Subject: Signal mask restore anomaly

From: Frank Jas <[email-masked]>
Date: Wed, 16 Aug 2000 12:25:36 -0700

Under certain circumstances, a signal blocked while a
signal handler is executing will not be restored.

This is my kernel info:

Linux 2.2.6-15apmac #1 Mon May 31 03:54:09 EDT 1999 ppc unknown

The behavior does not occur under Intel versions (2.2.5-15smp).

Below is a program that tests out the use of signal masks
with sigaction().  It sets up a handler for SIGINT and
SIGTSTP.  The handler for SIGTSTP adds SIGINT to
the signal mask for that handler.  So while the SIGTSTP
handler is executing, occurences of SIGINT will be
blocked until the handler returns.  SIGTSTP will also
be blocked.

#define _POSIX_SOURCE 1
#define _GNU_SOURCE 1

#include <signal.h>
#include <errno.h>
#include <stdio.h>

void on_intr (signum)
	int signum ;
{
	printf("\n    Interrupted with signal %d. \n", signum) ;
}

void on_tstp (signum)
	int signum;
{
	printf ("\n     Handler for SIGTSTP wants text or ^C or ^Z: ") ;
	getchar () ;
}

void main ()
{
	struct sigaction  new_intr_action;
	struct sigaction  new_tstp_action;
	int    c ;

	sigemptyset (&new_intr_action.sa_mask);
	new_intr_action.sa_handler = on_intr;
	new_intr_action.sa_flags   = SA_RESTART;

	sigaction  (SIGINT, &new_intr_action, 0);

	sigemptyset (&new_tstp_action.sa_mask);
	sigaddset   (&new_tstp_action.sa_mask, SIGINT);
	new_tstp_action.sa_handler = on_tstp;
	new_tstp_action.sa_flags   = SA_RESTART;

	sigaction  (SIGTSTP, &new_tstp_action, 0);

	printf("Starting main input loop enter text or ^Z or ^C:\n") ;

	while ((c = getchar ()) != EOF) {
		putchar (c);
	}
}

The following execution sequence leads to the anomalous behavior.

Trigger the SIGTSTP handler with a ^Z
While executing the handler, deliver a ^Z and ^C
Hit return to return from the handler.
Another instance of the SIGTSTP handler should execute.
Hit return to return from that instance of the handler.
Notice that the SIGINT handler does not execute and should have.
Any subsequent SIGINT are apparently blocked.

Apparently the nested occurence of SIGTSTP effected the restore
of the SIGINT mask, and the handling of the pending SIGINT.

The same behavior will result using 'kill' from another window
to send the signals.

Please let me know if this is not the appropriate venue for this
post.

Frank Jas

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