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,  with  errno  set  to
       indicate the cause of the error.  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 terminal.

       ERANGE (getlogin_r)  The  length  of  the username, including the terminating null byte ('\0'), 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)

ATTRIBUTES

   Multithreading (see pthreads(7))
       The getlogin() function is not thread-safe.

       The getlogin_r() function is thread-safe.

       The cuserid() function is thread-safe with exceptions.  It is not  thread-safe  if  called  with  a  NULL
       parameter.

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 terminal.

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 terminal 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 3.54 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/.