Advanced Linux Programming Ch7

Jerry Mcguire · 07-27-2012, 04:01 AM

I was following http://www.advancedlinuxprogramming.com/ chapter 7 to play around with the file descriptors.

Here is the source code of a process that I'm looking into.

Code:

/* File: a.c */
#include <stdio.h>
#include <unistd.h>

int main( int argc, char *argv[] )
{
	char what[80];
	what[0] = what[79] = '\0';
	fprintf( stdout, "%d\n", (int)getpid() );
	do {
		fgets( what, 79, stdin );
	} while ( what[0] != 'Q' );

	return 0;
}

Compile this to a.out and run it:

Code:

$ ./a.out
26678

Start another terminal, and attempt to send some text to 26678:

Code:

$ echo "hello" >/proc/26678/fd/0
$ echo "Q"     >/proc/26678/fd/0

a.out in the first terminal doesn't quit. Instead the texts 'hello' and 'Q' are displayed in that terminal. Why doesn't a.out exit at 'Q'?

Thanks.

Jerry Mcguire · 07-27-2012, 04:26 AM

I think I understand why. It's because the fd/0,1,2 are all connected to the tty. I was simply sending 'hello' and 'Q' to the tty pointed by these fd's.

Code:

$ ll /proc/26678/fd/
total 0
lrwx------ 1 ... 64 Jul 27 17:23 0 -> /dev/pts/42
lrwx------ 1 ... 64 Jul 27 17:23 1 -> /dev/pts/42
lrwx------ 1 ... 64 Jul 27 17:23 2 -> /dev/pts/42