Each of the to functions and its associated macro, if any, converts a single character to another character.

The to functions and macro conversions are as follows.

To use the function versions of the to routines that are also defined as macros, either remove the macro definitions with #undef directives or do not include CTYPE.H. If you use the /Za compiler option, the compiler uses the function version of toupper or tolower. Declarations of the toupper and tolower functions are in STDLIB.H.

The __toascii routine sets all but the low-order 7 bits of c to 0, so that the converted value represents a character in the ASCII character set. If c already represents an ASCII character, c is unchanged.

The tolower and toupper routines:

• Are dependent on the LC_CTYPE category of the current locale (tolower calls isupper and toupper calls islower).

• Convert c if c represents a convertible letter of the appropriate case in the current locale and the opposite case exists for that locale. Otherwise, c is unchanged.

The _tolower and _toupper routines:

• Are locale-independent, much faster versions of tolower and toupper.

• Can be used only when isascii(c) and either isupper(c) or islower(c), respectively, are nonzero.

• Have undefined results if c is not an ASCII letter of the appropriate case for converting.

The towlower and towupper functions return a converted copy of c if and only if both of the following conditions are nonzero. Otherwise, c is unchanged.

• c is a wide character of the appropriate case (that is, for which iswupper or iswlower, respectively, is nonzero).

• There is a corresponding wide character of the target case (that is, for which iswlower or iswupper, respectively, is nonzero).

// crt_toupper.c  
/* This program uses toupper and tolower to  
 * analyze all characters between 0x0 and 0x7F. It also  
 * applies _toupper and _tolower to any code in this  
 * range for which these functions make sense.  
 */  
  
#include <ctype.h>  
#include <string.h>  
  
char msg[] = "Some of THESE letters are Capitals.";  
char *p;  
  
int main( void )  
{  
   printf( "%s\n", msg );  
  
   /* Reverse case of message. */  
   for( p = msg; p < msg + strlen( msg ); p++ )  
   {  
      if( islower( *p ) )  
         putchar( _toupper( *p ) );  
      else if( isupper( *p ) )  
         putchar( _tolower( *p ) );  
      else  
         putchar( *p );  
   }  
}  

Some of THESE letters are Capitals.  
sOME OF these LETTERS ARE cAPITALS.  

Data Conversion
Locale
is, isw Routines