Provided by: manpages-dev_4.15-1_all 
NAME
getlogin, getlogin_r, cuserid - get username
SYNOPSIS
#include <unistd.h>
char *getlogin(void);
int getlogin_r(char *buf, 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 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.
RETURN VALUE
getlogin() returns a pointer to the username when successful, and NULL on failure, with
errno set to indicate the cause of 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 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.)
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 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.
CONFORMING TO
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.
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-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().
SEE ALSO
logname(1), geteuid(2), getuid(2), utmp(5)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.