freebsd-nq/share/doc/psd/04.uprog/p6

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.\"	@(#)p6	8.1 (Berkeley) 6/8/93
.\"
.NH
SIGNALS \(em INTERRUPTS AND ALL THAT
.PP
This section is concerned with how to
deal gracefully with signals from
the outside world (like interrupts), and with program faults.
Since there's nothing very useful that
can be done from within C about program
faults, which arise mainly from illegal memory references
or from execution of peculiar instructions,
we'll discuss only the outside-world signals:
.IT interrupt ,
which is sent when the
.UC DEL
character is typed;
.IT quit ,
generated by the
.UC FS
character;
.IT hangup ,
caused by hanging up the phone;
and
.IT terminate ,
generated by the
.IT kill
command.
When one of these events occurs,
the signal is sent to
.IT  all 
processes which were started
from the corresponding terminal;
unless other arrangements have been made,
the signal
terminates the process.
In the
.IT quit
case, a core image file is written for debugging
purposes.
.PP
The routine which alters the default action
is
called
.UL signal .
It has two arguments: the first specifies the signal, and the second
specifies how to treat it.
The first argument is just a number code, but the second is the
address is either a function, or a somewhat strange code
that requests that the signal either be ignored, or that it be
given the default action.
The include file
.UL signal.h
gives names for the various arguments, and should always be included
when signals are used.
Thus
.P1
#include <signal.h>
 ...
signal(SIGINT, SIG_IGN);
.P2
causes interrupts to be ignored, while
.P1
signal(SIGINT, SIG_DFL);
.P2
restores the default action of process termination.
In all cases,
.UL signal
returns the previous value of the signal.
The second argument to
.UL signal
may instead be the name of a function
(which has to be declared explicitly if
the compiler hasn't seen it already).
In this case, the named routine will be called
when the signal occurs.
Most commonly this facility is used
to allow the program to clean up
unfinished business before terminating, for example to
delete a temporary file:
.P1
#include <signal.h>

main()
{
	int onintr();

	if (signal(SIGINT, SIG_IGN) != SIG_IGN)
		signal(SIGINT, onintr);

	/* Process ... */

	exit(0);
}

onintr()
{
	unlink(tempfile);
	exit(1);
}
.P2
.PP
Why the test and the double call to
.UL signal ?
Recall that signals like interrupt are sent to
.ul
all
processes started from a particular terminal.
Accordingly, when a program is to be run
non-interactively
(started by
.UL & ),
the shell turns off interrupts for it
so it won't be stopped by interrupts intended for foreground processes.
If this program began by announcing that all interrupts were to be sent
to the
.UL onintr
routine regardless,
that would undo the shell's effort to protect it
when run in the background.
.PP
The solution, shown above, is to test the state of interrupt handling,
and to continue to ignore interrupts if they are already being ignored.
The code as written
depends on the fact that
.UL signal
returns the previous state of a particular signal.
If signals were already being ignored, the process should continue to ignore them;
otherwise, they should be caught.
.PP
A more sophisticated program may wish to intercept
an interrupt and interpret it as a request
to stop what it is doing
and return to its own command-processing loop.
Think of a text editor:
interrupting a long printout should not cause it
to terminate and lose the work
already done.
The outline of the code for this case is probably best written like this:
.P1
#include <signal.h>
#include <setjmp.h>
jmp_buf	sjbuf;

main()
{
	int (*istat)(), onintr();

	istat = signal(SIGINT, SIG_IGN);	/* save original status */
	setjmp(sjbuf);	/* save current stack position */
	if (istat != SIG_IGN)
		signal(SIGINT, onintr);

