Cannot capture SIGPIPE

Cannot capture SIGPIPE

[Parvez Pathan]

Hi All,
  I have  a server program which connect to clients c1 and c2. The server reads from c1 and writes to c2. Whenever c2 dies(or is killed) I get a SIGPIPE at the server. I have tried to capture the SIGPIPE but cant trap it. Some part of my code is as follows:

<code>
void cleanup (int x) { printf ("Quitting..."); exit(0); }
void back_to_work (int x) { printf ("Not Quitting..."); return; }

main ()
{
	signal (SIGINT, cleanup);
	signal (SIGPIPE, back_to_work);

	//rest of the program
}
</code>

The server is in a infinite loop. The server cannot capture a SIGPIPE given to it by the kill command (kill -PIPE <server pid>) also. Am I missing out anything??
Please help.

Thanks,
Parvez


____________________________________________________________
Get advanced SPAM filtering on Webmail or POP Mail ... Get Lycos Mail!
http://login.mail.lycos.com/r/referral?aid=27005

Re: Cannot capture SIGPIPE

[Glynn Clements]

Parvez Pathan wrote:

> I have a server program which connect to clients c1 and c2. The server
> reads from c1 and writes to c2. Whenever c2 dies(or is killed) I get a
> SIGPIPE at the server. I have tried to capture the SIGPIPE but cant
> trap it. Some part of my code is as follows:
> 
> <code>
> void cleanup (int x) { printf ("Quitting..."); exit(0); }
> void back_to_work (int x) { printf ("Not Quitting..."); return; }

This won't print at the point that the signal is received if stdout is
line buffered (which is the default). Add a newline or call fflush()
to force the output to be written.

However, printf() itself isn't necessarily safe to use within a signal
handler. It would be better to use write(), e.g.

	void back_to_work (int x) {
		const char msg[] = "Not Quitting...\n";
		write(STDOUT_FILENO, msg, strlen(msg));
	}

More generally, very few functions are safe to use within a signal
handler.

Where possible, a signal handler should just set a flag variable
(which must be declared "volatile"), with the real work being
performed outside of the handler when the flag is set, e.g.

	static volatile int sigpipe;

	void handle_sigpipe (int signo) { sigpipe = 1; }

	...

		for (;;)
		{
			if (sigpipe)
			{
				deal_with_sigpipe();
				sigpipe = 0;
			}
			...

-- 
Glynn Clements <glynn.clements@virgin.net>

RE: Cannot capture SIGPIPE

[romesh sahu]

oh i m sorry for my earlier mail...
i tried sending PIPE instead SIGPIPE...its the same thing happening...both SIGPIPE and PIPE work fine..the process is catching the signal and going to back_to_work and prints the "Not Quitting" message.

Regards,
Romesh

-----Original Message-----
From: linux-c-programming-owner@vger.kernel.org
[mailto:linux-c-programming-owner@vger.kernel.org]On Behalf Of Parvez
Pathan
Sent: Wednesday, February 04, 2004 1:23 PM
To: linux-c-programming@vger.kernel.org
Subject: Cannot capture SIGPIPE


Hi All,
  I have  a server program which connect to clients c1 and c2. The server reads from c1 and writes to c2. Whenever c2 dies(or is killed) I get a SIGPIPE at the server. I have tried to capture the SIGPIPE but cant trap it. Some part of my code is as follows:

<code>
void cleanup (int x) { printf ("Quitting..."); exit(0); }
void back_to_work (int x) { printf ("Not Quitting..."); return; }

main ()
{
	signal (SIGINT, cleanup);
	signal (SIGPIPE, back_to_work);

	//rest of the program
}
</code>

The server is in a infinite loop. The server cannot capture a SIGPIPE given to it by the kill command (kill -PIPE <server pid>) also. Am I missing out anything??
Please help.

Thanks,
Parvez


____________________________________________________________
Get advanced SPAM filtering on Webmail or POP Mail ... Get Lycos Mail!
http://login.mail.lycos.com/r/referral?aid=27005
-
To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

RE: Cannot capture SIGPIPE

[Parvez Pathan]

Hi,
 Thanks for the reply. One question that has now started lingering my mind is that Is it correct to pass the kill signal to the process as kill  will always try to terminate the process, whereas I do not want to terminate the process.
  Secondly, I did have some luck. Ater every write I check for 2 conditions, the return val after write and is errno is set to EPIPE. But still my handler does not get called. Strange!!

