Question

This code comes from K&R. I have read it several times, but it still seems to escape my grasp.

#define BUFSIZE 100

char buf[BUFSIZE];
int bufp = 0;

int getch(void)
{
      return(bufp>0)?buf[--bufp]:getchar();
}

int ungetch(int c)
{
      if(bufp>=BUFSIZE)
            printf("too many characters");
      else buf[bufp++]=c;
}

The purpose of these two functions, so K&R says, is to prevent a program from reading too much input. i.e. without this code a function might not be able to determine it has read enough data without first reading too much. But I don’t understand how it works.

For example, consider getch().
As far as I can see this is the steps it takes:

1. check if bufp is greater than 0.
2. if so then return the char value of buf[–bufp].
3. else return getchar().

I would like to ask a more specific question, but I literally dont know how this code achieves what it is intended to achieve, so my question is: What is (a) the purpose and (b) the reasoning of this code?

Thanks in advance.

NOTE: For any K&R fans, this code can be found on page 79 (depending on your edition, I suppose)

Answer 1

(a) The purpose of this code is to be able to read a character and then “un-read” it if it turns out you accidentally read a character too many (with a max. of 100 characters to be “un-read”). This is useful in parsers with lookahead.

(b) getch reads from buf if it has contents, indicated by bufp>0. If buf is empty, it calls getchar. Note that it uses buf as a stack: it reads it from right-to-left.

ungetch pushes a character onto the stack buf after doing a check to see if the stack isn’t full.