...files.

Specifying a buffer size currently has no effect on systems that don't have setvbuf(). The interface to specify the buffer size is not done using a method that calls setvbuf(), because that may dump core when called after any I/O has been performed, and there's no reliable way to determine whether this is the case.

... undefined.

In the current implementation, local variable bindings cannot normally be affected this way, but variables retrieved from other scopes can be. This may change.