	/* main processing loop */
}
.P2
.P1
onintr()
{
	printf("\enInterrupt\en");
	longjmp(sjbuf);	/* return to saved state */
}
.P2
The include file
.UL setjmp.h
declares the type
.UL jmp_buf
an object in which the state
can be saved.
.UL sjbuf
is such an object; it is an array of some sort.
The
.UL setjmp
routine then saves
the state of things.
When an interrupt occurs,
a call is forced to the
.UL onintr
routine,
which can print a message, set flags, or whatever.
.UL longjmp
takes as argument an object stored into by
.UL setjmp ,
and restores control
to the location after the call to
.UL setjmp ,
so control (and the stack level) will pop back
to the place in the main routine where
the signal is set up and the main loop entered.
Notice, by the way, that
the signal
gets set again after an interrupt occurs.
This is necessary; most signals are automatically
reset to their default action when they occur.
.PP
Some programs that want to detect signals simply can't be stopped
at an arbitrary point,
for example in the middle of updating a linked list.
If the routine called on occurrence of a signal
sets a flag and then
returns instead of calling
.UL exit
or
.UL longjmp ,
execution will continue
at the exact point it was interrupted.
The interrupt flag can then be tested later.
.PP
There is one difficulty associated with this
approach.
Suppose the program is reading the
terminal when the interrupt is sent.
The specified routine is duly called; it sets its flag
and returns.
If it were really true, as we said
above, that ``execution resumes at the exact point it was interrupted,''
the program would continue reading the terminal
until the user typed another line.
This behavior might well be confusing, since the user
might not know that the program is reading;
he presumably would prefer to have the signal take effect instantly.
The method chosen to resolve this difficulty
is to terminate the terminal read when execution
resumes after the signal, returning an error code
which indicates what happened.
.PP
Thus programs which catch and resume
execution after signals should be prepared for ``errors''
which are caused by interrupted
system calls.
(The ones to watch out for are reads from a terminal,
.UL wait ,
and
.UL pause .)
A program
whose
.UL onintr
program just sets
.UL intflag ,
resets the interrupt signal, and returns,
should usually include code like the following when it reads
the standard input:
.P1
if (getchar() == EOF)
	if (intflag)
		/* EOF caused by interrupt */
	else
		/* true end-of-file */
.P2
.PP
A final subtlety to keep in mind becomes important
when signal-catching is combined with execution of other programs.
Suppose a program catches interrupts, and also includes
a method (like ``!'' in the editor)
whereby other programs can be executed.
Then the code should look something like this:
.P1
if (fork() == 0)
	execl(...);
signal(SIGINT, SIG_IGN);	/* ignore interrupts */
wait(&status);	/* until the child is done */
signal(SIGINT, onintr);	/* restore interrupts */
.P2
Why is this?
Again, it's not obvious but not really difficult.
Suppose the program you call catches its own interrupts.
If you interrupt the subprogram,
it will get the signal and return to its
main loop, and probably read your terminal.
But the calling program will also pop out of
its wait for the subprogram and read your terminal.
Having two processes reading
your terminal is very unfortunate,
since the system figuratively flips a coin to decide
who should get each line of input.
A simple way out is to have the parent program
ignore interrupts until the child is done.
This reasoning is reflected in the standard I/O library function
.UL system :
.P1
#include <signal.h>

system(s)	/* run command string s */
char *s;
{
	int status, pid, w;
	register int (*istat)(), (*qstat)();

	if ((pid = fork()) == 0) {
		execl("/bin/sh", "sh", "-c", s, 0);
		_exit(127);
	}
	istat = signal(SIGINT, SIG_IGN);
	qstat = signal(SIGQUIT, SIG_IGN);
	while ((w = wait(&status)) != pid && w != -1)
		;
	if (w == -1)
		status = -1;
	signal(SIGINT, istat);
	signal(SIGQUIT, qstat);
	return(status);
}
.P2
.PP
As an aside on declarations,
the function
.UL signal
obviously has a rather strange second argument.
It is in fact a pointer to a function delivering an integer,
and this is also the type of the signal routine itself.
The two values
.UL SIG_IGN
and
.UL SIG_DFL
have the right type, but are chosen so they coincide with
no possible actual functions.
For the enthusiast, here is how they are defined for the PDP-11;
the definitions should be sufficiently ugly
and nonportable to encourage use of the include file.
.P1
#define	SIG_DFL	(int (*)())0
#define	SIG_IGN	(int (*)())1
.P2