6aa1f5995a
Add Caldera license. Approved by: David Taylor <davidt@caldera.com> Make buildable under FreeBSD.
362 lines
10 KiB
Plaintext
362 lines
10 KiB
Plaintext
.\" Copyright (C) Caldera International Inc. 2001-2002. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions are
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.\" met:
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.\"
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.\" Redistributions of source code and documentation must retain the above
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.\" copyright notice, this list of conditions and the following
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.\" disclaimer.
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.\"
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.\" Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\"
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.\" This product includes software developed or owned by Caldera
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.\" International, Inc. Neither the name of Caldera International, Inc.
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.\" nor the names of other contributors may be used to endorse or promote
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.\" products derived from this software without specific prior written
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.\" permission.
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.\"
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.\" USE OF THE SOFTWARE PROVIDED FOR UNDER THIS LICENSE BY CALDERA
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.\" INTERNATIONAL, INC. AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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.\" DISCLAIMED. IN NO EVENT SHALL CALDERA INTERNATIONAL, INC. BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
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.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
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.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
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.\" OR OTHERWISE) RISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
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.\" IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.\" @(#)p6 8.1 (Berkeley) 6/8/93
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.\"
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.NH
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SIGNALS \(em INTERRUPTS AND ALL THAT
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.PP
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This section is concerned with how to
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deal gracefully with signals from
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the outside world (like interrupts), and with program faults.
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Since there's nothing very useful that
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can be done from within C about program
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faults, which arise mainly from illegal memory references
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or from execution of peculiar instructions,
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we'll discuss only the outside-world signals:
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.IT interrupt ,
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which is sent when the
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.UC DEL
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character is typed;
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.IT quit ,
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generated by the
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.UC FS
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character;
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.IT hangup ,
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caused by hanging up the phone;
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and
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.IT terminate ,
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generated by the
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.IT kill
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command.
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When one of these events occurs,
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the signal is sent to
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.IT all
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processes which were started
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from the corresponding terminal;
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unless other arrangements have been made,
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the signal
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terminates the process.
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In the
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.IT quit
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case, a core image file is written for debugging
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purposes.
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.PP
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The routine which alters the default action
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is
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called
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.UL signal .
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It has two arguments: the first specifies the signal, and the second
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specifies how to treat it.
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The first argument is just a number code, but the second is the
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address is either a function, or a somewhat strange code
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that requests that the signal either be ignored, or that it be
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given the default action.
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The include file
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.UL signal.h
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gives names for the various arguments, and should always be included
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when signals are used.
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Thus
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.P1
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#include <signal.h>
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...
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signal(SIGINT, SIG_IGN);
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.P2
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causes interrupts to be ignored, while
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.P1
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signal(SIGINT, SIG_DFL);
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.P2
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restores the default action of process termination.
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In all cases,
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.UL signal
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returns the previous value of the signal.
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The second argument to
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.UL signal
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may instead be the name of a function
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(which has to be declared explicitly if
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the compiler hasn't seen it already).
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In this case, the named routine will be called
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when the signal occurs.
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Most commonly this facility is used
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to allow the program to clean up
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unfinished business before terminating, for example to
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delete a temporary file:
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.P1
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#include <signal.h>
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main()
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{
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int onintr();
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if (signal(SIGINT, SIG_IGN) != SIG_IGN)
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signal(SIGINT, onintr);
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/* Process ... */
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exit(0);
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}
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onintr()
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{
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unlink(tempfile);
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exit(1);
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}
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.P2
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.PP
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Why the test and the double call to
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.UL signal ?
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Recall that signals like interrupt are sent to
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.ul
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all
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processes started from a particular terminal.
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Accordingly, when a program is to be run
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non-interactively
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(started by
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.UL & ),
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the shell turns off interrupts for it
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so it won't be stopped by interrupts intended for foreground processes.
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If this program began by announcing that all interrupts were to be sent
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to the
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.UL onintr
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routine regardless,
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that would undo the shell's effort to protect it
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when run in the background.
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.PP
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The solution, shown above, is to test the state of interrupt handling,
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and to continue to ignore interrupts if they are already being ignored.
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The code as written
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depends on the fact that
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.UL signal
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returns the previous state of a particular signal.
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If signals were already being ignored, the process should continue to ignore them;
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otherwise, they should be caught.
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.PP
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A more sophisticated program may wish to intercept
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an interrupt and interpret it as a request
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to stop what it is doing
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and return to its own command-processing loop.
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Think of a text editor:
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interrupting a long printout should not cause it
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to terminate and lose the work
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already done.
