Hallo Corinna,
thanks for the response. Yes, you are right, I missed the fact that the child
process inherits the file descriptors.
I took a look at the documentation of exec and just to be clear on the
descriptor/handler issue:
- file descriptor are passed down (basically always when calling the 'open'
function)
- socket descriptors are *not* passed down (not sure, wasn't in the
documentation)
- handles of semaphores, mutexes, memory maps, message queues are *not* passed
down.
- signal mask, scheduling policies (at leat when using SCHED_FIFO, SCHED_RR or
SCHED_SPORADIC) and directory streams are passed down
Is that correct?
Thanks
Manuel
> -----Original Message-----
> From: *** On Behalf Of Corinna Vinschen
> Sent: Friday, October 21, 2011 12:50 PM
> To: ***
> Subject: Re: 1.7.9: spawn brakes reopening of serial port
>
> On Oct 20 10:19, Manuel Wienand wrote:
> > Hallo,
> >
> > it seems that spawning a process brakes the reopening of a serial
> port
> > (when done during the execution of that process). The result of the
> > open() function is "Permission denied (13)" in that case.
>
> I had a look into your testcase. What you see is expected behaviour.
>
> On Windows, a serial port can only be opened once at a time, for
> exclusive access. You could have tried that without starting a second
> process:
>
> open ("/dev/ttyS0", O_RDWR); // succeeds
> open ("/dev/ttyS0", O_RDWR); // 2nd call fails with EACCES.
>
> Now I hear you say that you close the descriptor before trying to open
> it the second time, but you're missing the fact that on spawn/exec the
> child process inherits the open file descriptors from the parent
> process.
> So, even if you close the descriptor in the parent, it's still open in
> the child.
>
> Apparently you don't want the child process to inherit this descriptor,
> so you must set its "close-on-exec" flag. There are basically two ways
> to do that
>
> - fd = open ("/dev/ttyS0", O_RDWR | O_CLOEXEC);
>
> - fd = open ("/dev/ttyS0", O_RDWR);
> fcntl (fd, F_SETFD, fcntl (fd, F_GETFD) | FD_CLOEXEC);
>
> I guess it goes without saying that the close-on-exec flag has to be
> set before calling spawn/exec.
>
> And, btw., please use /dev/ttyS0 rather than /dev/com1, etc. These are
> the "official" names for the serial ports, while the "dev/comX" syntax
> is only supported for backward compatibility.
>
>
> Corinna
>
> --
> Corinna Vinschen Please, send mails regarding Cygwin
> to
> Cygwin Project Co-Leader cygwin AT cygwin DOT com
> Red Hat
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