Re: A question to lib/libc/gen/daemon.c

[Alexander Farber]

Thanks. I've also read the chapter about dup's from the Stevens book 
and realized, that you always have to check the fd number you're going 
to close() after a dup2()

But how can the argc be less than 1? When is it the case? After an exec()?

2005/8/22, Todd C. Miller [EMAIL PROTECTED]:
 Nope.  There is no guarantee that fds 0-2 are open when a program
 starts.  In that case, fd will fall in the range 0-2 and without
 the check we can close one of the descriptors 0-2.
 
 Bonus trivia:  There's also no guarantee that argc  0 when a program
 starts.  Lots of programs make bad assumptions...

A question to lib/libc/gen/daemon.c

[Alexander Farber]

In the file /usr/src/lib/libc/gen/daemon.c

   if (!noclose  (fd = open(_PATH_DEVNULL, O_RDWR, 0)) != -1) {
(void)dup2(fd, STDIN_FILENO);
(void)dup2(fd, STDOUT_FILENO);
(void)dup2(fd, STDERR_FILENO);
if (fd  2)
(void)close (fd);
}

is same as:

   if (!noclose  (fd = open(_PATH_DEVNULL, O_RDWR, 0)) != -1) {
(void)dup2(fd, 0);
(void)dup2(fd, 1);
(void)dup2(fd, 2);
if (fd  2)
(void)close (fd);
}

right? What is this last check (fd  2) needed for? Isn't fd always  2, 
because the first 3 are already taken by the STDxxx streams at the
program start?

Thanks
Alex

Re: A question to lib/libc/gen/daemon.c

[Hannah Schroeter]

Hello!

On Sun, Aug 21, 2005 at 09:54:06AM +0200, Alexander Farber wrote:
   if (!noclose  (fd = open(_PATH_DEVNULL, O_RDWR, 0)) != -1) {
(void)dup2(fd, 0);
(void)dup2(fd, 1);
(void)dup2(fd, 2);
if (fd  2)
(void)close (fd);
}

right? What is this last check (fd  2) needed for? Isn't fd always  2, 
because the first 3 are already taken by the STDxxx streams at the
program start?

What if any of the descriptors = 2 are closed before invocation
of daemon? Then fd will be = 2, and if you close it, the desired
state (/dev/null open on 0, 1, and 2) will not be achieved.

I.e. safety.

Thanks
Alex

Kind regards,

Hannah.

Re: A question to lib/libc/gen/daemon.c

[Todd C. Miller]

In message [EMAIL PROTECTED]
so spake Alexander Farber (alexander.farber):

 In the file /usr/src/lib/libc/gen/daemon.c
 
if (!noclose  (fd = open(_PATH_DEVNULL, O_RDWR, 0)) != -1) {
 (void)dup2(fd, STDIN_FILENO);
 (void)dup2(fd, STDOUT_FILENO);
 (void)dup2(fd, STDERR_FILENO);
 if (fd  2)
 (void)close (fd);
 }
 
 is same as:
 
if (!noclose  (fd = open(_PATH_DEVNULL, O_RDWR, 0)) != -1) {
 (void)dup2(fd, 0);
 (void)dup2(fd, 1);
 (void)dup2(fd, 2);
 if (fd  2)
 (void)close (fd);
 }
 
 right?

Right.

 What is this last check (fd  2) needed for? Isn't fd always  2, 
 because the first 3 are already taken by the STDxxx streams at the
 program start?

Nope.  There is no guarantee that fds 0-2 are open when a program
starts.  In that case, fd will fall in the range 0-2 and without
the check we can close one of the descriptors 0-2.

Bonus trivia:  There's also no guarantee that argc  0 when a program
starts.  Lots of programs make bad assumptions...

 - todd