Every user who can log in on the system is identified by a unique number called the user ID. Each process has an effective user ID which says which user's access permissions it has.
Users are classified into groups for access control purposes. Each process has one or more group ID values which say which groups the process can use for access to files.
The effective user and group IDs of a process collectively form its persona. This determines which files the process can access. Normally, a process inherits its persona from the parent process, but under special circumstances a process can change its persona and thus change its access permissions.
Each file in the system also has a user ID and a group ID. Access control works by comparing the user and group IDs of the file with those of the running process.
The system keeps a database of all the registered users, and another database of all the defined groups. There are library functions you can use to examine these databases.
Each user account on a computer system is identified by a user name (or login name) and user ID. Normally, each user name has a unique user ID, but it is possible for several login names to have the same user ID. The user names and corresponding user IDs are stored in a data base which you can access as described in section User Database.
Users are classified in groups. Each user name belongs to one default group and may also belong to any number of 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 group name and group ID. See section Group Database, for how to find information about a group ID or group name.
At any time, each process has an effective user ID, a effective group ID, and a set of supplementary group IDs. These IDs determine the privileges of the process. They are collectively called the persona of the process, because they determine "who it is" for purposes of access control.
Your login shell starts out with a persona which consists of your user ID, your default group ID, and your supplementary group IDs (if you are in more than one group). In normal circumstances, all your other processes inherit these values.
A process also has a real user ID which identifies the user who created the process, and a real group ID which identifies that user's default group. These values do not play a role in access control, so we do not consider them part of the persona. But they are also important.
Both the real and effective user ID can be changed during the lifetime of a process. See section Why Change the Persona of a Process?.
For details on how a process's effective user ID and group IDs affect its permission to access files, see section How Your Access to a File is Decided.
The effective user ID of a process also controls permissions for sending
signals using the kill function. See section Signaling Another Process.
Finally, there are many operations which can only be performed by a
process whose effective user ID is zero. A process with this user ID is
a privileged process. Commonly the user name root is
associated with user ID 0, but there may be other user names with this
ID.
The most obvious situation where it is necessary for a process to change
its user and/or group IDs is the login program. When
login starts running, its user ID is root. Its job is to
start a shell whose user and group IDs are those of the user who is
logging in. (To accomplish this fully, login must set the real
user and group IDs as well as its persona. But this is a special case.)
The more common case of changing persona is when an ordinary user program needs access to a resource that wouldn't ordinarily be accessible to the user actually running it.
For example, you may have a file that is controlled by your program but that shouldn't be read or modified directly by other users, either because it implements some kind of locking protocol, or because you want to preserve the integrity or privacy of the information it contains. This kind of restricted access can be implemented by having the program change its effective user or group ID to match that of the resource.
Thus, imagine a game program that saves scores in a file. The game
program itself needs to be able to update this file no matter who is
running it, but if users can write the file without going through the
game, they can give themselves any scores they like. Some people
consider this undesirable, or even reprehensible. It can be prevented
by creating a new user ID and login name (say, games) to own the
scores file, and make the file writable only by this user. Then, when
the game program wants to update this file, it can change its effective
user ID to be that for games. In effect, the program must
adopt the persona of games so it can write the scores file.
The ability to change the persona of a process can be a source of unintentional privacy violations, or even intentional abuse. Because of the potential for problems, changing persona is restricted to special circumstances.
You can't arbitrarily set your user ID or group ID to anything you want; only privileged processes can do that. Instead, the normal way for a program to change its persona is that it has been set up in advance to change to a particular user or group. This is the function of the setuid and setgid bits of a file's access mode. See section The Mode Bits for Access Permission.
When the setuid bit of an executable file is on, executing that file gives the process a third user ID: the file user ID. This ID is set to the owner ID of the file. The system then changes the effective user ID to the file user ID. The real user ID remains as it was. Likewise, if the setgid bit is on, the process is given a file group ID equal to the group ID of the file, and its effective group ID is changed to the file group ID.
If a process has a file ID (user or group), then it can at any time change its effective ID to its real ID and back to its file ID. Programs use this feature to relinquish their special privileges except when they actually need them. This makes it less likely that they can be tricked into doing something inappropriate with their privileges.
Portability Note: Older systems do not have file IDs.
To determine if a system has this feature, you can test the compiler
define _POSIX_SAVED_IDS. (In the POSIX standard, file IDs are
known as saved IDs.)
See section File Attributes, for a more general discussion of file modes and accessibility.
