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getlogin, getlogin_r, cuserid - get user name
int getlogin_r(char *buf, size_t bufsize);
char *cuserid(char *string);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
getlogin_r(): _REENTRANT || _POSIX_C_SOURCE >= 199506L
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 user name in the array buf of size
cuserid() returns a pointer to a string containing a user name
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 user name. L_cuserid is declared in
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
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 user name when successful, and NULL
on failure. getlogin_r() returns 0 when successful, and nonzero on
EMFILE The calling process already has the maximum allowed number of
ENFILE The system already has the maximum allowed number of open files.
ENXIO The calling process has no controlling tty.
ERANGE (getlogin_r) The length of the user name, including the
terminating null byte, 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.
password database file
(traditionally /etc/utmp; some libc versions used /var/adm/utmp)
getlogin() and getlogin_r() specified in POSIX.1-2001.
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 tty.
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 tty of our program need not
be the user who started it. Avoid getlogin() for security-related
Note that glibc does not follow the POSIX spec 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
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().
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