GETLOGIN(3) Linux Programmer s Manual GETLOGIN(3)

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(): _REENTRANT || _POSIX_C_SOURCE >= 199506L cuserid(): _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. getlogin_r() returns 0 when successful, and non-zero on failure.

ERRORS POSIX specifies

EMFILE The calling process already has the maximum allowed number of open files.

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 username, including the terminat- ing 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.

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)

CONFORMING TO 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.

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 tty of our program need not be the user who started it. Avoid getlogin() for security-related pur- poses.

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 geteuid(2), getuid(2), utmp(5)

COLOPHON This page is part of release 3.22 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at

GNU 2008-06-29 GETLOGIN(3)