tolower
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | Defined in header <ctype.h>
  | ||
| int tolower( int ch ); | ||
Converts the given character to lowercase.
[edit] Parameters
| ch | - | character to be converted | 
[edit] Return value
Lowercase version of ch or unmodified ch if no lowercase version is listed in the current C locale.
[edit] See also
| converts a character to uppercase (function) | |
| converts a wide character to lowercase (function) | |