354 lines
9.8 KiB
Groff
354 lines
9.8 KiB
Groff
.\" $NetBSD: sigaction.2,v 1.23 1999/12/11 19:35:48 msaitoh Exp $
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.\"
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.\" Copyright (c) 1980, 1990, 1993
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.\" The Regents of the University of California. 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
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. 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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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)sigaction.2 8.2 (Berkeley) 4/3/94
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.\"
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.Dd November 1, 1997
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.Dt SIGACTION 2
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.Os
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.Sh NAME
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.Nm sigaction
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.Nd software signal facilities
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.Fd #include <signal.h>
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.Bd -literal
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struct sigaction {
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void (*sa_handler)(int);
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sigset_t sa_mask;
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int sa_flags;
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};
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.Ed
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.Ft int
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.Fn sigaction "int sig" "const struct sigaction *act" "struct sigaction *oact"
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.Sh DESCRIPTION
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The system defines a set of signals that may be delivered to a process.
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Signal delivery resembles the occurrence of a hardware interrupt:
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the signal is blocked from further occurrence, the current process
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context is saved, and a new one is built.
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A process may specify a
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.Em handler
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to which a signal is delivered, or specify that a signal is to be
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.Em ignored .
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A process may also specify that a default action is to be taken
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by the system when a signal occurs.
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A signal may also be
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.Em blocked ,
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in which case its delivery is postponed until it is
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.Em unblocked .
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The action to be taken on delivery is determined at the time of delivery.
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Normally, signal handlers execute on the current stack of the process.
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This may be changed, on a per-handler basis, so that signals are
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taken on a special
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.Em "signal stack" .
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.Pp
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Signal routines execute with the signal that caused their
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invocation
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.Em blocked ,
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but other signals may yet occur.
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A global
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.Em "signal mask"
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defines the set of signals currently blocked from delivery
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to a process.
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The signal mask for a process is initialized from that of its parent
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(normally empty).
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It may be changed with a
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.Xr sigprocmask 2
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call, or when a signal is delivered to the process.
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.Pp
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When a signal
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condition arises for a process, the signal is added to a set of
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signals pending for the process.
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If the signal is not currently
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.Em blocked
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by the process then it is delivered to the process.
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Signals may be delivered any time a process enters the operating system
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(e.g., during a system call, page fault or trap, or clock interrupt).
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If multiple signals are ready to be delivered at the same time,
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any signals that could be caused by traps are delivered first.
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Additional signals may be processed at the same time, with each
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appearing to interrupt the handlers for the previous signals
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before their first instructions.
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The set of pending signals is returned by the
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.Xr sigpending 2
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function.
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When a caught signal
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is delivered, the current state of the process is saved,
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a new signal mask is calculated (as described below),
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and the signal handler is invoked.
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The call to the handler is arranged so that if the signal handling
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routine returns normally the process will resume execution in the
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context from before the signal's delivery.
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If the process wishes to resume in a different context, then it
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must arrange to restore the previous context itself.
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.Pp
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When a signal is delivered to a process a new signal mask is
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installed for the duration of the process' signal handler
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(or until a
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.Xr sigprocmask 2
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call is made).
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This mask is formed by taking the union of the current signal mask,
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the signal to be delivered, and
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the signal mask associated with the handler to be invoked,
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.Em sa_mask .
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.Pp
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.Fn sigaction
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assigns an action for a specific signal.
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If
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.Fa act
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is non-zero, it
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specifies an action
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.Pf ( Dv SIG_DFL ,
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.Dv SIG_IGN ,
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or a handler routine) and mask
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to be used when delivering the specified signal.
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If
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.Fa oact
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is non-zero, the previous handling information for the signal
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is returned to the user.
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.Pp
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Once a signal handler is installed, it remains installed
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until another
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.Fn sigaction
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call is made, or an
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.Xr execve 2
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is performed.
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A signal-specific default action may be reset by
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setting
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.Fa sa_handler
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to
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.Dv SIG_DFL .
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Alternately, if the
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.Dv SA_RESETHAND
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bit is set the default action will be reinstated when the signal
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is first posted.
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The defaults are process termination, possibly with core dump;
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no action; stopping the process; or continuing the process.
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See the signal list below for each signal's default action.
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If
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.Fa sa_handler
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is set to
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.Dv SIG_DFL ,
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the default action for the signal is to discard the signal,
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and if a signal is pending,
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the pending signal is discarded even if the signal is masked.
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If
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.Fa sa_handler
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is set to
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.Dv SIG_IGN ,
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current and pending instances
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of the signal are ignored and discarded.
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.Pp
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Options may be specified by setting
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.Em sa_flags .
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If the
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.Dv SA_NOCLDSTOP
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bit is set when installing a catching function
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for the
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.Dv SIGCHLD
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signal,
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the
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.Dv SIGCHLD
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signal will be generated only when a child process exits,
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not when a child process stops.
