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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | FILES | ATTRIBUTES | VERSIONS | STANDARDS | STANDARDS | BUGS | SEE ALSO | COLOPHON |
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getlogin(3) Library Functions Manual getlogin(3)
getlogin, getlogin_r, cuserid - get username
Standard C library (libc, -lc)
#include <unistd.h>
char *getlogin(void);
int getlogin_r(size_t bufsize;
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
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.
OpenBSD has getlogin() and setlogin(), and a username associated
with a session, even if it has no controlling terminal.
getlogin()
getlogin_r()
POSIX.1-2008.
cuserid()
None.
getlogin()
getlogin_r():
POSIX.1-2001. OpenBSD.
cuserid()
System V, POSIX.1-1988. Removed in POSIX.1-1990. SUSv2.
Removed in POSIX.1-2001.
System V has a cuserid() function which uses the real user
ID rather than the effective user ID.
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 getlogin(3)
Pages that refer to this page: logname(1)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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