Header File 

ctype.h  

Category 

Conversion Routines 

Prototype 

int _tolower(int ch); 

Description 

_tolower is a macro that does the same conversion as tolower, except that it should be used only when ch is known to be uppercase (AZ). 

To use _tolower, you must include ctype.h. 

Return Value 

_tolower returns the converted value of ch if it is uppercase; 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] = _tolower(string[i]);
       }
   }
   printf("%s\n",string);
   return 0;
}

Portability