Here are detailed descriptions of the functions for reading the user and group IDs of a process, both real and effective. To use these facilities, you must include the header files `sys/types.h' and `unistd.h'.
unsigned int.
unsigned int.
getuid function returns the real user ID of the process.
getgid function returns the real group ID of the process.
geteuid function returns the effective user ID of the process.
getegid function returns the effective group ID of the process.
getgroups function is used to inquire about the supplementary
group IDs of the process. Up to count of these group IDs are
stored in the array groups; the return value from the function is
the number of group IDs actually stored. If count is smaller than
the total number of supplementary group IDs, then getgroups
returns a value of -1 and errno is set to EINVAL.
If count is zero, then getgroups just returns the total
number of supplementary group IDs. On systems that do not support
supplementary groups, this will always be zero.
Here's how to use getgroups to read all the supplementary group
IDs:
gid_t *
read_all_groups (void)
{
int ngroups = getgroups (0, NULL);
gid_t *groups
= (gid_t *) xmalloc (ngroups * sizeof (gid_t));
int val = getgroups (ngroups, groups);
if (val < 0)
{
free (groups);
return NULL;
}
return groups;
}
This section describes the functions for altering the user ID (real and/or effective) of a process. To use these facilities, you must include the header files `sys/types.h' and `unistd.h'.
The seteuid function returns a value of 0 to indicate
successful completion, and a value of -1 to indicate an error.
The following errno error conditions are defined for this
function:
EINVAL
EPERM
Older systems (those without the _POSIX_SAVED_IDS feature) do not
have this function.
If the process is not privileged, and the system supports the
_POSIX_SAVED_IDS feature, then this function behaves like
seteuid.
The return values and error conditions are the same as for seteuid.
-1, it means
not to change the real user ID; likewise if euid is -1, it
means not to change the effective user ID.
The setreuid function exists for compatibility with 4.3 BSD Unix,
which does not support file IDs. You can use this function to swap the
effective and real user IDs of the process. (Privileged processes are
not limited to this particular usage.) If file IDs are supported, you
should use that feature instead of this function. See section Enabling and Disabling Setuid Access.
The return value is 0 on success and -1 on failure.
The following errno error conditions are defined for this
function:
EPERM
This section describes the functions for altering the group IDs (real and effective) of a process. To use these facilities, you must include the header files `sys/types.h' and `unistd.h'.
seteuid, if the process is privileged it may
change its effective group ID to any value; if it isn't, but it has a
file group ID, then it may change to its real group ID or file group ID;
otherwise it may not change its effective group ID.
Note that a process is only privileged if its effective user ID is zero. The effective group ID only affects access permissions.
The return values and error conditions for setegid are the same
as those for seteuid.
This function is only present if _POSIX_SAVED_IDS is defined.
If the process is not privileged, then setgid behaves like
setegid.
The return values and error conditions for setgid are the same
as those for seteuid.
-1, it
means not to change the real group ID; likewise if egid is
-1, it means not to change the effective group ID.
The setregid function is provided for compatibility with 4.3 BSD
Unix, which does not support file IDs. You can use this function to
swap the effective and real group IDs of the process. (Privileged
processes are not limited to this usage.) If file IDs are supported,
you should use that feature instead of using this function.
See section Enabling and Disabling Setuid Access.
The return values and error conditions for setregid are the same
as those for setreuid.
setuid and setgid behave differently depending on whether
the effective user ID at the time is zero. If it is not zero, they
behave like seteuid and setegid. If it is, they change
both effective and real IDs and delete the file ID. To avoid confusion,
we recommend you always use seteuid and setegid except
when you know the effective user ID is zero and your intent is to change
the persona permanently. This case is rare--most of the programs that
need it, such as login and su, have already been written.
Note that if your program is setuid to some user other than root,
there is no way to drop privileges permanently.
The system also lets privileged processes change their supplementary
group IDs. To use setgroups or initgroups, your programs
should include the header file `grp.h'.
This function returns 0 if successful and -1 on error.
The following errno error conditions are defined for this
function:
EPERM
initgroups function sets the process's supplementary group
IDs to be the normal default for the user name user. If gid
is not -1, it includes that group also.
This function works by scanning the group database for all the groups
user belongs to. It then calls setgroups with the list it
has constructed.
The return values and error conditions are the same as for
setgroups.
A typical setuid program does not need its special access all of the time. It's a good idea to turn off this access when it isn't needed, so it can't possibly give unintended access.
If the system supports the _POSIX_SAVED_IDS feature, you can
accomplish this with seteuid. When the game program starts, its
real user ID is jdoe, its effective user ID is games, and
its saved user ID is also games. The program should record both
user ID values once at the beginning, like this:
user_user_id = getuid (); game_user_id = geteuid ();
Then it can turn off game file access with
seteuid (user_user_id);
and turn it on with
seteuid (game_user_id);
Throughout this process, the real user ID remains jdoe and the
file user ID remains games, so the program can always set its
effective user ID to either one.
