Why does this C code have unexpected behavior

Question

I have written the following simple C code, which compiles and runs fine. However it behaves in a way that I don't understand. I type a character, it prints it on the screen. But when I press the return key it prints the whole line. So if I type the letters a, b and c, abc is printed on the command line twice. WHy does it do this?

#include <stdio.h>


int main(){


    int c;
    while((c=getchar())!=EOF){
        putchar(c);
    }

return 0;

}

score 5 · Answer 1 · answered Mar 09 '12 at 14:44

5

Because your terminal is line buffered.

It doesn't send data to your program until it encounters a newline, though it will echo the character to the screen so you can see they key you hit.

answered Mar 09 '12 at 14:44

nos

223,662
58
417
506

score 4 · Accepted Answer · answered Mar 09 '12 at 14:44

It's your terminal, not the program.

When you press a key, your terminal prints it, but doesn't pass it to the program.

When you press enter, the terminal pass the whole line to the program, and the program prints the line.

EDIT: if you use Unix/Linux/etc, you can write stty -icanon -echo to disable that terminal behavior. The -echo turns off the printing, and the -icanon turns off the buffering.

score 0 · Answer 3 · answered Mar 09 '12 at 14:45

What you are seeing is a combination of a few things.

When you type a character, unless you expressedly stop it from happening, it will print to the screen.
Most print statements won't actually print anything until a new line is printed.

So, the text you are seeing is coming from the type commands, but the putchar() string printed is happening all at once.

score 0 · Answer 4 · edited May 23 '17 at 10:24

You're reading from the standard input stream which is line buffered.

Try this alternate code and you'll understand better what is going on:

#include <stdio.h>
int main(){
    int c;
    while((c=getchar())!=EOF){
        printf("got char %c\n", c);
    }
    return 0;
}

You need non-line buffered input. This is going to depend on your platform, but here is an answer on Linux: non-buffering stdin reading

Why does this C code have unexpected behavior

4 Answers4