628 lines
25 KiB
HTML
628 lines
25 KiB
HTML
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<HEAD>
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<link rel="SHORTCUT ICON" href="http://www.cons.org/favicon.ico">
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<TITLE>Proper handling of SIGINT/SIGQUIT [http://www.cons.org/cracauer/sigint.html]</TITLE>
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<!-- Created by: GNU m4 using $Revision: 1.20 $ of crawww.m4lib on 11-Feb-2005 -->
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<BODY BGCOLOR="#fff8e1">
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<CENTER><H2>Proper handling of SIGINT/SIGQUIT</H2></CENTER>
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<img src=linie.png width="100%" alt=" ">
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<P>
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<table border=1 cellpadding=4>
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<tr><th valign=top align=left>Abstract: </th>
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<td valign=top align=left>
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In UNIX terminal sessions, you usually have a key like
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<code>C-c</code> (Control-C) to immediately end whatever program you
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have running in the foreground. This should work even when the program
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you called has called other programs in turn. Everything should be
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aborted, giving you your command prompt back, no matter how deep the
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call stack is.
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<p>Basically, it's trivial. But the existence of interactive
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applications that use SIGINT and/or SIGQUIT for other purposes than a
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complete immediate abort make matters complicated, and - as was to
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expect - left us with several ways to solve the problems. Of course,
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existing shells and applications follow different ways.
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<P>This Web pages outlines different ways to solve the problem and
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argues that only one of them can do everything right, although it
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means that we have to fix some existing software.
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</td></tr><tr><th valign=top align=left>Intended audience: </th>
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<td valign=top align=left>Programmers who implement programs that catch SIGINT/SIGQUIT.
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<BR>Programmers who implements shells or shell-like programs that
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execute batches of programs.
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<p>Users who have problems problems getting rid of runaway shell
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scripts using <code>Control-C</code>. Or have interactive applications
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that don't behave right when sending SIGINT. Examples are emacs'es
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that die on Control-g or shellscript statements that sometimes are
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executed and sometimes not, apparently not determined by the user's
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intention.
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</td></tr><tr><th valign=top align=left>Required knowledge: </th>
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<td valign=top align=left>You have to know what it means to catch SIGINT or SIGQUIT and how
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processes are waiting for other processes (children) they spawned.
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</td></tr></table>
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<img src=linie.png width="100%" alt=" ">
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<H3>Basic concepts</H3>
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What technically happens when you press Control-C is that all programs
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running in the foreground in your current terminal (or virtual
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terminal) get the signal SIGINT sent.
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<p>You may change the key that triggers the signal using
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<code>stty</code> and running programs may remap the SIGINT-sending
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key at any time they like, without your intervention and without
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asking you first.
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<p>The usual reaction of a running program to SIGINT is to exit.
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However, not all program do an exit on SIGINT, programs are free to
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use the signal for other actions or to ignore it at all.
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<p>All programs running in the foreground receive the signal. This may
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be a nested "stack" of programs: You started a program that started
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another and the outer is waiting for the inner to exit. This nesting
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may be arbitrarily deep.
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<p>The innermost program is the one that decides what to do on SIGINT.
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It may exit, do something else or do nothing. Still, when the user hit
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SIGINT, all the outer programs are awaken, get the signal and may
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react on it.
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<H3>What we try to achieve</H3>
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The problem is with shell scripts (or similar programs that call
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several subprograms one after another).
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<p>Let us consider the most basic script:
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<PRE>
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#! /bin/sh
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program1
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program2
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</PRE>
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and the usual run looks like this:
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<PRE>
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$ sh myscript
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[output of program1]
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[output of program2]
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$
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</PRE>
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<p>Let us assume that both programs do nothing special on SIGINT, they
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just exit.
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<p>Now imagine the user hits C-c while a shellscript is executing its
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first program. The following programs receive SIGINT: program1 and
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also the shell executing the script. program1 exits.
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<p>But what should the shell do? If we say that it is only the
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innermost's programs business to react on SIGINT, the shell will do
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nothing special (not exit) and it will continue the execution of the
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script and run program2. But this is wrong: The user's intention in
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hitting C-c is to abort the whole script, to get his prompt back. If
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he hits C-c while the first program is running, he does not want
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program2 to be even started.
