Atari FTP/ATARI_FTP_0693.zip/ATARI_FTP_0693/Mint/mint095b.zoo/doc/signal.doc

Text File | 1992-05-17 | 21KB | 472 lines

An Introduction to Signals Under MiNT

MiNT introduces the new (to TOS) concept of a signal. If you're
familiar with Unix/Posix signals, then MiNT signals will will be
easy to learn; but note that there are some (not so subtle) differences
between MiNT and Unix!

What is a Signal?

A signal is a small non-negative integer that represents an exceptional event;
something that is very urgent. It's somewhat like an interrupt or
exception to the CPU, only it's implemented in the operating system
instead of in the hardware. Like many exceptions (bus errors, etc.) signals
are usually fatal. Most signals can be caught by programs (so that a
program-defined routine is called) or ignored; if a signal is caught or
ignored, it is no longer fatal. Signals can also be blocked; a
blocked signal is not acted upon until it is unblocked.

A signal is said to be sent to a process when the exceptional
condition related to that signal occurs, or when another process sends
the signal with the Pkill system call. The signal is said to
be delivered to the process when that process wakes up and
begins to take whatever actions are appropriate for the signal. Note
that there may be a considerable time interval between the sending of
a signal and its delivery. For example, if process A has blocked the
SIGHUP signal (signal 1), then no SIGHUP will be delivered to it until
it has unblocked that signal, even if process B sends it SIGHUP with
the Pkill system call. Note also that a signal is not
necessarily delivered the same number of times that it is sent. If both
process B and process C send SIGHUP to process A, when process A
unblocks SIGHUP only one SIGHUP will be delivered. This is because signals
are like flags; once a flag has been set (the signal is sent) setting
the flag again will have no effect until it has been cleared (the signal
is delivered).

What Signals Are There?

There are 32 possible signals, 0-31. Not all of these have been assigned
a meaning under MiNT. Here are the ones that have been given a meaning;
we give the symbolic name for the signal, the corresponding integer,
and the "traditional" meaning for the process that the signal is sent to.
Any signal not listed here should be considered as "reserved" and should not
be used by applications.

Unless otherwise noted, the default action for signals is to terminate
the process.

#define    SIGNULL        0    [no default action, signal is never delivered]
This isn't really a signal at all; it is never delivered to processes
and has no effect. It exists only so that processes can test to see if
a particular child process has exited, by attempting to send SIGNULL
to the child. If the child exists, the attempt will succeed but nothing
will be done. If the child has terminated, the caller will get an error.
It is not possible to catch or block this signal, since it is never
sent to processes anyway.

#define SIGHUP        1
"The terminal that you're connected to is no longer valid." This signal
is commonly sent by, for example, window managers when the user has
closed the window. Processes should not attempt any I/O to their
controlling terminal after receiving this signal, and indeed should probably
exit unless the user has specifically asked them to continue.

#define SIGINT        2
"Please stop what you're doing." This signal is sent when the user
presses control-C. It usually means that the process should exit; interactive
processes may wish to catch SIGINT so that the user can use it to
break out of time-consuming tasks.

#define SIGQUIT        3
"Stop what you're doing, something's gone wrong!" This signal is sent when
the user presses control-\. It usually indicates a desire to immediately
abort the process because of an error that the user has noticed. It is
generally thought to be "stronger" than SIGINT, and exiting (perhaps after
cleaning up data structures) is an appropriate response to this.

#define SIGILL        4
"An illegal instruction has been encountered." This corresponds to the
680x0 illegal instruction trap, and usually indicates a very serious error;
catching this signal is generally unwise.

#define SIGTRAP        5
"The single-step trace trap has been encountered." This corresponds to the
680x0 trace trap, and is usually activated (and handled) by debuggers;
user programs shouldn't catch this.

#define SIGABRT        6
"An awful error has occured." This is commonly sent by the abort library
function, and indicates that something has gone very, very wrong (for
example, data structures have been unexpectedly corrupted). It is unlikely
that anything useful can be done after this signal is sent; programs
should not normally catch or ignore SIGABRT.

#define SIGPRIV        7
"Privilege violation." An attempt has been made to execute an instruction
in user mode that is normally restricted to supervisor mode; this corresponds
to the 680x0 privilege violation exception, and indicates a serious error.

#define SIGFPE        8
"Division by zero or a floating point error has occured." Processes may
ignore or catch this signal (which corresponds to the 680x0 division by zero
trap) and deal with it as they see fit.