Thanks 
Parvez
--

--------- Original Message ---------

DATE: Wed, 4 Feb 2004 14:43:16 
From: "romesh sahu" <r.sahu@zensar.com>
To: <linux-c-programming@vger.kernel.org>
Cc: <parvezp@lycos.com>

>oh i m sorry for my earlier mail...
>i tried sending PIPE instead SIGPIPE...its the same thing happening...both SIGPIPE and PIPE work fine..the process is catching the signal and going to back_to_work and prints the "Not Quitting" message.
>
>Regards,
>Romesh
>
>-----Original Message-----
>From: linux-c-programming-owner@vger.kernel.org
>[mailto:linux-c-programming-owner@vger.kernel.org]On Behalf Of Parvez
>Pathan
>Sent: Wednesday, February 04, 2004 1:23 PM
>To: linux-c-programming@vger.kernel.org
>Subject: Cannot capture SIGPIPE
>
>
>Hi All,
>  I have  a server program which connect to clients c1 and c2. The server reads from c1 and writes to c2. Whenever c2 dies(or is killed) I get a SIGPIPE at the server. I have tried to capture the SIGPIPE but cant trap it. Some part of my code is as follows:
>
><code>
>void cleanup (int x) { printf ("Quitting..."); exit(0); }
>void back_to_work (int x) { printf ("Not Quitting..."); return; }
>
>main ()
>{
>	signal (SIGINT, cleanup);
>	signal (SIGPIPE, back_to_work);
>
>	//rest of the program
>}
></code>
>
>The server is in a infinite loop. The server cannot capture a SIGPIPE given to it by the kill command (kill -PIPE <server pid>) also. Am I missing out anything??
>Please help.
>
>Thanks,
>Parvez
>
>
>____________________________________________________________
>Get advanced SPAM filtering on Webmail or POP Mail ... Get Lycos Mail!
>http://login.mail.lycos.com/r/referral?aid=27005
>-
>To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
>the body of a message to majordomo@vger.kernel.org
>More majordomo info at  http://vger.kernel.org/majordomo-info.html
>



____________________________________________________________
Get advanced SPAM filtering on Webmail or POP Mail ... Get Lycos Mail!
http://login.mail.lycos.com/r/referral?aid=27005

RE: Cannot capture SIGPIPE

[Colovic (ext_evosoft) Aleksandar]

Hi,

good comment from Glynn. But I would additionaly declare the global variable
sigpipe as:

static volatile sig_atomic_t sigpipe;

... to be on the safe side.
In this example (as coded) it think it isn't necessary.

Aleksandar Colovic

-----Original Message-----
From: linux-c-programming-owner@vger.kernel.org
[mailto:linux-c-programming-owner@vger.kernel.org]On Behalf Of Glynn
Clements
Sent: Mittwoch, 4. Februar 2004 14:16
To: parvezp@lycos.com
Cc: linux-c-programming@vger.kernel.org
Subject: Re: Cannot capture SIGPIPE



Parvez Pathan wrote:

> I have a server program which connect to clients c1 and c2. The server
> reads from c1 and writes to c2. Whenever c2 dies(or is killed) I get a
> SIGPIPE at the server. I have tried to capture the SIGPIPE but cant
> trap it. Some part of my code is as follows:
> 
> <code>
> void cleanup (int x) { printf ("Quitting..."); exit(0); }
> void back_to_work (int x) { printf ("Not Quitting..."); return; }

This won't print at the point that the signal is received if stdout is
line buffered (which is the default). Add a newline or call fflush()
to force the output to be written.

However, printf() itself isn't necessarily safe to use within a signal
handler. It would be better to use write(), e.g.

	void back_to_work (int x) {
		const char msg[] = "Not Quitting...\n";
		write(STDOUT_FILENO, msg, strlen(msg));
	}

More generally, very few functions are safe to use within a signal
handler.

Where possible, a signal handler should just set a flag variable
(which must be declared "volatile"), with the real work being
performed outside of the handler when the flag is set, e.g.

	static volatile int sigpipe;

	void handle_sigpipe (int signo) { sigpipe = 1; }

	...

		for (;;)
		{
			if (sigpipe)
			{
				deal_with_sigpipe();
				sigpipe = 0;
			}
			...

-- 
Glynn Clements <glynn.clements@virgin.net>
-
To unsubscribe from this list: send the line "unsubscribe
linux-c-programming" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

RE: Cannot capture SIGPIPE

[Glynn Clements]

Colovic (ext_evosoft) Aleksandar wrote:

> good comment from Glynn. But I would additionaly declare the global variable
> sigpipe as:
> 
> static volatile sig_atomic_t sigpipe;
> 
> ... to be on the safe side.
> In this example (as coded) it think it isn't necessary.

Good point. Yes, technically it should be sig_atomic_t; this type of
scenario is the essential reason for the existence of that type.

However, in practice, very few platforms would have an "int" type
which can't be written atomically. The most likely situation where the
distinction between int and sig_atomic_t would be important would be a
16-bit CPU which, for compatibility reasons, has chosen to use a
32-bit int.

In that situation, an operation which updates an int variable could be
pre-empted midway. Even then, it wouldn't matter if only the bottom
word was ever changed (as is the case here).

But it serves to illustrate the point that you have to be very careful
about what you put into a signal handler.

-- 
Glynn Clements <glynn.clements@virgin.net>