Implementing scanf using read()

Question

I want to implement scanf function using read system call, but I have a problem - when you use read you have to specify how many bits it will read, and I am not sure what number to use. I have for example implemented flag %b for binary numbers, %x for hex etc, should I just use a constant amount of bits like 256 ?

Answer 1

You will probably have to implement a buffering layer. When you call read() , you get a number of bytes from the stream. From the point of view of the system, those bytes are now gone and will not be returned by a subsequent call to read() . The problem is, if you have a format specifier like %d , you can't know when you are done reading the number until you actually read the next non-digit byte. You'll have to store that already-read input somewhere so you can use it later, for example for a following %s .

Answer 2

基本上，您必须自己进行缓冲，因为您选择的任何数字在某些情况下可能太大（除非您选择的东西小得可笑），而在其他情况下可能太小（例如， 很多空白），所以没有权利数。

Answer 3

read() , without a layer of buffering code, will not work to implement scanf() as scanf() needs the ability to "unget" characters back to stdin .

Instead, scanf() could be implemented with fgetc()/unget() , but it is a great deal to implement a complete scanf() , no matter what route is taken.