Conventional daemons

[Steve Summit]

> [Why do daemons open / as stdin/stdout/stderr?]
> The real question was, why open anything? Surely there's
> nothing functionally useful about opening stdin/stdout on
> "/" and it could be a potential hazard if ported...

Another reason for keeping file descriptors 0, 1, and 2 open is
that it's remarkably easy to write code that depends on it. 
Consider the following fragment, which intends to determine the
load average by inspecting the output of "uptime":

	int fdpair[2], pid;
	pipe(fdpair);
	pid = fork();
	if(pid == 0)
		{
		close(1);
		dup(fdpair[1]);		/* intends to become 1 */
		execl("/usr/ucb/uptime", "uptime", 0);
		}
	close(fdpair[1]);
	read(fdpair[0], buf, BUFSIZ);
	while(wait(0) != pid);
	.
	.
	.

(Please don't nit-pick this program; I am aware of at least eight
things wrong with it, but I didn't want to obscure the example
with all of the error-checking that a production program would
require.)

The problem with this program is that the sequence

	close(1);
	dup(fdpair[1]);

will fail miserably if fdpair[1] happens to be 1, as in fact
would be the case if this program were run with no file
descriptors initially open.

Using dup2() might help this example, and of course the best thing
to do would be to use popen(), which is written so as to avoid
this problem.

When writing code that forks and execs other programs with my own
attached input and output, I frequently find myself writing
extremely strange code, such as the following:

	fd = open("/dev/littleredridinghood", 2);

	if(fd != 0)
		{
		close(0);
		dup(fd);
		}

	if(fd != 1)
		{
		close(1);
		dup(fd);
		}

	if(fd != 2)
		{
		close(2);
		dup(fd);
		}

	if(fd > 2)
		close(fd);

	execl("/big/bad", "wolf", 0);

The big bad wolf program is given little red riding hood as
standard input, output, and error.  The checks of fd against 0,
1, and 2 are in case it is already 0, 1, or 2, which would happen
if this program were invoked under some daemon which didn't leave
0, 1, and 2 open on something.

I think that one of the main reasons that well-written daemons do
leave 0, 1, and 2 open on something is there are probably a lot
of programs out there that weren't written by somebody paranoid
enough to check other file descriptors against 0 before closing
0. ("But 0 is always standard input, right?")

                                         Steve Summit
                                         tektronix!copper!stevesu