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<p>here is what would happen if the shell doesn't do anything:
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<PRE>
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$ sh myscript
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[first half of program1's output]
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C-c [users presses C-c]
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[second half of program1's output will not be displayed]
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[output of program2 will appear]
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</PRE>
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<p>Consider a more annoying example:
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<pre>
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#! /bin/sh
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# let's assume there are 300 *.dat files
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for file in *.dat ; do
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dat2ascii $dat
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done
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</pre>
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If your shell wouldn't end if the user hits <code>C-c</code>,
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<code>C-c</code> would just end <strong>one</strong> dat2ascii run and
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the script would continue. Thus, you had to hit <code>C-c</code> up to
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300 times to end this script.
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<H3>Alternatives to do so</H3>
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<p>There are several ways to handle abortion of shell scripts when
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SIGINT is received while a foreground child runs:
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<menu>
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<li>As just outlined, the shellscript may just continue, ignoring the
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fact that the user hit <code>C-c</code>. That way, your shellscript -
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including any loops - would continue and you had no chance of aborting
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it except using the kill command after finding out the outermost
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shell's PID. This "solution" will not be discussed further, as it is
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obviously not desirable.
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<p><li>The shell itself exits immediately when it receives SIGINT. Not
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only the program called will exit, but the calling (the
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script-executing) shell. The first variant is to exit the shell (and
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therefore discontinuing execution of the script) immediately, while
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the background program may still be executing (remember that although
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the shell is just waiting for the called program to exit, it is woken
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up and may act). I will call the way of doing things the "IUE" (for
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"immediate unconditional exit") for the rest of this document.
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<p><li>As a variant of the former, when the shell receives SIGINT
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while it is waiting for a child to exit, the shell does not exit
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immediately. but it remembers the fact that a SIGINT happened. After
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the called program exits and the shell's wait ends, the shell will
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exit itself and hence discontinue the script. I will call the way of
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doing things the "WUE" (for "wait and unconditional exit") for the
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rest of this document.
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<p><li>There is also a way that the calling shell can tell whether the
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called program exited on SIGINT and if it ignored SIGINT (or used it
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for other purposes). As in the <sl>WUE</sl> way, the shell waits for
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the child to complete. It figures whether the program was ended on
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SIGINT and if so, it discontinue the script. If the program did any
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other exit, the script will be continued. I will call the way of doing
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things the "WCE" (for "wait and cooperative exit") for the rest of
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this document.
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</menu>
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<H3>The problem</H3>
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On first sight, all three solutions (IUE, WUE and WCE) all seem to do
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what we want: If C-c is hit while the first program of the shell
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script runs, the script is discontinued. The user gets his prompt back
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immediately. So what are the difference between these way of handling
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SIGINT?
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<p>There are programs that use the signal SIGINT for other purposes
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than exiting. They use it as a normal keystroke. The user is expected
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to use the key that sends SIGINT during a perfectly normal program
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run. As a result, the user sends SIGINT in situations where he/she
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does not want the program or the script to end.
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<p>The primary example is the emacs editor: C-g does what ESC does in
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other applications: It cancels a partially executed or prepared
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operation. Technically, emacs remaps the key that sends SIGINT from
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C-c to C-g and catches SIGINT.
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<p>Remember that the SIGINT is sent to all programs running in the
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foreground. If emacs is executing from a shell script, both emacs and
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the shell get SIGINT. emacs is the program that decides what to do:
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Exit on SIGINT or not. emacs decides not to exit. The problem arises
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when the shell draws its own conclusions from receiving SIGINT without
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consulting emacs for its opinion.
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<p>Consider this script:
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<PRE>
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#! /bin/sh
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emacs /tmp/foo
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cp /tmp/foo /home/user/mail/sent
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</PRE>
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<p>If C-g is used in emacs, both the shell and emacs will received
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SIGINT. Emacs will not exit, the user used C-g as a normal editing
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keystroke, he/she does not want the script to be aborted on C-g.
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<p>The central problem is that the second command (cp) may
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unintentionally be killed when the shell draws its own conclusion
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about the user's intention. The innermost program is the only one to
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judge.
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<H3>One more example</H3>
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<p>Imagine a mail session using a curses mailer in a tty. You called
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your mailer and started to compose a message. Your mailer calls emacs.
