GETLOGIN
Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
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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():
_POSIX_C_SOURCE >= 199506L
cuserid():
Since glibc 2.24:
(_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
|| __GNU_SOURCE
Up to and including glibc 2.23:
_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 per-process limit on the number of open file descriptors has been reached.
- ENFILE
-
The system-wide limit on the total number of open files has been reached.
- ENXIO
-
The calling process has no controlling terminal.
- ERANGE
-
(getlogin_r)
The length of the username, including the terminating null byte (aq\0aq),
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
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
getlogin()
| Thread safety |
MT-Unsafe race:getlogin race:utent
sig:ALRM timer locale
|
|
getlogin_r()
| Thread safety |
MT-Unsafe race:utent sig:ALRM timer
locale
|
|
cuserid()
| Thread safety | MT-Unsafe race:cuserid/!string locale
|
In the above table,
utent
in
race:utent
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 could occur.
getlogin()
and
getlogin_r()
call those functions,
so we use race:utent to remind users.
CONFORMING TO
getlogin()
and
getlogin_r():
POSIX.1-2001, POSIX.1-2008.
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
logname(1),
geteuid(2),
getuid(2),
utmp(5)
COLOPHON
This page is part of release 4.13 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
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- FILES
-
- ATTRIBUTES
-
- CONFORMING TO
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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