Section: Linux Programmer's Manual (3)
getlogin, getlogin_r, cuserid - get username
int getlogin_r(char *buf, size_t bufsize);
char *cuserid(char *string);
Feature Test Macro Requirements for glibc (see
_POSIX_C_SOURCE >= 199506L
Since glibc 2.24:
(_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
Up to and including glibc 2.23:
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
returns this same username in the array
returns a pointer to a string containing a username
associated with the effective user ID of the process.
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.
string is statically allocated and might be overwritten on subsequent
calls to this function or to
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
or the user who logged in this session
(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.
returns a pointer to the username when successful,
and NULL on failure, with
set to indicate the cause of the error.
returns 0 when successful, and nonzero on failure.
The per-process limit on the number of open file descriptors has been reached.
The system-wide limit on the total number of open files has been reached.
The calling process has no controlling terminal.
The length of the username, including the terminating null byte ('\0'),
is larger than
Linux/glibc also has:
There was no corresponding entry in the utmp-file.
Insufficient memory to allocate passwd structure.
Standard input didn't refer to a terminal.
password database file
some libc versions used /var/adm/utmp)
For an explanation of the terms used in this section, see
MT-Unsafe race:getlogin race:utent
sig:ALRM timer locale
MT-Unsafe race:utent sig:ALRM timer
||Thread safety||MT-Unsafe race:cuserid/!string locale|
In the above table,
signifies that if any of the functions
are used in parallel in different threads of a program,
then data races could occur.
call those functions,
so we use race:utent to remind users.
System V has a
function which uses the real
user ID rather than the effective user ID.
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.
and a username
associated with a session, even if it has no controlling terminal.
Unfortunately, it is often rather easy to fool
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.
for security-related purposes.
Note that glibc does not follow the POSIX specification and uses
(Other recent systems, like SunOS 5.8 and HP-UX 11.11 and FreeBSD 4.8
all return the login name also when
Nobody knows precisely what
does; avoid it in portable programs.
Or avoid it altogether: use
instead, if that is
what you meant.
Do not use
This page is part of release 5.07 of the Linux
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