#define SIGKILL        9        [cannot be blocked or caught]
"Die!" This signal will never be seen by a process; nor can processes
block it. Sending SIGKILL is a way to be sure of killing a run-away
process. Use it only as a last resort, since it gives the process no
chance to clean up and exit gracefully.

#define SIGBUS        10
"Bus error." Corresponds to the 680x0 bus error exception, and indicates
a very serious error; programs should generally not attempt to ignore
or catch this signal.

#define SIGSEGV        11
"Illegal memory reference." Corresponds to the 680x0 address error
exception, and indicates a very serious error; catching or ignoring this
signal is not recommended.

#define SIGSYS        12
"Bad argument to a system call." This signal is sent when an illegal
(usually, out of range) parameter is sent to a system call, and when that
system call does not have any "nice" way to report errors. For example,
Super(0L) when the system is already in supervisor mode causes SIGSYS to
be raised. Note that the kernel does not always detect illegal/out of
range arguments to system calls, only sometimes.

#define SIGPIPE        13
"A pipe you were writing to has no readers." Programs may catch this signal
and attempt to exit gracefully after receiving it; note that exiting is
appropriate because the standard output is probably no longer connected
to anything.

#define SIGALRM        14
"The alarm you set earlier has happened." This signal is sent to processes
when the alarm clock set by Talarm (q.v.) expires. It's very
common to catch this signal and from the signal handler jump to a known
point in the program; for example, to indicate a timeout while attempting
to communicate over a serial line.

#define SIGTERM        15
"Please die." This is a polite form of SIGKILL (#9). Programs should
respect this nice request; they may want to catch the signal to perform
some cleanup actions, but they should then exit (since if they don't,
the user will probably get mad and send SIGKILL later anyway...). This
is the signal that is sent when a process is dragged to the trashcan on
the desktop.

#define SIGSTOP        17        [default action: suspend the process]
"Suspend yourself." This signal is sent to a process when it should be
stopped temporarily. SIGSTOP is used primarily by debuggers and similar
programs; suspensions requested directly by the user usually are signalled
by SIGTSTP (q.v.) This signal cannot be ignored, blocked, or caught.

#define SIGTSTP        18        [default action: suspend the process]
"The user is asking you to suspend yourself." This signal is sent immediately
when the user presses the control-Z key, and is sent when the process tries
to read a control-Y key in cooked mode (for a delayed stop). In both cases,
the process should suspend itself. Since this is the default action, no
special handling is normally required, although some programs may wish to
save the screen state and restore it when they are unsuspended.

#define SIGCONT        19        [default action: continue a stopped
                     process]
"You are being restarted after having been suspended." This signal is
sent by shells to resume a suspended process. If the process is not
suspended, the signal does nothing. This signal cannot be blocked, but
it *is* possible to install a handler for it (this is rarely necessary,
though).

#define SIGCHLD        20        [default action: no action taken]
"One of your children has been suspended or has exited." This signal is
sent by the kernel to the parent of any process that is terminated (either
because of a signal or by a Pterm, Pterm0, or Ptermres system call) or
which is suspended because of a signal. Programs that are concerned with the
status of their children (for example, shells) may wish to catch this signal;
after a SIGCHLD has been received, the Pwait3 system call may be
used to determine exactly which child has exited or been suspended.
Note that the Psigaction system call may be used to force SIGCHLD to be
delivered only when a child process terminates (so that suspension of
child processes, for example via job control, does not raise SIGCHLD.

#define SIGTTIN        21        [default action: suspended the process]
"Attempt to read from a terminal you don't own." This signal is sent to
any process that attempts to do input from a terminal with a different
process group than their own. Usually, this happens if the user has started
the job in the background; the process will be suspended until the user
explicitly brings it to the foreground with the appropriate shell command
(at which time the shell will reset the terminal's process group and
send the stopped process a SIGCONT signal to tell it to continue).
[NOTE: in fact, SIGTTIN and SIGTTOU are sent to all processes in the same
process group as the process that attempted to do the i/o; this is for
compatibility with Unix, and it simplifies the implementation of job
control shells.]

#define SIGTTOU        22        [default action: suspend the process]
"Attempt to write to a terminal that you don't own." Similar to SIGTTIN (q.v.).
Processes should normally respect the user's job control and should
override or ignore SIGTTOU only in situations where a very critical error
has occured and a message must be printed immediately.

