Using fflush(stdin)
c, fflush, stdin
Solution
Simple: this is undefined behavior, since `fflush` is meant to be called on an output stream. This is an excerpt from the C standard:
int fflush(FILE *ostream);
ostream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.
So it's not a question of "how bad" this is. `fflush(stdin)` is simply not portable, so you should not use it if you want your code to be portable between compilers.
Problem
So a quick Google search for `fflush(stdin)` for clearing the input buffer reveals numerous websites warning against using it. And yet that's exactly how my CS professor taught the class to do it. How bad is using `fflush(stdin)`? Should I really abstain from using it, even though my professor is using it and it seems to work flawlessly?