The toupper() function has as a domain a type int, the value of which is representable as an unsigned char or the value of EOF. If the argument has any other value, the argument is returned unchanged. If the argument of toupper() represents a lower-case letter, and there exists a corresponding upper-case letter (as defined by character type information in the program locale category LC_CTYPE), the result is the corresponding upper-case letter. All other arguments in the domain are returned unchanged.

On successful completion, toupper() returns the upper-case letter corresponding to the argument passed.

No errors are defined.

See attributes(5) for descriptions of the following attributes:

_toupper(3C), setlocale(3C), attributes(5)