getlogin(3p) — Linux manual page


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

PROLOG         top

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

NAME         top

       getlogin, getlogin_r — get login name

SYNOPSIS         top

       #include <unistd.h>

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

DESCRIPTION         top

       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         top

       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

ERRORS         top

       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

       The following sections are informative.

EXAMPLES         top

   Getting the User Login Name S
       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);


       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         top

       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:


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


       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.



SEE ALSO         top

       getpwnam(3p), getpwuid(3p), geteuid(3p), getuid(3p)

       The Base Definitions volume of POSIX.1‐2017, limits.h(0p),

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017                      GETLOGIN(3P)

Pages that refer to this page: unistd.h(0p)logname(1p)endgrent(3p)endpwent(3p)getpwuid(3p)