Is this the proper way to flush the C input stream?

Question

Well I been doing a lot of searching on google and on here about how to flush the input stream properly. All I hear is mixed arguments about how fflush() is undefined for th

Answer 1

It turns out to be platform dependent.

The fflush() cannot have an input stream as a parameter because according to the c standard, IT'S UNDEFINED BEHAVIOR since the behavior is not defined anywhere.

• On Windows, there is a defined behavior for fflush() and it does what you need it to do.

• On Linux, there is fpurge(3) which does what you want it to do too.

The best way is to simply read all characters in a loop until

1. A newline character is found.
2. EOF is returned from getchar().

like your clearInputBuf() function.

Note that flushing an output stream means writing all unwritten data to the stream, the data that is still in a buffer waiting to be flushed. But reading all the unread bytes from a stream, does not have the same meaning.

That's why it doesn't make sense to fflush() an input stream. On the other hand fpurge() is designed specifically for this, and it's name is a better choice because you want to clear the input stream and start fresh. The problem is, it's not a standard function.

Reading fpurge(3) should clarify why fflush(stdin) is undefined behavior, and why an implementation like the one on Windows doesn't make sense because it makes fflush() behave differently with different inputs. That's like making c compliant with PHP.