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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(): _REENTRANT || _POSIX_C_SOURCE >= 199506L
cuserid(): _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. getlogin_r() returns 0 when successful, and nonzero on failure.
POSIX specifies
EMFILE The calling process already has the maximum allowed number of open
files.
ENFILE The system already has the maximum allowed number of open files.
ENXIO The calling process has no controlling tty.
ERANGE (getlogin_r) The length of the username, including the terminating null
byte, 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)
getlogin() and getlogin_r() specified in POSIX.1-2001.
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 tty.
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 tty 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().
geteuid(2), getuid(2), utmp(5)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2008-06-29 GETLOGIN(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface