#include <ctype.h>
int
tolower (int c
);
The tolower function converts an upper case letter to the corresponding lower case letter.
If the argument is a character for which isupper is true and there is a corresponding character for which islower is true, the tolower function returns the corresponding character; otherwise, the argument is returned unchanged.
#include <ctype.h>
int
toupper (int c
);
The toupper function converts an lower case letter to the corresponding upper case letter.
If the argument is a character for which islower is true and there is a corresponding character for which isupper is true, the toupper function returns the corresponding character; otherwise, the argument is returned unchanged.