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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | FILES | ATTRIBUTES | CONFORMING TO | BUGS | SEE ALSO | COLOPHON |
GETLOGIN(3) Linux Programmer's Manual GETLOGIN(3)
getlogin, getlogin_r, cuserid - get username
#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
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.
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.
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 ('\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.)
/etc/passwd
password database file
/var/run/utmp
(traditionally /etc/utmp; some libc versions used
/var/adm/utmp)
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.
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.
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().
logname(1), geteuid(2), getuid(2), utmp(5)
This page is part of release 4.12 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/.
GNU 2016-10-08 GETLOGIN(3)