getlogin(3) — Linux manual page


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

NAME         top

       getlogin, getlogin_r, cuserid - get username

SYNOPSIS         top

       #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(): _POSIX_C_SOURCE >= 199506L

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

DESCRIPTION         top

       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

       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

       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.

RETURN VALUE         top

       getlogin() returns a pointer to the username when successful, and
       NULL on failure, with errno set to indicate the cause of the error.
       getlogin_r() returns 0 when successful, and nonzero on failure.

ERRORS         top

       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.)

FILES         top

              password database file

              (traditionally /etc/utmp; some libc versions used

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │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.

CONFORMING TO         top

       getlogin() and getlogin_r(): POSIX.1-2001, POSIX.1-2008.

       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 terminal.

BUGS         top

       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().

SEE ALSO         top

       logname(1), geteuid(2), getuid(2), utmp(5)

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

GNU                              2019-03-06                      GETLOGIN(3)