GETLOGIN(3)						     Linux Programmer's Manual						       GETLOGIN(3)

NAME

       getlogin, getlogin_r, cuserid - get username

SYNOPSIS

       #include <unistd.h>

       char *getlogin(void);
       int getlogin_r(char *buf, size_t bufsize);

       #include <stdio.h>

       char *cuserid(char *string);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       getlogin_r(): _REENTRANT || _POSIX_C_SOURCE >= 199506L
       cuserid(): _XOPEN_SOURCE

DESCRIPTION

       getlogin()  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().

       getlogin_r() returns this same username in the array buf of size bufsize.

       cuserid()  returns  a  pointer  to a string containing a username associated with the effective user 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 macro L_cuserid is an integer constant that indicates how long an array you might need to store a username.	L_cuserid is  declared	in
       <stdio.h>.

       These  functions  let  your  program identify positively the user who is running (cuserid()) or the user who logged in this session (getlo-
       gin()).	(These can differ when set-user-ID programs are involved.)

       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.

RETURN VALUE

       getlogin()  returns a pointer to the username when successful, and NULL on failure.  getlogin_r() returns 0 when successful, and nonzero on
       failure.

ERRORS

       POSIX specifies

       EMFILE The calling process already has the maximum allowed number of open files.

       ENFILE The system already has the maximum allowed number of open files.

       ENXIO  The calling process has no controlling tty.

       ERANGE (getlogin_r) The length of the username, including the terminating null byte, is larger than bufsize.

       Linux/glibc also has

       ENOENT There was no corresponding entry in the utmp-file.

       ENOMEM Insufficient memory to allocate passwd structure.

       ENOTTY Standard input didn't refer to a terminal.  (See BUGS.)

FILES

       /etc/passwd
	      password database file

       /var/run/utmp
	      (traditionally /etc/utmp; some libc versions used /var/adm/utmp)

CONFORMING TO

       getlogin() and getlogin_r() specified in POSIX.1-2001.

       System V has a cuserid() function which uses the real user ID rather than the effective user ID.  The cuserid() function  was  included	in
       the 1988 version of POSIX, but removed from the 1990 version.  It was present in SUSv2, but removed in POSIX.1-2001.

       OpenBSD has getlogin() and setlogin(), and a username associated with a session, even if it has no controlling tty.

BUGS

       Unfortunately,  it  is  often  rather  easy to fool getlogin().	Sometimes it does not work at all, because some program messed up the utmp
       file.  Often, it gives only the first 8 characters of the login name.  The user currently logged in on the controlling tty of  our  program
       need not be the user who started it.  Avoid getlogin() for security-related purposes.

       Note  that glibc does not follow the POSIX specification and uses stdin instead of /dev/tty.  A bug.  (Other recent systems, like SunOS 5.8
       and HP-UX 11.11 and FreeBSD 4.8 all return the login name also when stdin is redirected.)

       Nobody knows precisely what cuserid() does; avoid it in portable programs.  Or avoid it altogether:  use  getpwuid(geteuid())  instead,	if
       that is what you meant.	Do not use cuserid().

SEE ALSO

       geteuid(2), getuid(2), utmp(5)

COLOPHON

       This  page is part of release 3.25 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

GNU
								    2008-06-29							       GETLOGIN(3)