2fe8fb192f
There is important information about booting non-ack images in docs/UPDATING. ack/aout-format images can't be built any more, and booting clang/ELF-format ones is a little different. Updating to the new boot monitor is recommended. Changes in this commit: . drop boot monitor -> allowing dropping ack support . facility to copy ELF boot files to /boot so that old boot monitor can still boot fairly easily, see UPDATING . no more ack-format libraries -> single-case libraries . some cleanup of OBJECT_FMT, COMPILER_TYPE, etc cases . drop several ack toolchain commands, but not all support commands (e.g. aal is gone but acksize is not yet). . a few libc files moved to netbsd libc dir . new /bin/date as minix date used code in libc/ . test compile fix . harmonize includes . /usr/lib is no longer special: without ack, /usr/lib plays no kind of special bootstrapping role any more and bootstrapping is done exclusively through packages, so releases depend even less on the state of the machine making them now. . rename nbsd_lib* to lib* . reduce mtree
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.\" $NetBSD: sigaction.2,v 1.43 2006/06/03 18:23:52 christos 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. 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 June 3, 2006
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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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.In signal.h
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.Ft int
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.Fn sigaction "int sig" "const struct sigaction * restrict act" "struct sigaction * restrict 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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Signal masks are represented using the
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.Em sigset_t
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type; the
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.Xr sigsetops 3
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interface is used to modify such data.
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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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.Em "struct sigaction"
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includes the following members:
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.Bd -literal -offset indent
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void (*sa_sigaction)(int sig, siginfo_t *info, void *ctx);
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void (*sa_handler)(int sig);
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sigset_t sa_mask;
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int sa_flags;
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.Ed
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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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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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.Bl -tag -width SA_NOKERNINFO
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.It Dv SA_NODEFER
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If set, then the signal that caused the handler to be executed is not added
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to the list of block signals.
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Please note that
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.Fa sa_mask
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takes precedence over
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.Dv SA_NODEFER ,
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so that if the specified signal is blocked in
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.Fa sa_mask ,
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then
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.Dv SA_NODEFER
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will have no effect.
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.It Dv SA_NOCLDSTOP
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If 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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.It Dv SA_NOCLDWAIT
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If set, the system will not create a zombie when the child exits,
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but the child process will be automatically waited for.
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The same effect can be achieved by setting the signal handler for
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.Dv SIGCHLD
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to
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.Dv SIG_IGN .
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.It Dv SA_ONSTACK
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If set, 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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.It Dv SA_RESETHAND
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If set, the default action will be reinstated when the signal
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is first posted.
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.It Dv SA_RESTART
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Normally, 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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.It Dv SA_SIGINFO
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If set, the signal handler function will receive additional information
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about the caught signal.
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An alternative handler that gets passed additional arguments will
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be called which is named
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.Fa sa_sigaction .
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The
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.Ar sig
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argument of this handler contains the signal number that was caught.
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The
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.Ar info
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argument contains additional signal specific information which
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is listed in
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.Xr siginfo 2 .
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The
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.Ar ctx
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argument
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is a pointer to the
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.Xr ucontext 2
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context where the signal handler will return to.
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.It Dv SA_NOKERNINFO
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This flag is relevant only to
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.Dv SIGINFO ,
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and turns off printing kernel messages on the tty.
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It is similar to the
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.Dv NOKERNINFO
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flag in
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.Xr termios 4 .
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.El
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.Pp
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Only functions that are async-signal-safe can safely be used in signal
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handlers, see
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.Xr signal 7
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for a complete list.
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.Sh NOTES
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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 indicate the reason.
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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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.It Bq Er EINVAL
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The
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.Em sa_flags
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word contains bits other than
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.Dv SA_NOCLDSTOP ,
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.Dv SA_NOCLDWAIT ,
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.Dv SA_NODEFER ,
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.Dv SA_ONSTACK ,
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.Dv SA_RESETHAND ,
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.Dv SA_RESTART ,
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and
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.Dv SA_SIGINFO .
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.El
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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 siginfo 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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.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, available on most
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.Bx Ns \-derived
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systems.
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