getlogin(3) Library Functions Manual getlogin(3)
NAME
getlogin, getlogin_r, cuserid - get username
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
char *getlogin(void);
int getlogin_r(char buf[.bufsize], size_t bufsize);
#include <stdio.h>
char *cuserid(char *string);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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
DESCRIPTION
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-
size.
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
<stdio.h>.
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.
RETURN VALUE
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.
ERRORS
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 terminat-
ing 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.)
FILES
/etc/passwd
password database file
/var/run/utmp
(traditionally /etc/utmp; some libc versions used /var/adm/utmp)
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(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. getlo-
gin() and getlogin_r() call those functions, so we use race:utent to
remind users.
VERSIONS
OpenBSD has getlogin() and setlogin(), and a username associated with a
session, even if it has no controlling terminal.
STANDARDS
getlogin()
getlogin_r()
POSIX.1-2008.
cuserid()
None.
STANDARDS
getlogin()
getlogin_r():
POSIX.1-2001. OpenBSD.
cuserid()
System V, POSIX.1-1988. Removed in POSIX.1-1990. SUSv2. Re-
moved in POSIX.1-2001.
System V has a cuserid() function which uses the real user ID
rather than the effective user ID.
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 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-re-
lated 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 pro-
grams. Or avoid it altogether: use getpwuid(geteuid()) instead, if
that is what you meant. Do not use cuserid().
SEE ALSO
logname(1), geteuid(2), getuid(2), utmp(5)
Linux man-pages 6.7 2023-10-31 getlogin(3)
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