Thread: A version of getchar that does unbuffered input, K&R, chapter 8, read()

The idea is that read'ing is an expensive operation. Therefore, when you do a read, you may as well get several characters - that's the purpose of buf. BUFSIZ is a constant defined to be a well-chosen value for reading several characters at one time. For example, read'ing 1-2 characters at a time is probably too few, while reading 10 million at a time is probably too many.

Originally Posted by Ducol

How does the whole inputted array gets printed? It seems
that only one character is printed.

The user of your getchar function will only get one character at a time. Basically, the user doesn't have to care that there is buffering going on behind the scenes. You use this function just like you did back in Chapter 1.

Originally Posted by Ducol

I am not sure I understand the return statement:

Code:

 return (--n >= 0) ? (unsigned char) *bufp++ : EOF;

It may help to read this type of statement slowly, and break it into multiple statements. It basically works out to be the same as this:

Code:

--n;
if (n >= 0) {
    int ret = (unsigned char) *bufp;
    bufp++;
    return ret;
} else
    return EOF;

Originally Posted by c99tutorial

The idea is that read'ing is an expensive operation. Therefore, when you do a read, you may as well get several characters - that's the purpose of buf. BUFSIZ is a constant defined to be a well-chosen value for reading several characters at one time. For example, read'ing 1-2 characters at a time is probably too few, while reading 10 million at a time is probably too many.

Thanks a lot for your answer. I understand that. I also understand what the last return says, it's quite a clear statement. Maybe I have not clearly
asked my question, but you have confirmed my statement that there will be only one character printed at one call of this function.
But read() will receive a whole array of characters. If only one character gets printed, what happens to the rest of characters, and how does the program access all other characters to get the whole line printed?

For example, the following code will print the whole array:

Code:

int main(void) /*copy input to output*/
{
    char buf[BUFSIZE];
    int n;
    
    while ((n = read(0, buf, BUFSIZE)) > 0)
    {
        write(1, buf, n);
    }
    return 0;
}

It has a loop, and thus all character received by read() will be printed.