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 (getlogin()).  (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 success-
       ful, 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  sys-
       tems, 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 getp-
       wuid(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)