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<code>C-g</code> is a normal editing key in emacs. Technically it
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sends SIGINT (it was <code>C-c</code>, but emacs remapped the key) to
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<menu>
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<li>emacs
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<li>the shell between your mailer and emacs, the one from your mailers
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system("emacs /tmp/bla.44") command
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<li>the mailer itself
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<li>possibly another shell if your mailer was called by a shell script
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or from another application using system(3)
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<li>your interactive shell (which ignores it since it is interactive
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and hence is not relevant to this discussion)
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</menu>
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<p>If everyone just exits on SIGINT, you will be left with nothing but
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your login shell, without asking.
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<p>But for sure you don't want to be dropped out of your editor and
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out of your mailer back to the commandline, having your edited data
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and mailer status deleted.
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<p>Understand the difference: While <code>C-g</code> is used an a kind
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of abort key in emacs, it isn't the major "abort everything" key. When
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you use <code>C-g</code> in emacs, you want to end some internal emacs
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command. You don't want your whole emacs and mailer session to end.
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<p>So, if the shell exits immediately if the user sends SIGINT (the
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second of the four ways shown above), the parent of emacs would die,
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leaving emacs without the controlling tty. The user will lose it's
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editing session immediately and unrecoverable. If the "main" shell of
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the operating system defaults to this behavior, every editor session
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that is spawned from a mailer or such will break (because it is
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usually executed by system(3), which calls /bin/sh). This was the case
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in FreeBSD before I and Bruce Evans changed it in 1998.
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<p>If the shell recognized that SIGINT was sent and exits after the
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current foreground process exited (the third way of the four), the
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editor session will not be disturbed, but things will still not work
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right.
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<H3>A further look at the alternatives</H3>
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<p>Still considering this script to examine the shell's actions in the
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IUE, WUE and ICE way of handling SIGINT:
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<PRE>
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#! /bin/sh
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emacs /tmp/foo
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cp /tmp/foo /home/user/mail/sent
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</PRE>
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<p>The IUE ("immediate unconditional exit") way does not work at all:
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emacs wants to survive the SIGINT (it's a normal editing key for
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emacs), but its parent shell unconditionally thinks "We received
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SIGINT. Abort everything. Now.". The shell will exit even before emacs
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exits. But this will leave emacs in an unusable state, since the death
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of its calling shell will leave it without required resources (file
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descriptors). This way does not work at all for shellscripts that call
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programs that use SIGINT for other purposes than immediate exit. Even
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for programs that exit on SIGINT, but want to do some cleanup between
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the signal and the exit, may fail before they complete their cleanup.
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<p>It should be noted that this way has one advantage: If a child
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blocks SIGINT and does not exit at all, this way will get control back
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to the user's terminal. Since such programs should be banned from your
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system anyway, I don't think that weighs against the disadvantages.
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<p>WUE ("wait and unconditional exit") is a little more clever: If C-g
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was used in emacs, the shell will get SIGINT. It will not immediately
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exit, but remember the fact that a SIGINT happened. When emacs ends
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(maybe a long time after the SIGINT), it will say "Ok, a SIGINT
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happened sometime while the child was executing, the user wants the
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script to be discontinued". It will then exit. The cp will not be
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executed. But that's bad. The "cp" will be executed when the emacs
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session ended without the C-g key ever used, but it will not be
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executed when the user used C-g at least one time. That is clearly not
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desired. Since C-g is a normal editing key in emacs, the user expects
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the rest of the script to behave identically no matter what keys he
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used.
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<p>As a result, the "WUE" way is better than the "IUE" way in that it
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does not break SIGINT-using programs completely. The emacs session
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will end undisturbed. But it still does not support scripts where
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other actions should be performed after a program that use SIGINT for
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non-exit purposes. Since the behavior is basically undeterminable for
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the user, this can lead to nasty surprises.
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<p>The "WCE" way fixes this by "asking" the called program whether it
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exited on SIGINT or not. While emacs receives SIGINT, it does not exit
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on it and a calling shell waiting for its exit will not be told that
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it exited on SIGINT. (Although it receives SIGINT at some point in
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time, the system does not enforce that emacs will exit with
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"I-exited-on-SIGINT" status. This is under emacs' control, see below).
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<p>this still work for the normal script without SIGINT-using
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programs:</p>
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<PRE>
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#! /bin/sh
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program1
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program2
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</PRE>
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Unless program1 and program2 mess around with signal handling, the
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system will tell the calling shell whether the programs exited
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normally or as a result of SIGINT.
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<p>The "WCE" way then has an easy way to things right: When one called
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program exited with "I-exited-on-SIGINT" status, it will discontinue
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the script after this program. If the program ends without this
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status, the next command in the script is started.
