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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 stati-
cally 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 buf-
cuserid() returns a pointer to a string containing a username associ-
ated 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
These functions let your program identify positively the user who is
running (cuserid()) or the user who logged in this session (getlo-
gin()). (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. getlo-
gin_r() returns 0 when successful, and nonzero on failure.
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)
Multithreading (see pthreads(7))
The getlogin() function is not thread-safe.
The getlogin_r() function is thread-safe.
The cuserid() function is thread-safe with exceptions. It is not
thread-safe if called with a NULL parameter.
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 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-
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
Nobody knows precisely what cuserid() does; avoid it in portable pro-
grams. Or avoid it altogether: use getpwuid(geteuid()) instead, if
that is what you meant. Do not use cuserid().
geteuid(2), getuid(2), utmp(5)