Why can't use C standard I/O with sockets

Question

It's often said that one shouldn't use C standard I/O functions (like fprintf(), fscanf()) when working with sockets.

I can't understand why. I think if the reason was just in their buffered nature, one could just flush the output buffer each time he outputs, right?

Why everyone uses UNIX I/O functions instead? Are there any situations when the use of standard C functions is appropriate and correct?

Answer 1

You can certainly use stdio with sockets. You can even write a program that uses nothing but stdin and stdout, run it from inetd (which provides a socket on STDIN_FILENO and STDOUT_FILENO), and it works even though it doesn't contain any socket code at all.

What you can't do is mix buffered I/O with select or poll because there is no fselect or fpoll working on FILE *'s and you can't even implement one yourself because there's no standard way of querying a FILE * to find out whether its input buffer is empty.

As soon as you need to handle multiple connections, stdio is not good enough.

Answer 2

It's totally fine when you have simple scenario with one socket in blocking mode and your application protocol is text-based.

It quickly becomes a huge pain with more then one or non-blocking socket(s), with any sort of binary encoding, and with any real performance requirements.