_toupper — transliterate lowercase characters to uppercase

       Applications should use the toupper() function instead of the obsolescent _toupper() function.

       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                                      _TOUPPER(3POSIX)

       The  _toupper()  macro shall be equivalent to toupper() except that the application shall ensure that the
       argument c is a lowercase letter.

       No errors are defined.

       Thefollowingsectionsareinformative.

       None.

       The _toupper() function may be removed in a future version.

       _toupper — transliterate lowercase characters to uppercase

       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.

       None.

       Upon successful completion, _toupper() shall return the uppercase letter corresponding  to  the  argument
       passed.

islower(), toupper()

       The Base Definitions volume of POSIX.1‐2017, Chapter7, Locale, <ctype.h>

       #include <ctype.h>

       int _toupper(int c);