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Further, if the
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.Dv SA_ONSTACK
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bit is set in
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.Em sa_flags ,
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the system will deliver the signal to the process on a
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.Em "signal stack" ,
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specified with
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.Xr sigaltstack 2 .
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Finally, if the
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.Dv SA_NOCLDWAIT
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bit is set in
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.Em sa_flags ,
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the system will not create a zombie when the child exits, but the child
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process will be automatically waited for.
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.Pp
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If a signal is caught during the system calls listed below,
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the call may be forced to terminate
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with the error
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.Er EINTR ,
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the call may return with a data transfer shorter than requested,
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or the call may be restarted.
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Restarting of pending calls is requested
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by setting the
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.Dv SA_RESTART
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bit in
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.Ar sa_flags .
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The affected system calls include
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.Xr open 2 ,
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.Xr read 2 ,
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.Xr write 2 ,
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.Xr sendto 2 ,
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.Xr recvfrom 2 ,
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.Xr sendmsg 2
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and
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.Xr recvmsg 2
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on a communications channel or a slow device (such as a terminal,
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but not a regular file)
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and during a
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.Xr wait 2
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or
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.Xr ioctl 2 .
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However, calls that have already committed are not restarted,
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but instead return a partial success (for example, a short read count).
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.Pp
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After a
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.Xr fork 2
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or
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.Xr vfork 2
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all signals, the signal mask, the signal stack,
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and the restart/interrupt flags are inherited by the child.
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.Pp
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The
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.Xr execve 2
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system call reinstates the default
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action for all signals which were caught and
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resets all signals to be caught on the user stack.
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Ignored signals remain ignored;
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the signal mask remains the same;
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signals that restart pending system calls continue to do so.
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.Pp
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See
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.Xr signal 7
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for comprehensive list of supported signals.
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.Sh NOTE
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The mask specified in
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.Fa act
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is not allowed to block
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.Dv SIGKILL
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or
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.Dv SIGSTOP .
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This is enforced silently by the system.
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.Sh RETURN VALUES
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A 0 value indicates that the call succeeded.
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A \-1 return value indicates an error occurred and
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.Va errno
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is set to indicated the reason.
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.Sh EXAMPLE
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The handler routine can be declared:
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.Bd -literal -offset indent
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void
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handler(sig, code, scp)
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int sig, code;
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struct sigcontext *scp;
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.Ed
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.Pp
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Here
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.Fa sig
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is the signal number, into which the hardware faults and traps are
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mapped.
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.Fa code
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is a parameter that is either a constant
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or the code provided by the hardware.
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.Fa scp
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is a pointer to the
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.Fa sigcontext
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structure (defined in
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.Aq Pa signal.h ) ,
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used to restore the context from before the signal.
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.Pp
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For POSIX compliance, the
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.Fa sa_handler
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is declared to be (void (*)(int)) and the above handler will need to be
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casted to that type.
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Future versions of
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.Nx
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will replace the
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.Fa sigcontext
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interface with the
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.Fa siginfo
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interface.
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.Sh ERRORS
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.Fn sigaction
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will fail and no new signal handler will be installed if one
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of the following occurs:
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.Bl -tag -width Er
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.It Bq Er EFAULT
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Either
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.Fa act
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or
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.Fa oact
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points to memory that is not a valid part of the process
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address space.
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.It Bq Er EINVAL
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.Fa sig
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is not a valid signal number.
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.It Bq Er EINVAL
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An attempt is made to ignore or supply a handler for
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.Dv SIGKILL
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or
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.Dv SIGSTOP .
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.El
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.Sh STANDARDS
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The
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.Fn sigaction
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function conforms to
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.St -p1003.1-90 .
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The
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.Dv SA_ONSTACK
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and
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.Dv SA_RESTART
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flags are Berkeley extensions,
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as are the signals
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.Dv SIGTRAP ,
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.Dv SIGEMT ,
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.Dv SIGBUS ,
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.Dv SIGSYS ,
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.Dv SIGURG ,
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.Dv SIGIO ,
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.Dv SIGXCPU ,
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.Dv SIGXFSZ ,
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.Dv SIGVTALRM ,
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.Dv SIGPROF ,
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.Dv SIGWINCH ,
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and
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.Dv SIGINFO .
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These signals are available on most
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.Bx Ns \-derived
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systems.
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.Sh SEE ALSO
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.Xr kill 1 ,
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.Xr kill 2 ,
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.Xr ptrace 2 ,
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.Xr sigaltstack 2 ,
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.Xr sigprocmask 2 ,
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.Xr sigsuspend 2 ,
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.Xr setjmp 3 ,
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.Xr sigsetops 3 ,
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.Xr tty 4 ,
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.Xr signal 7
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