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

Well, right off the top, scanf("%d%d",&n,&k) will shove a value into n only and silently leave k unset - You'd see this if you checked the return value of scanf(), which tells you how many variables it filled. I think you want scanf("%d %d",&n,&k) with the space.

Second, n is the number of iterations to run, but you test for "n>0" yet never decrement it. Ergo, n>0 is always true and the loop won't exit.

As someone else mentioned, feeding stdin over a pipe causes the loop to exit because the end of stdin has an EOF, which causes fread() to return NULL, exiting the loop. You probably want to add an "n=n-1" or "n--" somewhere in there.

Next, in your sscanf, %n is not really a standard thing; I'm not sure what it's meant to do, but it may do nothing: scanf() generally stops parsing at the first unrecognized format identifier, which does nothing here (since you already got your data,) but is bad practice.

Finally, if performance is important, you'd be better off not using fread() etc at all, as they're not really high performance. Look at isdigit(3) and iscntrl(3) and think about how you could parse the numbers from a raw data buffer read with read(2).