On other systems that don't support file user IDs, you can
turn setuid access on and off by using setreuid to swap the real
and effective user IDs of the process, as follows:
setreuid (geteuid (), getuid ());
This special case is always allowed--it cannot fail.
Why does this have the effect of toggling the setuid access? Suppose a
game program has just started, and its real user ID is jdoe while
its effective user ID is games. In this state, the game can
write the scores file. If it swaps the two uids, the real becomes
games and the effective becomes jdoe; now the program has
only jdoe access. Another swap brings games back to
the effective user ID and restores access to the scores file.
In order to handle both kinds of systems, test for the saved user ID feature with a preprocessor conditional, like this:
#ifdef _POSIX_SAVED_IDS setuid (user_user_id); #else setreuid (geteuid (), getuid ()); #endif
Here's an example showing how to set up a program that changes its effective user ID.
This is part of a game program called caber-toss that manipulates
a file `scores' that should be writable only by the game program
itself. The program assumes that its executable file will be installed
with the setuid bit set and owned by the same user as the `scores'
file. Typically, a system administrator will set up an account like
games for this purpose.
The executable file is given mode 4755, so that doing an
`ls -l' on it produces output like:
-rwsr-xr-x 1 games 184422 Jul 30 15:17 caber-toss
The setuid bit shows up in the file modes as the `s'.
The scores file is given mode 644, and doing an `ls -l' on
it shows:
-rw-r--r-- 1 games 0 Jul 31 15:33 scores
Here are the parts of the program that show how to set up the changed
user ID. This program is conditionalized so that it makes use of the
file IDs feature if it is supported, and otherwise uses setreuid
to swap the effective and real user IDs.
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
/* Remember the effective and real UIDs. */
static uid_t euid, ruid;
/* Restore the effective UID to its original value. */
void
do_setuid (void)
{
int status;
#ifdef _POSIX_SAVED_IDS
status = seteuid (euid);
#else
status = setreuid (ruid, euid);
#endif
if (status < 0) {
fprintf (stderr, "Couldn't set uid.\n");
exit (status);
}
}
/* Set the effective UID to the real UID. */
void
undo_setuid (void)
{
int status;
#ifdef _POSIX_SAVED_IDS
status = seteuid (ruid);
#else
status = setreuid (euid, ruid);
#endif
if (status < 0) {
fprintf (stderr, "Couldn't set uid.\n");
exit (status);
}
}
/* Main program. */
int
main (void)
{
/* Remember the real and effective user IDs. */
ruid = getuid ();
euid = geteuid ();
undo_setuid ();
/* Do the game and record the score. */
...
}
Notice how the first thing the main function does is to set the
effective user ID back to the real user ID. This is so that any other
file accesses that are performed while the user is playing the game use
the real user ID for determining permissions. Only when the program
needs to open the scores file does it switch back to the file user ID,
like this:
/* Record the score. */
int
record_score (int score)
{
FILE *stream;
char *myname;
/* Open the scores file. */
do_setuid ();
stream = fopen (SCORES_FILE, "a");
undo_setuid ();
/* Write the score to the file. */
if (stream)
{
myname = cuserid (NULL);
if (score < 0)
fprintf (stream, "%10s: Couldn't lift the caber.\n", myname);
else
fprintf (stream, "%10s: %d feet.\n", myname, score);
fclose (stream);
return 0;
}
else
return -1;
}
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:
setuid programs with privileged user IDs such as
root unless it is absolutely necessary. If the resource is
specific to your particular program, it's better to define a new,
nonprivileged user ID or group ID just to manage that resource.
It's better if you can write your program to use a special group than a
special user.
exec functions in combination with
changing the effective user ID. Don't let users of your program execute
arbitrary programs under a changed user ID. Executing a shell is
especially bad news. Less obviously, the execlp and execvp
functions are a potential risk (since the program they execute depends
on the user's PATH environment variable).
If you must exec another program under a changed ID, specify an
absolute file name (see section File Name Resolution) for the executable,
and make sure that the protections on that executable and all
containing directories are such that ordinary users cannot replace it
with some other program.
You should also check the arguments passed to the program to make sure
they do not have unexpected effects. Likewise, you should examine the
environment variables. Decide which arguments and variables are safe,
and reject all others.