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<p>It is important to understand that a shell in "WCE" modus does not
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need to listen to the SIGINT signal at all. Both in the
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"emacs-then-cp" script and in the "several-normal-programs" script, it
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will be woken up and receive SIGINT when the user hits the
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corresponding key. But the shell does not need to react on this event
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and it doesn't need to remember the event of any SIGINT, either.
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Telling whether the user wants to end a script is done by asking that
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program that has to decide, that program that interprets keystrokes
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from the user, the innermost program.
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<H3>So everything is well with WCE?</H3>
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Well, almost.
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<p>The problem with the "WCE" modus is that there are broken programs
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that do not properly communicate the required information up to the
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calling program.
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<p>Unless a program messes with signal handling, the system does this
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automatically.
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<p>There are programs that want to exit on SIGINT, but they don't let
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the system do the automatic exit, because they want to do some
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cleanup. To do so, they catch SIGINT, do the cleanup and then exit by
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themselves.
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<p>And here is where the problem arises: Once they catch the signal,
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the system will no longer communicate the "I-exited-on-SIGINT" status
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to the calling program automatically. Even if the program exit
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immediately in the signal handler of SIGINT. Once it catches the
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signal, it has to take care of communicating the signal status
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itself.
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<p>Some programs don't do this. On SIGINT, they do cleanup and exit
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immediately, but the calling shell isn't told about the non-normal exit
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and it will call the next program in the script.
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<p>As a result, the user hits SIGINT and while one program exits, the
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shellscript continues. To him/her it looks like the shell fails to
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obey to his abortion command.
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<p>Both IUE or WUE shell would not have this problem, since they
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discontinue the script on their own. But as I said, they don't support
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programs using SIGINT for non-exiting purposes, no matter whether
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these programs properly communicate their signal status to the calling
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shell or not.
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<p>Since some shell in wide use implement the WUE way (and some even
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IUE), there is a considerable number of broken programs out there that
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break WCE shells. The programmers just don't recognize it if their
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shell isn't WCE.
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<H3>How to be a proper program</H3>
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<p>(Short note in advance: What you need to achieve is that
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WIFSIGNALED(status) is true in the calling program and that
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WTERMSIG(status) returns SIGINT.)
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<p>If you don't catch SIGINT, the system automatically does the right
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thing for you: Your program exits and the calling program gets the
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right "I-exited-on-SIGINT" status after waiting for your exit.
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<p>But once you catch SIGINT, you have to act.
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<p>Decide whether the SIGINT is used for exit/abort purposes and hence
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a shellscript calling this program should discontinue. This is
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hopefully obvious. If you just need to do some cleanup on SIGINT, but
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then exit immediately, the answer is "yes".
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<p>If so, you have to tell the calling program about it by exiting
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with the "I-exited-on-SIGINT" status.
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<p>There is no other way of doing this than to kill yourself with a
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SIGINT signal. Do it by resetting the SIGINT handler to SIG_DFL, then
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send yourself the signal.
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<PRE>
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void sigint_handler(int sig)
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{
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<do some cleanup>
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signal(SIGINT, SIG_DFL);
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kill(getpid(), SIGINT);
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}
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</PRE>
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Notes:
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<MENU>
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<LI>You cannot "fake" the proper exit status by an exit(3) with a
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special numeric value. People often assume this since the manuals for
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shells often list some return value for exactly this. But this is just
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a convention for your shell script. It does not work from one UNIX API
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program to another.
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<P>All that happens is that the shell sets the "$?" variable to a
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special numeric value for the convenience of your script, because your
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script does not have access to the lower-lever UNIX status evaluation
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functions. This is just an agreement between your script and the
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executing shell, it does not have any meaning in other contexts.
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<P><LI>Do not use kill(0, SIGINT) without consulting the manul for
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your OS implementation. I.e. on BSD, this would not send the signal to
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the current process, but to all processes in the group.
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<P><LI>POSIX 1003.1 allows all these calls to appear in signal
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handlers, so it is portable.
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</MENU>
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<p>In a bourne shell script, you can catch signals using the
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<code>trap</code> command. Here, the same as for C programs apply. If
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the intention of SIGINT is to end your program, you have to exit in a
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way that the calling programs "sees" that you have been killed. If
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you don't catch SIGINT, this happened automatically, but of you catch
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SIGINT, i.e. to do cleanup work, you have to end the program by
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killing yourself, not by calling exit.
