Thank you dear GL.Sam. You re right . I must write example code.

In my menu
at
case 1 : tolower ()
case 2 : delete_line()

Code:
case 1 :
                        
tolower();/* at these , i want to  all words lowercase in a file*/
break ;

case 2 :

delete_line() ;  /* delete line*/
break ;
"delete_line()" function is written by me and working perfect at "case 2" . But i want to call "tolower()" function at "case 1" for all words to lowercase in file (for example ; simple.txt) .

How can i succes this ? Thank you .