You should never use system in a privileged program, because it
invokes a shell.
setuid part of your program needs to access other files
besides the controlled resource, it should verify that the real user
would ordinarily have permission to access those files. You can use the
access function (see section How Your Access to a File is Decided) to check this; it
uses the real user and group IDs, rather than the effective IDs.
You can use the functions listed in this section to determine the login
name of the user who is running a process, and the name of the user who
logged in the current session. See also the function getuid and
friends (see section Reading the Persona of a Process). How this information is collected by
the system and how to control/add/remove information from the background
storage is described in section The User Accounting Database.
The getlogin function is declared in `unistd.h', while
cuserid and L_cuserid are declared in `stdio.h'.
getlogin function returns a pointer to a string containing the
name of the user logged in on the controlling terminal of the process,
or a null pointer if this information cannot be determined. The string
is statically allocated and might be overwritten on subsequent calls to
this function or to cuserid.
cuserid function returns a pointer to a string containing a
user name associated with the effective ID of the process. If
string is not a null pointer, it should be an array that can hold
at least L_cuserid characters; the string is returned in this
array. Otherwise, a pointer to a string in a static area is returned.
This string is statically allocated and might be overwritten on
subsequent calls to this function or to getlogin.
The use of this function is deprecated since it is marked to be withdrawn in XPG4.2 and has already been removed from newer revisions of POSIX.1.
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 section The Persona of a Process.) The user cannot do anything to fool these functions.
For most purposes, it is more useful to use the environment variable
LOGNAME to find out who the user is. This is more flexible
precisely because the user can set LOGNAME arbitrarily.
See section Standard Environment Variables.
Most Unix-like operating systems keep track of logged in users by maintaining a user accounting database. This user accounting database stores for each terminal, who has logged on, at what time, the process ID of the user's login shell, etc., etc., but also stores information about the run level of the system, the time of the last system reboot, and possibly more.
The user accounting database typically lives in `/etc/utmp', `/var/adm/utmp' or `/var/run/utmp'. However, these files should never be accessed directly. For reading information from and writing information to the user accounting database, the functions described in this section should be used.
These functions and the corresponding data structures are declared in the header file `utmp.h'.
exit_status data structure is used to hold information about
the exit status of processes marked as DEAD_PROCESS in the user
accounting database.
short int e_termination
short int e_exit
utmp data structure is used to hold information about entries
in the user accounting database. On the GNU system it has the following
members:
short int ut_type
EMPTY, RUN_LVL,
BOOT_TIME, OLD_TIME, NEW_TIME, INIT_PROCESS,
LOGIN_PROCESS, USER_PROCESS, DEAD_PROCESS or
ACCOUNTING.
pid_t ut_pid
char ut_line[]
char ut_id[]
char ut_user[]
char ut_host[]
struct exit_status ut_exit
DEAD_PROCESS.
long ut_session
struct timeval ut_tv
OLD_TIME this is
the time when the system clock changed, and for entries of type
NEW_TIME this is the time the system clock was set to.
int32_t ut_addr_v6[4]
The ut_type, ut_pid, ut_id, ut_tv, and
ut_host fields are not available on all systems. Portable
applications therefore should be prepared for these situations. To help
doing this the `utmp.h' header provides macros
_HAVE_UT_TYPE, _HAVE_UT_PID, _HAVE_UT_ID,
_HAVE_UT_TV, and _HAVE_UT_HOST if the respective field is
available. The programmer can handle the situations by using
#ifdef in the program code.
The following macros are defined for use as values for the
ut_type member of the utmp structure. The values are
integer constants.
EMPTY
RUN_LVL
BOOT_TIME
OLD_TIME
NEW_TIME
INIT_PROCESS
LOGIN_PROCESS
USER_PROCESS
DEAD_PROCESS
ACCOUNTING
The size of the ut_line, ut_id, ut_user and
ut_host arrays can be found using the sizeof operator.
Many older systems have, instead of an ut_tv member, an
ut_time member, usually of type time_t, for representing
the time associated with the entry. Therefore, for backwards
compatibility only, `utmp.h' defines ut_time as an alias for
ut_tv.tv_sec.
getutent, getutid or getutline to
read entries and pututline to write entries.
If the database is already open, it resets the input to the beginning of the database.
getutent function reads the next entry from the user
accounting database. It returns a pointer to the entry, which is
statically allocated and may be overwritten by subsequent calls to
getutent. You must copy the contents of the structure if you
wish to save the information or you can use the getutent_r
function which stores the data in a user-provided buffer.
