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(char buf[.bufsize], size_t bufsize);
#include <stdio.h>
char *cuserid(char *string);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


    _POSIX_C_SOURCE >= 199506L


    Since glibc 2.24:
        (_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
            || _GNU_SOURCE
    Up to and including glibc 2.23:

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:

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.
(getlogin_r) The length of the username, including the terminating null byte ('\0'), is larger than bufsize.

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. (See BUGS.)

password database file
(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.


POSIX.1-2001. OpenBSD.
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)

2024-06-15 Linux man-pages 6.9.1