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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():
           _POSIX_C_SOURCE >= 199506L

       cuserid():
           Since glibc 2.24:
               (_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
                   || _GNU_SOURCE
           Up to and including glibc 2.23:
               _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 informa‐
       tion 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.

RETURN VALUE
       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.

ERRORS
       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
       /etc/passwd
              password database file

       /var/run/utmp
              (traditionally /etc/utmp; some libc versions used /var/adm/utmp)

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
       β”‚Interface    β”‚ Attribute     β”‚ Value                                                                                                                            β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚getlogin()   β”‚ Thread safety β”‚ MT-Unsafe race:getlogin race:utent sig:ALRM timer locale                                                                         β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚getlogin_r() β”‚ Thread safety β”‚ MT-Unsafe race:utent sig:ALRM timer locale                                                                                       β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚cuserid()    β”‚ Thread safety β”‚ MT-Unsafe race:cuserid/!string locale                                                                                            β”‚
       β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜
       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
       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
       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 on‐
       ly 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
       logname(1), geteuid(2), getuid(2), utmp(5)

GNU                                                                            2021-03-22                                                                    GETLOGIN(3)