getlogin, getlogin_r, cuserid — get user name
#include <unistd.h>
char
*getlogin( |
void) ; |
int
getlogin_r( |
char * | buf, |
size_t | bufsize) ; |
#include <stdio.h>
char
*cuserid( |
char * | string) ; |
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 user name in the array buf
of size bufsize
.
cuserid
() returns a pointer
to a string containing a user name 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 user name. 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 user name when successful, and NULL on failure.
getlogin_r
() returns 0 when
successful, and non-zero on failure.
POSIX specifies
The calling process already has the maximum allowed number of open files.
The system already has the maximum allowed number of open files.
The calling process has no controlling tty.
(getlogin_r) The length of the user name, including
the terminating null byte, is larger than bufsize
.
Linux/glibc also has
There was no corresponding entry in the utmp-file.
Insufficient memory to allocate passwd structure.
/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 spec 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
().
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