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.\" Copyright 1995  James R. Van Zandt <jrv@vanzandt.mv.com>
.\"
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.\" 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 2015-03-02 "GNU" "Linux Programmer's Manual"
.SH NAME
getlogin, getlogin_r, cuserid \- get username
.SH SYNOPSIS
.B #include <unistd.h>
.sp
.B "char *getlogin(void);"
.br
.BI "int getlogin_r(char *" buf ", size_t " bufsize );
.sp
.B #include <stdio.h>
.sp
.BI "char *cuserid(char *" string );
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR getlogin_r ():
_REENTRANT || _POSIX_C_SOURCE\ >=\ 199506L
.br
.BR cuserid ():
_XOPEN_SOURCE
.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 calling process already has the maximum allowed number of open files.
.TP
.B ENFILE
The system already has the maximum allowed number of open files.
.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 .
.LP
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

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 ()
specified in POSIX.1-2001.

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.
.LP
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.
.LP
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.)
.LP
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 geteuid (2),
.BR getuid (2),
.BR utmp (5)