#define SIGXCPU        24
"Your CPU time limit has been exhausted." Sent to processes when they have
consumed more than the maximum number of milliseconds of CPU time allowed
by the Psetlimit() system call. The signal will continue to be sent to
the process until it exits; if a process does catch this signal, it should
do whatever clean up actions are necessary and then terminate.

#define SIGWINCH    28        [default action: no action taken]
"The window you were running in has changed size." This signal is sent
to processes by some window managers to indicate that the user has changed
the size of the window the process is running in. If the process cares
about the window size, it may catch this signal and use an Fcntl call
to inquire about the new window size when the signal is received.
(See the documentation for Fcntl for details.)

#define SIGUSR1        29
#define SIGUSR2        30

These two signals are reserved for applications, which may define whatever
meaning they wish for them. Note, however, that these signals do
terminate processes by default, so don't send them to a process which
isn't prepared to deal with them.


System Calls Dealing With Signals


WORD
Pkill( WORD pid, WORD sig )

If pid > 0, then the given signal (see the numbers above) is sent to the
process with that pid.
If pid == 0, then the given signal is sent to all members of the process
group of the process making the Pkill call. This includes, of course,
the process itself.
If pid < 0, the signal is sent to all members of process group (-pid).

Returns:
0     for successful sending of the signal
    (Note that if the current process is a recipient of the signal,
    and the signal proves to be fatal, then Pkill will never return.)
ERANGE    if "sig" is less than 0 or greater than 31
EFILNF    if pid > 0 and the indicated process has terminated or does not
    exist, or if pid < 0 and there are no processes in the given
    process group
EACCDN  if the sending process is not authorized to send signals to
    the specified receiving process or group (for example, they
    belong to different users)


#define SIG_DFL (0L)
#define SIG_IGN (1L)

LONG
Psignal( WORD sig, LONG handler )

Change the handling of the indicated signal.

If "handler" is SIG_DFL, then the default action for the signal will occur when
the signal is delivered to the current process.

If "handler" is SIG_IGN, then the signal will be ignored by the process, and
delivery of the signal will have no noticeable effect (in particular, the
signal will not interrupt the Pause system call, q.v.). If the signal
is pending at the time of the Psignal call, it is discarded.

If "handler" is any other value, it is assumed to be the address of a
user function that will be called when the signal is delivered to the
process. The user function is called with a single LONG argument on
the stack, which is the number of the signal being delivered (this is done
so that processes may use the same handler for a number of different
signals). While the signal is being handled, it is blocked from delivery;
thus, signal handling is "reliable" (unlike Version 7 and early System V
Unix implementations, in which delivery of a second signal while it
was being handled could kill the process). Also note that, unlike some
versions of Unix, the signal handling is *not* reset to the default action
before the handler is called; it remains set to the given signal handler.

The signal handler must either return (via a normal 680x0 rts instruction)
or call the Psigreturn system call to indicate when signal handling is
complete; in both cases, the signal will be unblocked. Psigreturn also
performs some internal clean-up of the kernel stack that is necessary if
the signal handler is not planning to return (for example, if the C
longjmp() function is to be used to continue execution at another point
in the program).

Signal handlers may make any GEMDOS, BIOS, or XBIOS system calls freely.
GEM AES and VDI calls should not be made in a signal handler.

Note that calling Psignal to change behavior of a signal has the side
effect of unmasking that signal, so that delivery is possible. This is done
so that processes may, while handling a signal, reset the behavior and
send themselves another instance of the signal, for example in order
to suspend themselves while handling a job control signal.

Signal handling is preserved across Pfork and Pvfork calls. Signals
that are ignored by the parent are also ignored by the child after a Pexec
call; signals that were being caught for handling in a function are reset
in the child to the default behavior.

Returns:
The old value of handler on success.
ERANGE    if sig < 1 or sig > 31
EACCDN    if sig cannot be caught by the user (i.e. SIGKILL or SIGSTOP)

Bugs:
Signal handling can be nested only a small (around 3) number of times,
i.e. if 4 signals are delivered to a process, and the process has established
handlers for all 4, and none of the handlers has returned or called
Psigreturn, then there is a very good chance of a stack overflow killing
the process off. In practice, this is unlikely to happen.


LONG
Psigblock( LONG amask )

Block receipt of some signals. The "amask" argument is added to the
current set of signals being masked, i.e. the new set of blocked signals
is the union of the old set and the set represented by amask. Sets of
blocked signals are represented by a 32 bit unsigned long quantity, with
bit (1L << sig) set if signal "sig" is to be blocked, and clear if not.

