Re: stdout redirected to a process's own stdin

From: Marc Rochkind (rochkind_at_basepath.com)
Date: 09/04/03 

Date: Wed, 03 Sep 2003 20:54:35 -0600

On Wed, 3 Sep 2003 14:38:48 -0600, Jason Creighton
<androflux@softhome.net.remove.to.reply> wrote:

[snip]

> #include <stdio.h>
>
> int main() {
> int chr;
> puts("Hello world!");

Probably does not put anything in the pipe because output is buffered.

> while((chr = getchar()) != EOF) {

Probably is waiting here for some input from the pipe.

> chr = (chr + 1) % 255; putc(chr, stdout);
> putc(chr, stderr);
> }
> return 0;
> }
>
> My thinking was that the puts("Hello world!") would get the loop started
> (ie, provide input for getchar() ) and then I could monitor what's it's
> doing from the output on stderr.
>

OK thinking... I wouldn't worry about this becoming a bad habit, but would
continue to do it as you feel the need. ;-)

> however:
>
> ~/prog/c$ ./loop adder
>
> ...hangs until I press control-C.
>
> I suspect this is due to a misunderstanding of how pipes work. Where did
> I go wrong?

Not really. The UNIX system has no way of knowing about any
misunderstanding you might have had. Most likely the system is just
behaving as directed by the program. ;-)

All kidding aside... turn off the output buffering with setbuf or flush the
buffer after the puts with a call to fflush.

(This is such a classic error that I put it in a book I wrote about 18
years ago.)

>
> Jason Creighton
>

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