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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       getlogin, getlogin_r -- get login name

SYNOPSIS
       #include <unistd.h>

       char *getlogin(void);
       int getlogin_r(char *name, size_t namesize);

DESCRIPTION
       The getlogin() function shall return a pointer to a string containing
       the user name associated by the login activity with the controlling
       terminal of the current process. If getlogin() returns a non-null
       pointer, then that pointer points to the name that the user logged in
       under, even if there are several login names with the same user ID.

       The getlogin() function need not be thread-safe.

       The getlogin___r() function shall put the name associated by the login
       activity with the controlling terminal of the current process in the
       character array pointed to by name.  The array is namesize characters
       long and should have space for the name and the terminating null
       character. The maximum size of the login name is {LOGIN_NAME_MAX}.

       If getlogin___r() is successful, name points to the name the user used at
       login, even if there are several login names with the same user ID.

       The getlogin() and getlogin___r() functions may make use of file
       descriptors 0, 1, and 2 to find the controlling terminal of the current
       process, examining each in turn until the terminal is found. If in this
       case none of these three file descriptors is open to the controlling
       terminal, these functions may fail. The method used to find the
       terminal associated with a file descriptor may depend on the file
       descriptor being open to the actual terminal device, not /dev/tty.

RETURN VALUE
       Upon successful completion, getlogin() shall return a pointer to the
       login name or a null pointer if the user's login name cannot be found.
       Otherwise, it shall return a null pointer and set errno to indicate the
       error.

       The application shall not modify the string returned. The returned
       pointer might be invalidated or the string content might be overwritten
       by a subsequent call to getlogin().  The returned pointer and the
       string content might also be invalidated if the calling thread is
       terminated.

       If successful, the getlogin___r() function shall return zero; otherwise,
       an error number shall be returned to indicate the error.

ERRORS
       These functions may fail if:

       EMFILE All file descriptors available to the process are currently
              open.

       ENFILE The maximum allowable number of files is currently open in the
              system.

       ENOTTY None of the file descriptors 0, 1, or 2 is open to the
              controlling terminal of the current process.

       ENXIO  The calling process has no controlling terminal.

       The getlogin___r() function may fail if:

       ERANGE The value of namesize is smaller than the length of the string
              to be returned including the terminating null character.

       The following sections are informative.

EXAMPLES
   Getting the User Login Name SS
       The following example calls the getlogin() function to obtain the name
       of the user associated with the calling process, and passes this
       information to the getpwnam() function to get the associated user
       database information.


           #include <unistd.h>
           #include <sys/types.h>
           #include <pwd.h>
           #include <stdio.h>
           ...
           char *lgn;
           struct passwd *pw;
           ...
           if ((lgn = getlogin()) == NULL || (pw = getpwnam(lgn)) == NULL) {
               fprintf(stderr, "Get of user information failed.\n"); exit(1);
               }

APPLICATION USAGE
       Three names associated with the current process can be determined:
       getpwuid(geteuid()) shall return the name associated with the effective
       user ID of the process; getlogin() shall return the name associated
       with the current login activity; and getpwuid(getuid()) shall return
       the name associated with the real user ID of the process.

       The getlogin___r() function is thread-safe and returns values in a user-
       supplied buffer instead of possibly using a static data area that may
       be overwritten by each call.

RATIONALE
       The getlogin() function returns a pointer to the user's login name. The
       same user ID may be shared by several login names. If it is desired to
       get the user database entry that is used during login, the result of
       getlogin() should be used to provide the argument to the getpwnam()
       function. (This might be used to determine the user's login shell,
       particularly where a single user has multiple login shells with
       distinct login names, but the same user ID.)

       The information provided by the cuserid() function, which was
       originally defined in the POSIX.1-1988 standard and subsequently
       removed, can be obtained by the following:


           getpwuid(geteuid())

       while the information provided by historical implementations of
       cuserid() can be obtained by:


           getpwuid(getuid())

       The thread-safe version of this function places the user name in a
       user-supplied buffer and returns a non-zero value if it fails. The non-
       thread-safe version may return the name in a static data area that may
       be overwritten by each call.

FUTURE DIRECTIONS
       None.

SEE ALSO
       getpwnam(), getpwuid(), geteuid(), getuid()

       The Base Definitions volume of POSIX.1-2017, <limits.h>, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                          GETLOGIN(3)
