Buffered reading from stdin using fread in C

Question

I am trying to efficiently read from the stdin by using setvbuf in `_IOFBF~ mode. I am new to buffering. I am looking for working examples

Answer 1

One thing that I find confusing is why you are both enabling full buffering within the stream object via the call to setvbuf and doing your own buffering by reading a full buffer into buf.

I understand the need to do buffering, but that is a bit overkill.

I'm going to recommend you stick with setvbuf and remove your own buffering. The reason why is that implementing your own buffering can be tricky. The problem is what will happen when a token (in your case a number) straddles the buffer boundary. For example, let's say your buffer is 8 bytes (9 bytes total for trailing NULL) and your input stream looks like

12345 12345

The first time you fill the buffer you get:

"12345 12"

while the second time you fill the buffer you get:

"345"

Proper buffering requires you to handle that case so you treat the buffer as the two numbers {12345, 12345} and not three numbers {12345, 12, 234}.

Since stdio handles that already for you, just use that. Continue to call setvbuf, get rid of the fread and use scanf to read individual numbers from the input stream.