A null pointer is returned in case no further entry is available.
ut_type member of the
id structure is one of RUN_LVL, BOOT_TIME,
OLD_TIME or NEW_TIME the entries match if the
ut_type members are identical. If the ut_type member of
the id structure is INIT_PROCESS, LOGIN_PROCESS,
USER_PROCESS or DEAD_PROCESS, the entries match if the
ut_type member of the entry read from the database is one of
these four, and the ut_id members match. However if the
ut_id member of either the id structure or the entry read
from the database is empty it checks if the ut_line members match
instead. If a matching entry is found, getutid returns a pointer
to the entry, which is statically allocated, and may be overwritten by a
subsequent call to getutent, getutid or getutline.
You must copy the contents of the structure if you wish to save the
information.
A null pointer is returned in case the end of the database is reached without a match.
The getutid function may cache the last read entry. Therefore,
if you are using getutid to search for multiple occurrences, it
is necessary to zero out the static data after each call. Otherwise
getutid could just return a pointer to the same entry over and
over again.
ut_type value is
LOGIN_PROCESS or USER_PROCESS, and whose ut_line
member matches the ut_line member of the line structure.
If it finds such an entry, it returns a pointer to the entry which is
statically allocated, and may be overwritten by a subsequent call to
getutent, getutid or getutline. You must copy the
contents of the structure if you wish to save the information.
A null pointer is returned in case the end of the database is reached without a match.
The getutline function may cache the last read entry. Therefore
if you are using getutline to search for multiple occurrences, it
is necessary to zero out the static data after each call. Otherwise
getutline could just return a pointer to the same entry over and
over again.
pututline function inserts the entry *utmp at
the appropriate place in the user accounting database. If it finds that
it is not already at the correct place in the database, it uses
getutid to search for the position to insert the entry, however
this will not modify the static structure returned by getutent,
getutid and getutline. If this search fails, the entry
is appended to the database.
The pututline function returns a pointer to a copy of the entry
inserted in the user accounting database, or a null pointer if the entry
could not be added. The following errno error conditions are
defined for this function:
EPERM
All the get* functions mentioned before store the information
they return in a static buffer. This can be a problem in multi-threaded
programs since the data returned for the request is overwritten by the
return value data in another thread. Therefore the GNU C Library
provides as extensions three more functions which return the data in a
user-provided buffer.
getutent_r is equivalent to the getutent function. It
returns the next entry from the database. But instead of storing the
information in a static buffer it stores it in the buffer pointed to by
the parameter buffer.
If the call was successful, the function returns 0 and the
pointer variable pointed to by the parameter result contains a
pointer to the buffer which contains the result (this is most probably
the same value as buffer). If something went wrong during the
execution of getutent_r the function returns -1.
This function is a GNU extension.
getutid the next entry matching
the information stored in id. But the result is stored in the
buffer pointed to by the parameter buffer.
If successful the function returns 0 and the pointer variable
pointed to by the parameter result contains a pointer to the
buffer with the result (probably the same as result. If not
successful the function return -1.
This function is a GNU extension.
getutline the next entry
matching the information stored in line. But the result is stored
in the buffer pointed to by the parameter buffer.
If successful the function returns 0 and the pointer variable
pointed to by the parameter result contains a pointer to the
buffer with the result (probably the same as result. If not
successful the function return -1.
This function is a GNU extension.
In addition to the user accounting database, most systems keep a number of similar databases. For example most systems keep a log file with all previous logins (usually in `/etc/wtmp' or `/var/log/wtmp').
For specifying which database to examine, the following function should be used.
utmpname function changes the name of the database to be
examined to file, and closes any previously opened database. By
default getutent, getutid, getutline and
pututline read from and write to the user accounting database.
The following macros are defined for use as the file argument:
The utmpname function returns a value of 0 if the new name
was successfully stored, and a value of -1 to indicate an error.
Note that utmpname does not try to open the database, and that
therefore the return value does not say anything about whether the
database can be successfully opened.
Specially for maintaining log-like databases the GNU C Library provides the following function:
updwtmp function appends the entry *utmp to the
database specified by wtmp_file. For possible values for the
wtmp_file argument see the utmpname function.
Portability Note: Although many operating systems provide a
subset of these functions, they are not standardized. There are often
subtle differences in the return types, and there are considerable
differences between the various definitions of struct utmp. When
programming for the GNU system, it is probably best to stick
with the functions described in this section. If however, you want your
program to be portable, consider using the XPG functions described in
section XPG User Accounting Database Functions, or take a look at the BSD compatible functions in
section Logging In and Out.
