23.18 errno

What value is the macro errno set to by the fgetpos or ftell function on failure? (ISO 7.9.9.1, (ISO 7.9.9.4)

Application defined.

What is the format of the messages generated by the perror function? (ISO 7.9.10.4)

The argument to perror, followed by a colon, followed by a text description of the value of errno.