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<p>Consider this example from FreeBSD's <code>mkdep</code>, which is a
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bourne shell script.
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<pre>
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TMP=_mkdep$$
|
|
trap 'rm -f $TMP ; trap 2 ; kill -2 $$' 1 2 3 13 15
|
|
</pre>
|
|
|
|
Yes, you have to do it the hard way. It's even more annoying in shell
|
|
scripts than in C programs since you can't "pre-delete" temporary
|
|
files (which isn't really portable in C, though).
|
|
|
|
<P>All this applies to programs in all languages, not only C and
|
|
bourne shell. Every language implementation that lets you catch SIGINT
|
|
should also give you the option to reset the signal and kill yourself.
|
|
|
|
<P>It is always desirable to exit the right way, even if you don't
|
|
expect your usual callers to depend on it, some unusual one will come
|
|
along. This proper exit status will be needed for WCE and will not
|
|
hurt when the calling shell uses IUE or WUE.
|
|
|
|
<H3>How to be a proper shell</H3>
|
|
|
|
All this applies only for the script-executing case. Most shells will
|
|
also have interactive modes where things are different.
|
|
|
|
<MENU>
|
|
|
|
<LI>Do nothing special when SIGINT appears while you wait for a child.
|
|
You don't even have to remember that one happened.
|
|
|
|
<P><LI>Wait for child to exit, get the exit status. Do not truncate it
|
|
to type char.
|
|
|
|
<P><LI>Look at WIFSIGNALED(status) and WTERMSIG(status) to tell
|
|
whether the child says "I exited on SIGINT: in my opinion the user
|
|
wants the shellscript to be discontinued".
|
|
|
|
<P><LI>If the latter applies, discontinue the script.
|
|
|
|
<P><LI>Exit. But since a shellscript may in turn be called by a
|
|
shellscript, you need to make sure that you properly communicate the
|
|
discontinue intention to the calling program. As in any other program
|
|
(see above), do
|
|
|
|
<PRE>
|
|
signal(SIGINT, SIG_DFL);
|
|
kill(getpid(), SIGINT);
|
|
</PRE>
|
|
|
|
</MENU>
|
|
|
|
<H3>Other remarks</H3>
|
|
|
|
Although this web page talks about SIGINT only, almost the same issues
|
|
apply to SIGQUIT, including proper exiting by killing yourself after
|
|
catching the signal and proper reaction on the WIFSIGNALED(status)
|
|
value. One notable difference for SIGQUIT is that you have to make
|
|
sure that not the whole call tree dumps core.
|
|
|
|
<H3>What to fight</H3>
|
|
|
|
Make sure all programs <em>really</em> kill themselves if they react
|
|
to SIGINT or SIGQUIT and intend to abort their operation as a result
|
|
of this signal. Programs that don't use SIGINT/SIGQUIT as a
|
|
termination trigger - but as part of normal operation - don't kill
|
|
themselves, but do a normal exit instead.
|
|
|
|
<p>Make sure people understand why you can't fake an exit-on-signal by
|
|
doing exit(...) using any numerical status.
|
|
|
|
<p>Make sure you use a shell that behaves right. Especially if you
|
|
develop programs, since it will help seeing problems.
|
|
|
|
<H3>Concrete examples how to fix programs:</H3>
|
|
<ul>
|
|
|
|
<li>The fix for FreeBSD's
|
|
<A HREF="http://www.freebsd.org/cgi/cvsweb.cgi/src/usr.bin/time/time.c.diff?r1=1.10&r2=1.11">time(1)</A>. This fix is the best example, it's quite short and clear and
|
|
it fixes a case where someone tried to fake signal exit status by a
|
|
numerical value. And the complete program is small.
|
|
|
|
<p><li>Fix for FreeBSD's
|
|
<A HREF="http://www.freebsd.org/cgi/cvsweb.cgi/src/usr.bin/truss/main.c.diff?r1=1.9&r2=1.10">truss(1)</A>.
|
|
|
|
<p><li>The fix for FreeBSD's
|
|
<A HREF="http://www.freebsd.org/cgi/cvsweb.cgi/src/usr.bin/mkdep/mkdep.gcc.sh.diff?r1=1.8.2.1&r2=1.8.2.2">mkdep(1)</A>, a shell script.
