tolower

    #include <ctype.h>
    int tolower( int char );

  Returns lower case type of the character

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
#include <ctype.h>
 
int main ( void )
{
  char string[30];
  /* write the Chars "i" and "t" lowercase in the buffer "string" */
  sprintf(string,"%c WHANT %cHIS IN LOWERCASE",tolower('I'), tolower('T'));
  printf("%s\n",string);
 
return 0;
}

  output:
  user@host:~$ ./tolower
  i WHANT tHIS IN LOWERCASE