These functions, described in the X/Open Portability Guide, are declared in the header file `utmpx.h'.
utmpx data structure contains at least the following members:
short int ut_type
EMPTY, RUN_LVL,
BOOT_TIME, OLD_TIME, NEW_TIME, INIT_PROCESS,
LOGIN_PROCESS, USER_PROCESS or DEAD_PROCESS.
pid_t ut_pid
char ut_line[]
char ut_id[]
char ut_user[]
struct timeval ut_tv
OLD_TIME this is
the time when the system clock changed, and for entries of type
NEW_TIME this is the time the system clock was set to.
On the GNU system, struct utmpx is identical to struct
utmp except for the fact that including `utmpx.h' does not make
visible the declaration of struct exit_status.
The following macros are defined for use as values for the
ut_type member of the utmpx structure. The values are
integer constants and are, on the GNU system, identical to the
definitions in `utmp.h'.
EMPTY
RUN_LVL
BOOT_TIME
OLD_TIME
NEW_TIME
INIT_PROCESS
LOGIN_PROCESS
USER_PROCESS
DEAD_PROCESS
The size of the ut_line, ut_id and ut_user arrays
can be found using the sizeof operator.
setutent. On the GNU system it is
simply an alias for setutent.
getutxent function is similar to getutent, but returns
a pointer to a struct utmpx instead of struct utmp. On
the GNU system it simply is an alias for getutent.
endutent. On the GNU system it is
simply an alias for endutent.
getutid, but uses struct utmpx
instead of struct utmp. On the GNU system it is simply an alias
for getutid.
getutid, but uses struct utmpx
instead of struct utmp. On the GNU system it is simply an alias
for getutline.
pututxline function is functionally identical to
pututline, but uses struct utmpx instead of struct
utmp. On the GNU system, pututxline is simply an alias for
pututline.
utmpxname function is functionally identical to
utmpname. On the GNU system, utmpxname is simply an
alias for utmpname.
You can translate between a traditional struct utmp and an XPG
struct utmpx with the following functions. On the GNU system,
these functions are merely copies, since the two structures are
identical.
getutmp copies the information, insofar as the structures are
compatible, from utmpx to utmp.
getutmpx copies the information, insofar as the structures are
compatible, from utmp to utmpx.
These functions, derived from BSD, are available in the separate `libutil' library, and declared in `utmp.h'.
Note that the ut_user member of struct utmp is called
ut_name in BSD. Therefore, ut_name is defined as an alias
for ut_user in `utmp.h'.
This function returns 0 on successful completion, and -1
on error.
login functions inserts an entry into the user accounting
database. The ut_line member is set to the name of the terminal
on standard input. If standard input is not a terminal login
uses standard output or standard error output to determine the name of
the terminal. If struct utmp has a ut_type member,
login sets it to USER_PROCESS, and if there is an
ut_pid member, it will be set to the process ID of the current
process. The remaining entries are copied from entry.
A copy of the entry is written to the user accounting log file.
The logout function returns 1 if the entry was successfully
written to the database, or 0 on error.
logwtmp function appends an entry to the user accounting log
file, for the current time and the information provided in the
ut_line, ut_name and ut_host arguments.
Portability Note: The BSD struct utmp only has the
ut_line, ut_name, ut_host and ut_time
members. Older systems do not even have the ut_host member.
This section describes how to search and scan the database of registered users. The database itself is kept in the file `/etc/passwd' on most systems, but on some systems a special network server gives access to it.
The functions and data structures for accessing the system user database are declared in the header file `pwd.h'.
passwd data structure is used to hold information about
entries in the system user data base. It has at least the following members:
char *pw_name
char *pw_passwd.
uid_t pw_uid
gid_t pw_gid
char *pw_gecos
char *pw_dir
char *pw_shell
You can search the system user database for information about a
specific user using getpwuid or getpwnam. These
functions are declared in `pwd.h'.
getpwuid.
A null pointer value indicates there is no user in the data base with user ID uid.
getpwuid in that it returns
information about the user whose user ID is uid. However, it
fills the user supplied structure pointed to by result_buf with
the information instead of using a static buffer. The first
buflen bytes of the additional buffer pointed to by buffer
are used to contain additional information, normally strings which are
pointed to by the elements of the result structure.
If a user with ID uid is found, the pointer returned in
result points to the record which contains the wanted data (i.e.,
result contains the value result_buf). If no user is found
or if an error occurred, the pointer returned in result is a null
pointer. The function returns zero or an error code. If the buffer
buffer is too small to contain all the needed information, the
error code ERANGE is returned and errno is set to
ERANGE.
getpwnam.
A null pointer return indicates there is no user named name.
getpwnam in that is returns
information about the user whose user name is name. However, like
getpwuid_r, it fills the user supplied buffers in
result_buf and buffer with the information instead of using
a static buffer.