Blocked signals remain blocked across Pfork and Pvfork calls. For Pexec
calls, children always start out with an empty set of blocked signals,
regardless of which signals are blocked in the parent.

Returns:
The old set of blocked signals (i.e. the set as it was before amask
was added to it).

NOTE: Certain signals (SIGKILL, SIGSTOP, SIGCONT) cannot be blocked;
if the corresponding bits are set in amask, the kernel will clear them
but will not report an error.


LONG
Psigsetmask( LONG mask )

Decide which signals are to be blocked from delivery. Unlike Psigblock
(which adds to the set of blocked signals) Psigsetmask changes the
entire set, replacing the old set of blocked signals with the one
specified in "mask". As with Psigblock, signal n is blocked from
delivery if bit (1L << n) is set in mask. Note that certain signals
cannot be blocked, and if the corresponding bits are set in the
mask the kernel will clear them.

Returns:
The old set of blocked signals.

Usage:
Typically, Psigblock and Psigsetmask are used together to temporarily
block signals, e.g.:

    oldmask = Psigblock( (1L << SIGINT) );
    ... do some things with SIGINT blocked from delivery ...
    (void) Psigsetmask(oldmask);



LONG
Psigpending()

Give an indication of what signals are pending (i.e. have been sent to
the process but not yet delivered, probably because they are blocked
from delivery).

Returns:
A 32 bit unsigned long representing the set of signals that are pending.
Signal n is pending if bit (1L << n) is set in the returned value.


void
Psigreturn()

Terminate signal handling. This call should be used by any signal
handler that is not planning to return to the kernel (i.e. if the
handler is going to execute a non-local jump to another point in the
program). It cleans up the signal delivery stack and unblocks the
signal that was being delivered. Calling Psigreturn when no signal
is being delivered is harmless.

Bugs:
Calling Psigreturn from a signal handler, and then actually returning
from that handler, is likely to produce extremely unpleasant results.
Don't do it.


void
Pause()

Wait until a signal is delivered. This call will return after any (non-fatal)
signal has been delivered to the process. Note that signals that are being
ignored are never delivered.


void
Psigpause( LONG mask )

Block the set of signals specified by "mask", and then wait until
a signal is delivered. This call will return after any (non-fatal)
signal has been delivered to the process. Before returning, the signal
mask is restored to its original value.



struct sigaction {
    LONG    sa_handler;
    LONG    sa_mask;
    WORD    sa_flags;
#define SA_NOCLDSTOP 1;
}

WORD
Psigaction( WORD sig, struct sigaction *act, struct sigaction *act )

Like Psignal, this call changes the handling of the indicated signal.
If "act" is non-zero, it is a pointer to a structure which describes
the new signal handling behavior, as follows:

sa_handler    is the function to be called when the signal is delivered,
        or SIG_DFL, or SIG_IGN. For further information about
        signal handling functions, see Psignal.
sa_mask        is a set of additional signals to mask while the signal
        is being delivered. The signal itself is always masked,
        but sometimes it might be desireable to mask some
        other signals as well; this allows a "prioritization"
        of signals
sa_flags    extra flags which can affect the behavior of the
        signal. This field should normally be set to 0.
        If "sig" is SIGCHLD and "flags" is SA_NOCLDSTOP,
        then the SIGCHLD signal will not be delivered to
        this process when one of its children are stopped.

If "oact" is non-zero, then the old signal handling behavior is
copied into "oact" and returned.

Returns:
0 on success
ERANGE    if sig < 1 or sig > 31
EACCDN    if sig cannot be caught by the user

Usage:
If "sig" is a valid signal, the call:
    foo = Psignal(sig, newfunc)
is equivalent to:
    struct sigaction newact, oact;
    newact->sa_mask=0L;
    newact->sa_flags=0;
    newact->sa_handler = newfunc;
    Psigaction(sig, &newact, &oact);
    foo = oact->sa_handler;


LONG
Talarm( LONG s )

If s > 0, schedule a SIGALRM signal to occur in s seconds. This alarm
will replace any previously scheduled alarm.
If s = 0, cancel any previously scheduled alarm.
If s < 0, inquire about a scheduled alarm but do not change it.

Returns:
If an alarm was previously scheduled, returns the number of seconds remaining
until that previously scheduled alarm. Otherwise, returns 0.

Bugs:
Internal calculations are done in milliseconds, not seconds, so the returned
value is not exactly accurate.
For the same reason, setting an alarm more than 2 million seconds or so
into the future will not work correctly.