GETLOGIN(3) Linux Programmer's Manual GETLOGIN(3)
NAME
getlogin, getlogin_r, cuserid - get user name
SYNOPSIS
#include <unistd.h>
char *getlogin(void);
int getlogin_r(char *buf, size_t bufsize);
#include <stdio.h>
char *cuserid(char *string(3,n));
DESCRIPTION
getlogin returns a pointer to a string(3,n) containing the name of the user
logged in(1,8) on the controlling terminal of the process, or a null pointer
if(3,n) this information cannot be determined. The string(3,n) is statically
allocated and might be overwritten on subsequent calls to this function
or to cuserid.
getlogin_r returns this same user name in(1,8) the array buf of size buf-
size.
cuserid returns a pointer to a string(3,n) containing a user name associated
with the effective user ID of the process. If string(3,n) is not a null
pointer, it should be an array that can hold at least L_cuserid charac-
ters; the string(3,n) is returned in(1,8) this array. Otherwise, a pointer to a
string(3,n) in(1,8) a static area is returned. This string(3,n) is statically allo-
cated 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 user name. L_cuserid is declared in(1,8)
stdio.h.
These functions let your program identify positively the user who is
running (cuserid) or the user who logged in(1,8) this session (getlogin).
(These can differ when setuid 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(7,n,1 builtins) LOGNAME arbitrarily.
RETURN VALUE
getlogin returns a pointer to the user name when successful, and NULL
on failure. getlogin_r returns 0 when successful, and nonzero on fail-
ure.
ERRORS
POSIX specifies
EMFILE The calling process already has the maximum allowed number of
open(2,3,n) files.
ENFILE The system already has the maximum allowed number of open(2,3,n) files.
ENXIO The calling process has no controlling tty.
ERANGE (getlogin_r) The length of the user name, including final NUL,
is larger than bufsize.
Linux/glibc also has
ENOENT There was no corresponding entry in(1,8) the utmp-file.
ENOMEM Insufficient memory to allocate passwd(1,5) structure.
FILES
/etc/passwd(1,5) password database file(1,n)
/var/run/utmp (traditionally /etc/utmp;
some libc versions used /var/adm/utmp)
CONFORMING TO
POSIX.1. System V has a cuserid function which uses the real user ID
rather than the effective user ID. The cuserid function was included in(1,8)
the 1988 version(1,3,5) of POSIX, but removed from the 1990 version.
OpenBSD has getlogin and setlogin, and a username associated with a
session, even if(3,n) it has no controlling tty.
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(1,3,5) name. The user
currently logged in(1,8) on the controlling tty(1,4) of our program need not be
the user who started it. Avoid getlogin() for security-related pur-
poses.
Note that glibc does not follow the POSIX spec and uses stdin instead
of /dev/tty(1,4). A bug. (Other recent systems, like SunOS 5.8 and HPUX
11.11 and FreeBSD 4.8 all return the login(1,3,5) name also when stdin is
redirected.)
Nobody knows precisely what cuserid() does - avoid it in(1,8) portable pro-
grams - avoid it altogether - use getpwuid(geteuid()) instead, if(3,n) that
is what you meant. DO NOT USE cuserid().
SEE ALSO
geteuid(2), getuid(2)
Linux 2.4 2003-08-24 GETLOGIN(3)