#include <ctype.h>
int tolower(int c); /* Convert c to lowercase if appropriate */
int toupper(int c); /* Convert c to uppercase if appropriate */
The tolower() function converts c to a lowercase letter, if possible. Conversely, the toupper() function converts c to an uppercase letter, if possible.
If successful, tolower() and toupper() return the corresponding character, as defined in the IBM-1047 code page, if such a character exists.
If unsuccessful, tolower() and toupper() return the unchanged value c.