. But in contrast to the @code{getgrent} function this function is reentrant since the result is placed in the user supplied structure pointed to by @var{result_buf}. Additional data, normally the strings pointed to by the elements of the result structure, are placed in the additional buffer or length @var{buflen} starting at @var{buffer}. If the function returns zero @var{result} points to the structure with the wanted data (normally this is in @var{result_buf}). If errors occurred the return value is non-zero and @var{result} contains a null pointer. @end deftypefun @comment grp.h @comment SVID, BSD @deftypefun void endgrent (void) This function closes the internal stream used by @code{getgrent} or @code{getgrent_r}. @end deftypefun @node Netgroup Database, Database Example, Group Database, Users and Groups @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, Lookup Netgroup, Netgroup Database, Netgroup Database @subsection Netgroup Data @cindex Netgroup Sometimes it is useful group users according to other criterias like the ones used in the @xref{Group Database}. E.g., it is useful to associate a certain group of users with a certain machine. On the other hand grouping of host names is not supported so far. In Sun Microsystems SunOS appeared a new kind of database, the netgroup database. It allows to group hosts, users, and domain freely, giving them individual names. More concrete: a netgroup is a list of triples consisting of a host name, a user name, and a domain name, where any of the entries can be a wildcard entry, matching all inputs. A last possibility is that names of other netgroups can also be given in the list specifying a netgroup. So one can construct arbitrary hierarchies without loops. Sun's implementation allows netgroups only for the @code{nis} or @code{nisplus} service @pxref{Services in the NSS configuration}. The implementation in the GNU C library has no such restriction. An entry in either of the input services must have the following form: @smallexample @var{groupname} ( @var{groupname} | @code{(}@var{hostname}@code{,}@var{username}@code{,}@code{domainname}@code{)} )+ @end smallexample Any of the fields in the triple can be empty which means anything matches. While describing the functions we will see that the opposite case is useful as well. I.e., there may be entries which will not match any input. For entries like a name consisting of the single character @code{-} shall be used. @node Lookup Netgroup, Netgroup Membership, Netgroup Data, Netgroup Database @subsection Looking up one Netgroup The lookup functions for netgroups are a bit different to all other system database handling functions. Since a single netgroup can contain many entries a two-step process is needed. First a single netgroup is selected and then one can iterate over all entries in this netgroup. These functions are declared in @file{netdb.h}. @comment netdb.h @deftypefun int setnetgrent (const char *@var{netgroup}) A call to this function initializes the internal state of the library to allow following calls of the @code{getnetgrent} iterate over all entries in the netgroup with name @var{netgroup}. When the call is successful (i.e., when a netgroup with this name exist) the return value is @code{1}. When the return value is @code{0} no netgroup of this name is known or some other error occurred. @end deftypefun It is important to remember that there is only one single state for iterating the netgroups. Even if the programmer uses the @code{getnetgrent_r} function the result is not really reentrant since always only one single netgroup at a time can be processed. If the program needs to process more than one netgroup simultaneously she must protect this by using external locking. This problem was introduced in the original netgroups implementation in SunOS and since we must stay compatible it is not possible to change this. Some other functions also use the netgroups state. Currently these are the @code{innetgr} function and parts of the implementation of the @code{compat} service part of the NSS implementation. @comment netdb.h @deftypefun int getnetgrent (char **@var{hostp}, char **@var{userp}, char **@var{domainp}) This function returns the next unprocessed entry of the currently selected netgroup. The string pointers, which addresses are passed in the arguments @var{hostp}, @var{userp}, and @var{domainp}, will contain after a successful call pointers to appropriate strings. If the string in the next entry is empty the pointer has the value @code{NULL}. The returned string pointers are only valid unless no of the netgroup related functions are called. The return value is @code{1} if the next entry was successfully read. A value of @code{0} means no further entries exist or internal errors occurred. @end deftypefun @comment netdb.h @deftypefun int getnetgrent_r (char **@var{hostp}, char **@var{userp}, char **@var{domainp}, char *@var{buffer}, int @var{buflen}) This function is similar to @code{getnetgrent} with only one exception: the strings the three string pointers @var{hostp}, @var{userp}, and @var{domainp} point to, are placed in the buffer of @var{buflen} bytes starting at @var{buffer}. This means the returned values are valid even after other netgroup related functions are called. The return value is @code{1} if the next entry was successfully read and the buffer contains enough room to place the strings in it. @code{0} is returned in case no more entries are found, the buffer is too small, or internal errors occurred. This function is a GNU extension. The original implementation in the SunOS libc does not provide this function. @end deftypefun @comment netdb.h @deftypefun void endnetgrent (void) This function free all buffers which were allocated to process the last selected netgroup. As a result all string pointers returned by calls to @code{getnetgrent} are invalid afterwards. @end deftypefun @node Netgroup Membership, , Lookup Netgroup, Netgroup Database @subsection Testing for Netgroup Membership It is often not necessary to scan the whole netgroup since often the only interesting question is whether a given entry is part of the selected netgroup. @comment netdb.h @deftypefun int innetgr (const char *@var{netgroup}, const char *@var{host}, const char *@var{user}, const char *@var{domain}) This function tests whether the triple specified by the parameters @var{hostp}, @var{userp}, and @var{domainp} is part of the netgroup @var{netgroup}. Using this function has the advantage that @enumerate @item no other netgroup function can use the global netgroup state since internal locking is used and @item the function is implemented more efficiently than successive calls to the other @code{set}/@code{get}/@code{endnetgrent} functions. @end enumerate Any of the pointers @var{hostp}, @var{userp}, and @var{domainp} can be @code{NULL} which means any value is excepted in this position. This is also true for the name @code{-} which should not match any other string otherwise. The return value is @code{1} if an entry matching the given triple is found in the netgroup. The return value is @code{0} if the netgroup itself is not found, the netgroup does not contain the triple or internal errors occurred. @end deftypefun @node Database Example, , Netgroup Database, Users and Groups @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