.\" Copyright 1995  James R. Van Zandt <jrv@vanzandt.mv.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.\" Changed Tue Sep 19 01:49:29 1995, aeb: moved from man2 to man3
.\"  added ref to /etc/utmp, added BUGS section, etc.
.\" modified 2003 Walter Harms, aeb - added getlogin_r, note on stdin use
.TH GETLOGIN 3 2017-09-15 "GNU" "Linux Programmer's Manual"
.SH NAME
getlogin, getlogin_r, cuserid \- get username
.SH SYNOPSIS
.B #include <unistd.h>
.PP
.B "char *getlogin(void);"
.br
.BI "int getlogin_r(char *" buf ", size_t " bufsize );
.PP
.B #include <stdio.h>
.PP
.BI "char *cuserid(char *" string );
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.BR getlogin_r ():
.\" Deprecated: _REENTRANT ||
_POSIX_C_SOURCE\ >=\ 199506L
.PP
.BR cuserid ():
.nf
    Since glibc 2.24:
        (_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
        || __GNU_SOURCE
    Up to and including glibc 2.23:
        _XOPEN_SOURCE
.fi
.SH DESCRIPTION
.BR 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
.BR cuserid ().
.PP
.BR getlogin_r ()
returns this same username in the array
.I buf
of size
.IR bufsize .
.PP
.BR cuserid ()
returns a pointer to a string containing a username
associated with the effective user ID of the process.
If \fIstring\fP
is not a null pointer, it should be an array that can hold at least
\fBL_cuserid\fP 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
.BR getlogin ().
.PP
The macro \fBL_cuserid\fP is an integer constant that indicates how
long an array you might need to store a username.
\fBL_cuserid\fP is declared in \fI<stdio.h>\fP.
.PP
These functions let your program identify positively the user who is
running
.RB ( cuserid ())
or the user who logged in this session
.RB ( getlogin ()).
(These can differ when set-user-ID programs are involved.)
.PP
For most purposes, it is more useful to use the environment variable
\fBLOGNAME\fP to find out who the user is.
This is more flexible
precisely because the user can set \fBLOGNAME\fP arbitrarily.
.SH RETURN VALUE
.BR getlogin ()
returns a pointer to the username when successful,
and NULL on failure, with
.I errno
set to indicate the cause of the error.
.BR getlogin_r ()
returns 0 when successful, and nonzero on failure.
.SH ERRORS
POSIX specifies
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached.
.TP
.B ENFILE
The system-wide limit on the total number of open files has been reached.
.TP
.B ENXIO
The calling process has no controlling terminal.
.TP
.B ERANGE
(getlogin_r)
The length of the username, including the terminating null byte (\(aq\\0\(aq),
is larger than
.IR bufsize .
.PP
Linux/glibc also has
.TP
.B ENOENT
There was no corresponding entry in the utmp-file.
.TP
.B ENOMEM
Insufficient memory to allocate passwd structure.
.TP
.B ENOTTY
Standard input didn't refer to a terminal.
(See BUGS.)
.SH FILES
.TP
\fI/etc/passwd\fP
password database file
.TP
\fI/var/run/utmp\fP
(traditionally \fI/etc/utmp\fP;
some libc versions used \fI/var/adm/utmp\fP)
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR getlogin ()
T}	Thread safety	T{
MT-Unsafe race:getlogin race:utent
.br
sig:ALRM timer locale
T}
T{
.BR getlogin_r ()
T}	Thread safety	T{
MT-Unsafe race:utent sig:ALRM timer
.br
locale
T}
T{
.BR cuserid ()
T}	Thread safety	MT-Unsafe race:cuserid/!string locale
.TE
.sp 1
In the above table,
.I utent
in
.I race:utent
signifies that if any of the functions
.BR setutent (3),
.BR getutent (3),
or
.BR endutent (3)
are used in parallel in different threads of a program,
then data races could occur.
.BR getlogin ()
and
.BR getlogin_r ()
call those functions,
so we use race:utent to remind users.
.SH CONFORMING TO
.BR getlogin ()
and
.BR getlogin_r ():
POSIX.1-2001, POSIX.1-2008.
.PP
System V has a
.BR cuserid ()
function which uses the real
user ID rather than the effective user ID.
The
.BR 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.
.PP
OpenBSD has
.BR getlogin ()
and
.BR setlogin (),
and a username
associated with a session, even if it has no controlling terminal.
.SH BUGS
Unfortunately, it is often rather easy to fool
.BR 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
.BR getlogin ()
for security-related purposes.
.PP
Note that glibc does not follow the POSIX specification and uses
.I stdin
instead of
.IR /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
.I stdin
is redirected.)
.PP
Nobody knows precisely what
.BR cuserid ()
does; avoid it in portable programs.
Or avoid it altogether: use
.I getpwuid(geteuid())
instead, if that is
what you meant.
.B Do not use
.BR cuserid ().
.SH SEE ALSO
.BR logname (1),
.BR geteuid (2),
.BR getuid (2),
.BR utmp (5)
.SH COLOPHON
This page is part of release 4.16 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.