The return values are the same as for getpwuid_r.
This section explains how a program can read the list of all users in the system, one user at a time. The functions described here are declared in `pwd.h'.
You can use the fgetpwent function to read user entries from a
particular file.
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.
fgetpwent in that it reads the next
user entry from stream. But the result is returned in the
structure pointed to by result_buf. The
first buflen bytes of the additional buffer pointed to by
buffer are used to contain additional information, normally
strings which are pointed to by the elements of the result structure.
The stream must correspond to a file in the same format as the standard password database file.
If the function returns zero result points to the structure with the wanted data (normally this is in result_buf). If errors occurred the return value is nonzero and result contains a null pointer.
The way to scan all the entries in the user database is with
setpwent, getpwent, and endpwent.
getpwent and
getpwent_r use to read the user database.
getpwent function reads the next entry from the stream
initialized by setpwent. It returns a pointer to the entry. The
structure is statically allocated and is rewritten on subsequent calls
to getpwent. You must copy the contents of the structure if you
wish to save the information.
A null pointer is returned when no more entries are available.
getpwent in that it returns the next
entry from the stream initialized by setpwent. Like
fgetpwent_r, it uses the user-supplied buffers in
result_buf and buffer to return the information requested.
The return values are the same as for fgetpwent_r.
getpwent or
getpwent_r.
*p to the stream
stream, in the format used for the standard user database
file. The return value is zero on success and nonzero on failure.
This function exists for compatibility with SVID. We recommend that you
avoid using it, because it makes sense only on the assumption that the
struct passwd structure has no members except the standard ones;
on a system which merges the traditional Unix data base with other
extended information about users, adding an entry using this function
would inevitably leave out much of the important information.
The function putpwent is declared in `pwd.h'.
This section describes how to search and scan the database of registered groups. The database itself is kept in the file `/etc/group' on most systems, but on some systems a special network service provides access to it.
The functions and data structures for accessing the system group database are declared in the header file `grp.h'.
group structure is used to hold information about an entry in
the system group database. It has at least the following members:
char *gr_name
gid_t gr_gid
char **gr_mem
You can search the group database for information about a specific
group using getgrgid or getgrnam. These functions are
declared in `grp.h'.
getgrgid.
A null pointer indicates there is no group with ID gid.
getgrgid in that it returns
information about the group whose group ID is gid. However, it
fills the user supplied structure pointed to by result_buf with
the information instead of using a static buffer. The first
buflen bytes of the additional buffer pointed to by buffer
are used to contain additional information, normally strings which are
pointed to by the elements of the result structure.
If a group with ID gid is found, the pointer returned in
result points to the record which contains the wanted data (i.e.,
result contains the value result_buf). If no group is found
or if an error occurred, the pointer returned in result is a null
pointer. The function returns zero or an error code. If the buffer
buffer is too small to contain all the needed information, the
error code ERANGE is returned and errno is set to
ERANGE.
getgrnam.
A null pointer indicates there is no group named name.
getgrnam in that is returns
information about the group whose group name is name. Like
getgrgid_r, it uses the user supplied buffers in
result_buf and buffer, not a static buffer.
The return values are the same as for getgrgid_r
ERANGE.
This section explains how a program can read the list of all groups in the system, one group at a time. The functions described here are declared in `grp.h'.
You can use the fgetgrent function to read group entries from a
particular file.
fgetgrent function reads the next entry from stream.
It returns a pointer to the entry. The structure is statically
allocated and is overwritten on subsequent calls to fgetgrent. You
must copy the contents of the structure if you wish to save the
information.
The stream must correspond to a file in the same format as the standard group database file.
fgetgrent in that it reads the next
user entry from stream. But the result is returned in the
structure pointed to by result_buf. The first buflen bytes
of the additional buffer pointed to by buffer are used to contain
additional information, normally strings which are pointed to by the
elements of the result structure.
This stream must correspond to a file in the same format as the standard group database file.
If the function returns zero result points to the structure with the wanted data (normally this is in result_buf). If errors occurred the return value is non-zero and result contains a null pointer.
The way to scan all the entries in the group database is with
setgrent, getgrent, and endgrent.
getgrent or getgrent_r.
getgrent function reads the next entry from the stream
initialized by setgrent. It returns a pointer to the entry. The
structure is statically allocated and is overwritten on subsequent calls
to getgrent. You must copy the contents of the structure if you
wish to save the information.
getgrent in that it returns the next
entry from the stream initialized by setgrent. Like
fgetgrent_r, it places the result in user-supplied buffers
pointed to result_buf and buffer.
