Section (3) getlogin
getlogin, getlogin_r, cuserid — get username
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
getlogin_r() returns this
same username in the array
buf of size
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
L_cuserid is an
integer constant that indicates how long an array you might
need to store a username.
L_cuserid is declared in
These functions let your program identify positively the
user who is running (
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
find out who the user is. This is more flexible precisely
because the user can set
getlogin() returns a pointer
to the username when successful, and NULL on failure, with
errno set to indicate the cause
of the error.
returns 0 when successful, and nonzero on failure.
The per-process limit on the number of open file descriptors has been reached.
The system-wide limit on the total number of open files has been reached.
The calling process has no controlling terminal.
(getlogin_r) The length of the username, including the terminating null byte (_zsingle_quotesz_ _zsingle_quotesz_), is larger than
Linux/glibc also has:
There was no corresponding entry in the utmp-file.
Insufficient memory to allocate passwd structure.
Standard input didn_zsingle_quotesz_t refer to a terminal. (See BUGS.)
password database file
/etc/utmp; some libc versions used
For an explanation of the terms used in this section, see attributes(7).
MT-Unsafe race:getlogin race:utent
sig:ALRM timer locale
MT-Unsafe race:utent sig:ALRM timer
||Thread safety||MT-Unsafe race:cuserid/!string locale|
In the above table,
signifies that if any of the functions setutent(3), getutent(3), or endutent(3) are used in
parallel in different threads of a program, then data races
getlogin_r() call those
functions, so we use race:utent to remind users.
System V has a
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.
setlogin(), and a username
associated with a session, even if it has no controlling
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.
Note that glibc does not follow the POSIX specification
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
geteuid()) instead, if that is what you
meant. Do not use
This page is part of release 5.04 of the Linux
man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
Copyright 1995 James R. Van Zandt <jrvvanzandt.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