GETLOGIN(3) Linux Programmer's Manual GETLOGIN(3)
getlogin, getlogin_r, cuserid - get username
int getlogin_r(char *buf, size_t bufsize);
char *cuserid(char *string);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
getlogin_r(): _REENTRANT || _POSIX_C_SOURCE >= 199506L
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 effec-
tive 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 arbi-
getlogin() returns a pointer to the username when successful, and NULL on failure. getlo-
gin_r() returns 0 when successful, and nonzero on failure.
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.)
password database file
(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 char-
acters of the login name. The user currently logged in on the controlling tty of our pro-
gram 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
geteuid(2), getuid(2), utmp(5)
This page is part of release 3.25 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at http://www.ker-
GNU 2008-06-29 GETLOGIN(3)