Re: avoid errors when printing to socket

> https://perldoc.perl.org/functions/print says that 'print' would return
true
> if successful and doesn't say what it returns otherwise.  It also says
that
> "Printing to a closed pipe or socket will generate a SIGPIPE signal."

Looks like print returns 1 if it succeeds, undef if not:
$ perl -wE 'my $res = print "hi mom\n"; say $res'
hi mom
1
$ perl -wE 'my $res = print OUT "hi mom\n"; say $res'
Name "main::OUT" used only once: possible typo at -e line 1.
print() on unopened filehandle OUT at -e line 1.
Use of uninitialized value $res in say at -e line 1.
$ perl -E 'open(OUT, ">", STDERR) ;my $res = print OUT "hi mom\n"; say $res'
1
$ perl -wE 'open(OUT, ">", STDERR); close OUT ;my $res = print OUT "hi
mom\n"; say $res'
print() on closed filehandle OUT at -e line 1.
Use of uninitialized value $res in say at -e line 1.

>So I tried to install a signal handler, but either I did that wrong, or no
signal was generated.  What can I do?

Well, seeing a SIGPIPE would mean it's failed already, so it's not going to
get you any further. Showing your code might help in debugging.

On Thu, Sep 30, 2021 at 12:44 PM hw <[email protected]> wrote:

> Hi,
>
> I have a program in which I'm creating an UDP socket with IO::Socket::INET
> to
> a device on my LAN.  Opening the socket yields no error.  Writing to the
> socket with 'print' works fine.  The device sends data back, and receiving
> the
> data works fine.
>
> But it doesn't work when the device is offline, like when I pull the
> network
> cable the device is connected to.  Of course it's obvious that it doesn't
> work, and I'm getting an error message:
>
>
> print() on closed filehandle GEN0 at ...
>
>
> My program keeps running nonetheless, which is ok.
>
> Now the problem is that my program needs a way to detect whether the
> device is
> offline or not so I can make it try to reconnect.
>
> Fortunately, IO::Socket has a 'connected' method, yet unfortunately, the
> method can't really tell whether the socket is actually open or not
> (especially with UDP sockets, I guess).
>
> But then, there must be a way to figure this out because otherwise 'print'
> wouldn't give an error message when the socket is not open.
>
> So how can I either find out in advance if a 'print' will be successful,
> or how
> can I find out if 'print' was successful or not?  It's like I'm seeing the
> error message and there isn't anything I could do about it.
>
> https://perldoc.perl.org/functions/print says that 'print' would return
> true
> if successful and doesn't say what it returns otherwise.  It also says
> that
> "Printing to a closed pipe or socket will generate a SIGPIPE signal."
>
> So I tried to install a signal handler, but either I did that wrong, or no
> signal was generated.  What can I do?
>
>
>
>
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-- 

a

Andy Bach,
[email protected]
608 658-1890 cell
608 261-5738 wk