c - Theory Behind getchar() and putchar() Functions

Question

I'm working through "The C Programming Language" by K&R and example 1.5 has stumped me:

#include <stdio.h>

/* copy input to output; 1st version */
int main(int argc, char *argv[])
{
    int c;

    while ((c = getchar()) != EOF)
        putchar(c);

    return 0;
}

I understand that 'getchar()' takes a character for 'putchar()' to display. However, when I run the program in terminal, why is it that I can pass an entire line of characters for 'putchar()' to display?

Answer 1

Because your terminal is line-buffered. getchar() and putchar() still only work on single characters but the terminal waits with submitting the characters to the program until you've entered a whole line. Then getchar() gets the character from that buffer one-by-one and putchar() displays them one-by-one.

Addition: that the terminal is line-buffered means that it submits input to the program when a newline character is encountered. It is usually more efficient to submit blocks of data instead of one character at a time. It also offers the user a chance to edit the line before pressing enter.

Note: Line buffering can be turned off by disabling canonical mode for the terminal and calling setbuf with NULL on stdin.