PROLOG

       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

       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().

       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 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);
               }

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.1‐2008, <limits.h>, <unistd.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1,  2013  Edition,  Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013  by  the
       Institute  of  Electrical  and  Electronics  Engineers,  Inc and The Open Group.  (This is
       POSIX.1-2008 with the  2013  Technical  Corrigendum  1  applied.)  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.unix.org/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 .