|
|
|
|
|
|
<p><li>Fix for FreeBSD's make(1), <A HREF="http://www.freebsd.org/cgi/cvsweb.cgi/src/usr.bin/make/job.c.diff?r1=1.9&r2=1.10">part 1</A>,
|
|
<A HREF="http://www.freebsd.org/cgi/cvsweb.cgi/src/usr.bin/make/compat.c.diff?r1=1.10&r2=1.11">part 2</A>.
|
|
|
|
</ul>
|
|
|
|
<H3>Testsuite for shells</H3>
|
|
|
|
I have a collection of shellscripts that test shells for the
|
|
behavior. See my <A HREF="download/">download dir</A> to get the newest
|
|
"sh-interrupt" files, either as a tarfile or as individual file for
|
|
online browsing. This isn't really documented, besides from the
|
|
comments the scripts echo.
|
|
|
|
<H3>Appendix 1 - table of implementation choices</H3>
|
|
|
|
<table border cellpadding=2>
|
|
|
|
<tr valign=top>
|
|
<th>Method sign</th>
|
|
<th>Does what?</th>
|
|
<th>Example shells that implement it:</th>
|
|
<th>What happens when a shellscript called emacs, the user used
|
|
<code>C-g</code> and the script has additional commands in it?</th>
|
|
<th>What happens when a shellscript called emacs, the user did not use
|
|
<code>C-c</code> and the script has additional commands in it?</th>
|
|
<th>What happens if a non-interactive child catches SIGINT?</th>
|
|
<th>To behave properly, children must do what?</th>
|
|
</tr>
|
|
|
|
<tr valign=top align=left>
|
|
<td>IUE</td>
|
|
<td>The shell executing a script exits immediately if it receives
|
|
SIGINT.</td>
|
|
<td>4.4BSD ash (ash), NetBSD, FreeBSD prior to 3.0/22.8</td>
|
|
<td>The editor session is lost and subsequent commands are not
|
|
executed.</td>
|
|
<td>The editor continues as normal and the subsequent commands are
|
|
executed. </td>
|
|
<td>The scripts ends immediately, returning to the caller even before
|
|
the current foreground child of the shell exits. </td>
|
|
<td>It doesn't matter what the child does or how it exits, even if the
|
|
child continues to operate, the shell returns. </td>
|
|
</tr>
|
|
|
|
<tr valign=top align=left>
|
|
<td>WUE</td>
|
|
<td>If the shell executing a script received SIGINT while a foreground
|
|
process was running, it will exit after that child's exit.</td>
|
|
<td>pdksh (OpenBSD /bin/sh)</td>
|
|
<td>The editor continues as normal, but subsequent commands from the
|
|
script are not executed.</td>
|
|
<td>The editor continues as normal and subsequent commands are
|
|
executed. </td>
|
|
<td>The scripts returns to its caller after the current foreground
|
|
child exits, no matter how the child exited. </td>
|
|
<td>It doesn't matter how the child exits (signal status or not), but
|
|
if it doesn't return at all, the shell will not return. In no case
|
|
will further commands from the script be executed. </td>
|
|
</tr>
|
|
|
|
<tr valign=top align=left>
|
|
<td>WCE</td>
|
|
<td>The shell exits if a child signaled that it was killed on a
|
|
signal (either it had the default handler for SIGINT or it killed
|
|
itself). </td>
|
|
<td>bash (Linux /bin/sh), most commercial /bin/sh, FreeBSD /bin/sh
|
|
from 3.0/2.2.8.</td>
|
|
<td>The editor continues as normal and subsequent commands are
|
|
executed. </td>
|
|
<td>The editor continues as normal and subsequent commands are
|
|
executed. </td>
|
|
<td>The scripts returns to its caller after the current foreground
|
|
child exits, but only if the child exited with signal status. If
|
|
the child did a normal exit (even if it received SIGINT, but catches
|
|
it), the script will continue. </td>
|
|
<td>The child must be implemented right, or the user will not be able
|
|
to break shell scripts reliably.</td>
|
|
</tr>
|
|
|
|
</table>
|
|
|
|
<P><img src=linie.png width="100%" alt=" ">
|
|
<BR>©2005 Martin Cracauer <cracauer @ cons.org>
|
|
<A HREF="http://www.cons.org/cracauer/">http://www.cons.org/cracauer/</A>
|
|
<BR>Last changed: $Date: 2005/02/11 21:44:43 $
|
|
</BODY></HTML>
|