TOUPPER Section: Linux Programmer's Manual (3) Updated: April 4, 2025 Index   NAME toupper, tolower - convert letter to upper or lower case   SYNOPSIS #include <ctype.h> int toupper (int c); int tolower (int c);   DESCRIPTION toupper() converts the letter c to upper case, if possible. tolower() converts the letter c to lower case, if possible.   RETURN VALUE The value returned is that of the converted letter, or c if the conversion was not possible.   CONFORMING TO ANSI - C, BSD 4.3   BUGS The details of what constitutes an uppercase or lowercase letter depend on the current locale. For example, the default C locale does not know about umlauts, so no conversion is done for them. In some non - English locales, there are lowercase letters with no corresponding uppercase equivalent; the German sharp s is one example.   SEE ALSO   Index NAME SYNOPSIS DESCRIPTION RETURN VALUE CONFORMING TO BUGS SEE ALSO This document was created by using the manual pages. Time: 11:58:07 GMT, March 24, 2025