##### Synopsis

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

##### Description

tolower converts an uppercase letter to a corresponding lowercase letter.
If the argument c is a character for which isupper is true, tolower returns the corresponding lowercase letter; otherwise, the argument is returned unchanged.

##### Portability

tolower conforms to ISO/IEC 9899:1990 (C90) and ISO/IEC 9899:1999 (C99).