Question

what is the use of fflush(stdin)in c language

Answer 1

I believe it flushes the input stream (meaning, clears it out).

It is implemented as sometimes invisible characters (new lines/line breaks) get stuck in there. There are other ways to handle these characters without flushes.

Answer 2

fflush(stdout) is very useful for making sure that all buffered output has been flushed out (usually to a disk file or the terminal). If you don't use it, then Odd Things can happen (for instance, a prompt appearing *after* you do the corresponding read from the terminal, or the last bit of output from the program never actually making it into a disk file).

Since fflush() is only defined for "output" streams, it's a good way to generate an EBADF (bad file descriptor) for calling it on a non-output stream.