Programming Forums > Application Development > C

o0zi · May 22nd, 2005, 8:48 AM

I have a piece of code that looks like this:

---example.c---

#include <stdio.h>

int main() {
        char c;
        while ((c = getchar()) && (c != EOF))
                ;
        putchar('\n');
        c = getchar();
        printf("%c\n", c);
}

Entering input from the keyboard, this program works fine.
However, if I then redirect input to this program (e.g. dmesg | ./example), then I get a garbage character printed to the console, and the program does not wait for input from the keyboard.
I assume this is something to do with resetting stdin - how can I pass input back to the keyboard when stdin has been set to receive input from a redirect?

Thanks,
Rob

Nuticulus · May 22nd, 2005, 9:21 AM

If you're piping stdout of dmesg to the stdin of ./example, can you provide stdin from the keyboard also? Just wondering.

Edit: thinking about it, if you've reached EOF, getchar() will return EOF instantly, because there's absolutely nothing left to be read. And you can't just magically give it input from stdin, because that comes from the program piping input, not your keyboard.

I'm puzzled as to why you made this program. Why not just pipe output to /dev/null?

osvaldomarques · May 23rd, 2005, 11:25 PM

Hi Rob,

In that example, you have a semicolon just after the while expression. It ends the while block; so this program reads all the contents of the input and discards it until you get an eof; then it prints an "new line", reads the input again, probably eof also, and prints it.

When you are typing you see the characters because the console is in cannonical mode, echoing them as it passes to the program. When you pipe another content to the standard input of your program, you don't see the characters because there is no echo as they doesn't come from the terminal.

So, you will see just the garbage at the end of the file. For this program do what I suppose you intend to, we would to change it to

(Toggle Plain Text)

#include <stdio.h>

int main() {
        char c;
        while ((c = getchar()) && (c != EOF))
                printf("%c\n", c);
        putchar('\n');
}

The colateral effect is when typing you will see 2 characters for each typing; one from echo and one from printf(). Then you would need to program the console to do not echo the char when the standard input is from the console.

Regards,

Osvaldo.

May 22nd, 2005, 8:48 AM	#1
o0zi Newbie Join Date: May 2005 Posts: 1 Rep Power: 0	Resetting stdin after EOF I have a piece of code that looks like this: ---example.c--- (Toggle Plain Text) #include <stdio.h> int main() { char c; while ((c = getchar()) && (c != EOF)) ; putchar('\n'); c = getchar(); printf("%c\n", c); } #include <stdio.h> int main() { char c; while ((c = getchar()) && (c != EOF)) ; putchar('\n'); c = getchar(); printf("%c\n", c); } Entering input from the keyboard, this program works fine. However, if I then redirect input to this program (e.g. dmesg \| ./example), then I get a garbage character printed to the console, and the program does not wait for input from the keyboard. I assume this is something to do with resetting stdin - how can I pass input back to the keyboard when stdin has been set to receive input from a redirect? Thanks, Rob

May 22nd, 2005, 9:21 AM	#2
Nuticulus Programmer Join Date: May 2005 Location: England Posts: 61 Rep Power: 4	If you're piping stdout of dmesg to the stdin of ./example, can you provide stdin from the keyboard also? Just wondering. Edit: thinking about it, if you've reached EOF, getchar() will return EOF instantly, because there's absolutely nothing left to be read. And you can't just magically give it input from stdin, because that comes from the program piping input, not your keyboard. I'm puzzled as to why you made this program. Why not just pipe output to /dev/null? __________________ http://www.nuticulus.net/hackmysig/sig.PNG Give my site a chance, you know you want to ;-) Google will solve 99% of your problems. Including those with your sex life. Caution, may <strike>contain</strike> be nuts. Last edited by Nuticulus; May 22nd, 2005 at 9:38 AM.

May 23rd, 2005, 11:25 PM	#3
osvaldomarques Newbie Join Date: May 2005 Location: Rio de Janeiro - Brazil Posts: 2 Rep Power: 0	Hi Rob, In that example, you have a semicolon just after the while expression. It ends the while block; so this program reads all the contents of the input and discards it until you get an eof; then it prints an "new line", reads the input again, probably eof also, and prints it. When you are typing you see the characters because the console is in cannonical mode, echoing them as it passes to the program. When you pipe another content to the standard input of your program, you don't see the characters because there is no echo as they doesn't come from the terminal. So, you will see just the garbage at the end of the file. For this program do what I suppose you intend to, we would to change it to (Toggle Plain Text) #include <stdio.h> int main() { char c; while ((c = getchar()) && (c != EOF)) printf("%c\n", c); putchar('\n'); } #include <stdio.h> int main() { char c; while ((c = getchar()) && (c != EOF)) printf("%c\n", c); putchar('\n'); } The colateral effect is when typing you will see 2 characters for each typing; one from echo and one from printf(). Then you would need to program the console to do not echo the char when the standard input is from the console. Regards, Osvaldo.

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Programming Forums > Application Development > C

Resetting stdin after EOF