to Functions

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

tolower, _tolower, towlower
toupper, _toupper, towupper


The to functions and macro conversions are as follows.

Routine Macro Description
__toascii __toascii Converts c to ASCII character
tolower tolower Converts c to lowercase if appropriate
_tolower _tolower Converts c to lowercase
towlower None Converts c to corresponding wide-character lowercase letter
toupper toupper Converts c to uppercase if appropriate
_toupper _toupper Converts c to uppercase
towupper None Converts c to corresponding wide-character uppercase letter

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 ) );
         putchar( *p );
Some of THESE letters are Capitals.

See also

Data Conversion
is, isw Routines