Header File 

ctype.h  

Category 

Conversion Routines 

Prototype 

int _toupper(int ch); 

Description 

Translates characters to uppercase. 

_toupper is a macro that does the same conversion as toupper, except that it should be used only when ch is known to be lowercase (a to z). 

To use _toupper, you must include ctype.h. 

Return Value 

_toupper returns the converted value of ch if it is lowercase; otherwise, the result is undefined. 

Example  

#include <string.h>
#include <stdio.h>
#include <ctype.h>
int main(void)
{
   int length, i;
   char *string = "this is a string.";
   length = strlen(string);
   for (i = 0; i < length; i++) {
      if ((string[i] >= 'a') && (string[i] <= 'z')){
         string[i] = _toupper(string[i]);
         }
   }
   printf("%s\n",string);
   return 0;
}

Portability