If the function returns zero result contains a pointer to the data (normally equal to result_buf). If errors occurred the return value is non-zero and result contains a null pointer.
getgrent or
getgrent_r.
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.
#include <grp.h>
#include <pwd.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
int
main (void)
{
uid_t me;
struct passwd *my_passwd;
struct group *my_group;
char **members;
/* Get information about the user ID. */
me = getuid ();
my_passwd = getpwuid (me);
if (!my_passwd)
{
printf ("Couldn't find out about user %d.\n", (int) me);
exit (EXIT_FAILURE);
}
/* Print the information. */
printf ("I am %s.\n", my_passwd->pw_gecos);
printf ("My login name is %s.\n", my_passwd->pw_name);
printf ("My uid is %d.\n", (int) (my_passwd->pw_uid));
printf ("My home directory is %s.\n", my_passwd->pw_dir);
printf ("My default shell is %s.\n", my_passwd->pw_shell);
/* Get information about the default group ID. */
my_group = getgrgid (my_passwd->pw_gid);
if (!my_group)
{
printf ("Couldn't find out about group %d.\n",
(int) my_passwd->pw_gid);
exit (EXIT_FAILURE);
}
/* Print the information. */
printf ("My default group is %s (%d).\n",
my_group->gr_name, (int) (my_passwd->pw_gid));
printf ("The members of this group are:\n");
members = my_group->gr_mem;
while (*members)
{
printf (" %s\n", *(members));
members++;
}
return EXIT_SUCCESS;
}
Here is some output from this program:
I am Throckmorton Snurd. My login name is snurd. My uid is 31093. My home directory is /home/fsg/snurd. My default shell is /bin/sh. My default group is guest (12). The members of this group are: friedman tami
Sometimes it is useful to group users according to other criteria (see section 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 grouping hosts, users, and domains freely, giving them individual names. To be more concrete, a netgroup is a list of triples consisting of a host name, a user name, and a domain name where any of the entries can be a wildcard entry matching all inputs. A last possibility is that names of other netgroups can also be given in the list specifying a netgroup. So one can construct arbitrary hierarchies without loops.
Sun's implementation allows netgroups only for the nis or
nisplus service, see section Services in the NSS configuration File. The
implementation in the GNU C library has no such restriction. An entry
in either of the input services must have the following form:
groupname ( groupname |(hostname,username,domainname))+
Any of the fields in the triple can be empty which means anything
matches. While describing the functions we will see that the opposite
case is useful as well. I.e., there may be entries which will not
match any input. For entries like this, a name consisting of the single
character - shall be used.
The lookup functions for netgroups are a bit different to all other system database handling functions. Since a single netgroup can contain many entries a two-step process is needed. First a single netgroup is selected and then one can iterate over all entries in this netgroup. These functions are declared in `netdb.h'.
getnetgrent to iterate over all entries
in the netgroup with name netgroup.
When the call is successful (i.e., when a netgroup with this name exists)
the return value is 1. When the return value is 0 no
netgroup of this name is known or some other error occurred.
It is important to remember that there is only one single state for
iterating the netgroups. Even if the programmer uses the
getnetgrent_r function the result is not really reentrant since
always only one single netgroup at a time can be processed. If the
program needs to process more than one netgroup simultaneously she
must protect this by using external locking. This problem was
introduced in the original netgroups implementation in SunOS and since
we must stay compatible it is not possible to change this.
Some other functions also use the netgroups state. Currently these are
the innetgr function and parts of the implementation of the
compat service part of the NSS implementation.
NULL.
The returned string pointers are only valid if none of the netgroup
related functions are called.
The return value is 1 if the next entry was successfully read. A
value of 0 means no further entries exist or internal errors occurred.
getnetgrent with only one exception:
the strings the three string pointers hostp, userp, and
domainp point to, are placed in the buffer of buflen bytes
starting at buffer. This means the returned values are valid
even after other netgroup related functions are called.
The return value is 1 if the next entry was successfully read and
the buffer contains enough room to place the strings in it. 0 is
returned in case no more entries are found, the buffer is too small, or
internal errors occurred.
This function is a GNU extension. The original implementation in the SunOS libc does not provide this function.
getnetgrent are invalid afterwards.
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.
set/get/endnetgrent functions.
Any of the pointers hostp, userp, and domainp can be
NULL which means any value is accepted in this position. This is
also true for the name - which should not match any other string
otherwise.
The return value is 1 if an entry matching the given triple is
found in the netgroup. The return value is 0 if the netgroup
itself is not found, the netgroup does not contain the triple or
internal errors occurred.
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