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The outline of the code for this case is probably best written like this:
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.P1
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#include <signal.h>
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#include <setjmp.h>
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jmp_buf sjbuf;
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main()
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{
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int (*istat)(), onintr();
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istat = signal(SIGINT, SIG_IGN); /* save original status */
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setjmp(sjbuf); /* save current stack position */
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if (istat != SIG_IGN)
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signal(SIGINT, onintr);
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/* main processing loop */
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}
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.P2
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.P1
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onintr()
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{
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printf("\enInterrupt\en");
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longjmp(sjbuf); /* return to saved state */
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}
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.P2
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The include file
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.UL setjmp.h
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declares the type
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.UL jmp_buf
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an object in which the state
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can be saved.
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.UL sjbuf
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is such an object; it is an array of some sort.
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The
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.UL setjmp
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routine then saves
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the state of things.
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When an interrupt occurs,
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a call is forced to the
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.UL onintr
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routine,
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which can print a message, set flags, or whatever.
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.UL longjmp
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takes as argument an object stored into by
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.UL setjmp ,
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and restores control
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to the location after the call to
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.UL setjmp ,
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so control (and the stack level) will pop back
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to the place in the main routine where
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the signal is set up and the main loop entered.
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Notice, by the way, that
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the signal
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gets set again after an interrupt occurs.
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This is necessary; most signals are automatically
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reset to their default action when they occur.
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.PP
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Some programs that want to detect signals simply can't be stopped
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at an arbitrary point,
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for example in the middle of updating a linked list.
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If the routine called on occurrence of a signal
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sets a flag and then
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returns instead of calling
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.UL exit
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or
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.UL longjmp ,
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execution will continue
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at the exact point it was interrupted.
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The interrupt flag can then be tested later.
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.PP
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There is one difficulty associated with this
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approach.
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Suppose the program is reading the
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terminal when the interrupt is sent.
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The specified routine is duly called; it sets its flag
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and returns.
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If it were really true, as we said
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above, that ``execution resumes at the exact point it was interrupted,''
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the program would continue reading the terminal
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until the user typed another line.
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This behavior might well be confusing, since the user
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might not know that the program is reading;
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he presumably would prefer to have the signal take effect instantly.
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The method chosen to resolve this difficulty
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is to terminate the terminal read when execution
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resumes after the signal, returning an error code
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which indicates what happened.
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.PP
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Thus programs which catch and resume
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execution after signals should be prepared for ``errors''
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which are caused by interrupted
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system calls.
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(The ones to watch out for are reads from a terminal,
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.UL wait ,
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and
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.UL pause .)
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A program
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whose
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.UL onintr
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program just sets
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.UL intflag ,
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resets the interrupt signal, and returns,
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should usually include code like the following when it reads
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the standard input:
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.P1
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if (getchar() == EOF)
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if (intflag)
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/* EOF caused by interrupt */
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else
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/* true end-of-file */
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.P2
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.PP
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A final subtlety to keep in mind becomes important
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when signal-catching is combined with execution of other programs.
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Suppose a program catches interrupts, and also includes
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a method (like ``!'' in the editor)
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whereby other programs can be executed.
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Then the code should look something like this:
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.P1
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if (fork() == 0)
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execl(...);
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signal(SIGINT, SIG_IGN); /* ignore interrupts */
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wait(&status); /* until the child is done */
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signal(SIGINT, onintr); /* restore interrupts */
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.P2
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Why is this?
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Again, it's not obvious but not really difficult.
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Suppose the program you call catches its own interrupts.
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If you interrupt the subprogram,
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it will get the signal and return to its
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main loop, and probably read your terminal.
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But the calling program will also pop out of
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its wait for the subprogram and read your terminal.
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Having two processes reading
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your terminal is very unfortunate,
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since the system figuratively flips a coin to decide
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who should get each line of input.
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A simple way out is to have the parent program
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ignore interrupts until the child is done.
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This reasoning is reflected in the standard I/O library function
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.UL system :
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.P1
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#include <signal.h>
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system(s) /* run command string s */
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char *s;
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{
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int status, pid, w;
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register int (*istat)(), (*qstat)();
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if ((pid = fork()) == 0) {
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execl("/bin/sh", "sh", "-c", s, 0);
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_exit(127);
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}
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istat = signal(SIGINT, SIG_IGN);
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qstat = signal(SIGQUIT, SIG_IGN);
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while ((w = wait(&status)) != pid && w != -1)
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;
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if (w == -1)
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status = -1;
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signal(SIGINT, istat);
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signal(SIGQUIT, qstat);
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return(status);
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}
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.P2
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.PP
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As an aside on declarations,
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the function
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.UL signal
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obviously has a rather strange second argument.
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It is in fact a pointer to a function delivering an integer,
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and this is also the type of the signal routine itself.
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The two values
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.UL SIG_IGN
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and
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.UL SIG_DFL
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have the right type, but are chosen so they coincide with
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no possible actual functions.
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For the enthusiast, here is how they are defined for the PDP-11;
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the definitions should be sufficiently ugly
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and nonportable to encourage use of the include file.
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.P1
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#define SIG_DFL (int (*)())0
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#define SIG_IGN (int (*)())1
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.P2
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