getlogin, getlogin_r, cuserid - get username
Standard C library (
libc,
-lc)
#include <unistd.h>
char *getlogin(void);
int getlogin_r(char buf[.bufsize], size_t bufsize);
#include <stdio.h>
char *cuserid(char *string);
getlogin_r():
_POSIX_C_SOURCE >= 199506L
cuserid():
Since glibc 2.24:
(_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
|| _GNU_SOURCE
Up to and including glibc 2.23:
_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, with
errno set to indicate the error.
getlogin_r()
returns 0 when successful, and nonzero on failure.
POSIX specifies:
- EMFILE
- The per-process limit on the number of open file
descriptors has been reached.
- ENFILE
- The system-wide limit on the total number of open files has
been reached.
- ENXIO
- The calling process has no controlling terminal.
- ERANGE
- (getlogin_r) The length of the username, including the
terminating null byte ('\0'), 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)
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
getlogin () |
Thread safety |
MT-Unsafe race:getlogin race:utent sig:ALRM timer locale |
getlogin_r () |
Thread safety |
MT-Unsafe race:utent sig:ALRM timer locale |
cuserid () |
Thread safety |
MT-Unsafe race:cuserid/!string locale |
In the above table,
utent in
race:utent signifies that if any of
the functions
setutent(3),
getutent(3), or
endutent(3)
are used in parallel in different threads of a program, then data races could
occur.
getlogin() and
getlogin_r() call those functions, so we
use race:utent to remind users.
getlogin() and
getlogin_r(): POSIX.1-2001, POSIX.1-2008.
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-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().
logname(1),
geteuid(2),
getuid(2),
utmp(5)