577 lines
16 KiB
Groff
577 lines
16 KiB
Groff
.\" 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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.\" From: @(#)sigaction.2 8.2 (Berkeley) 4/3/94
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.\" $FreeBSD$
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.\"
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.Dd April 3, 1994
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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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union {
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void (*__sa_handler) __P((int));
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void (*__sa_sigaction) __P((int, struct __siginfo *,
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void *));
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} __sigaction_u; /* signal handler */
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int sa_flags; /* see signal options below */
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sigset_t sa_mask; /* signal mask to apply */
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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 normally blocked from further occurrence, the current process
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context is saved, and a new one is built. 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
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of delivery.
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Normally, signal handlers execute on the current stack
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of the process. This may be changed, on a per-handler basis,
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so that signals are taken on a special
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.Em "signal stack" .
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.Pp
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Signal routines normally 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. The signal mask for a process is initialized
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from that of its parent (normally empty). It
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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. The call to the handler
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is arranged so that if the signal handling routine returns
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normally the process will resume execution in the context
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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
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call is made).
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This mask is formed by taking the union of the current signal mask set,
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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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.Pp
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.Fn Sigaction
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assigns an action for a signal specified by
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.Fa sig .
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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 normally 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
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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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The meaning of the various bits is as follows:
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.Bl -tag -offset indent -width SA_RESETHANDXX
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.It Dv SA_NOCLDSTOP
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If this 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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.It Dv SA_NOCLDWAIT
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If this bit is set when calling
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.Fn sigaction
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for the
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.Dv SIGCHLD
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signal, the system will not create zombie processes when children of
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the calling process exit. If the calling process subsequently issues
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a
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.Xr wait 2
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(or equivalent), it blocks until all of the calling process's child
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processes terminate, and then returns a value of -1 with errno set to
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.Er ECHILD .
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.It Dv SA_ONSTACK
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If this bit is set, the system will deliver the signal to the process
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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_NODEFER
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If this bit is set, further occurrences of the delivered signal are
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not masked during the execution of the handler.
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.It Dv SA_RESETHAND
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If this bit is set, the handler is reset back to
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.Dv SIG_DFL
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at the moment the signal is delivered.
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.It Dv SA_SIGINFO
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If this bit is set, the handler function is assumed to be pointed to
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by the sa_sigaction member of struct sigaction and should match the
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prototype shown above or as below in
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.Sx EXAMPLES .
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This bit should not be set when assigning
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.Dv SIG_DFL
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or
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.Dv SIG_IGN .
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.El
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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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Restart 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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.Xr Execve 2
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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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The following is a list of all signals
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with names as in the include file
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.Aq Pa signal.h :
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.Bl -column SIGVTALARMXX "create core imagexxx"
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.It Sy " NAME " " Default Action " " Description"
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.It Dv SIGHUP No " terminate process" " terminal line hangup"
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.It Dv SIGINT No " terminate process" " interrupt program"
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.It Dv SIGQUIT No " create core image" " quit program"
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.It Dv SIGILL No " create core image" " illegal instruction"
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.It Dv SIGTRAP No " create core image" " trace trap"
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.It Dv SIGABRT No " create core image" Xr abort 3
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call (formerly
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.Dv SIGIOT )
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.It Dv SIGEMT No " create core image" " emulate instruction executed"
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.It Dv SIGFPE No " create core image" " floating-point exception"
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.It Dv SIGKILL No " terminate process" " kill program"
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.It Dv SIGBUS No " create core image" " bus error"
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.It Dv SIGSEGV No " create core image" " segmentation violation"
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.It Dv SIGSYS No " create core image" " non-existent system call invoked"
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.It Dv SIGPIPE No " terminate process" " write on a pipe with no reader"
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.It Dv SIGALRM No " terminate process" " real-time timer expired"
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.It Dv SIGTERM No " terminate process" " software termination signal"
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.It Dv SIGURG No " discard signal" " urgent condition present on socket"
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.It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)"
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.It Dv SIGTSTP No " stop process" " stop signal generated from keyboard"
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.It Dv SIGCONT No " discard signal" " continue after stop"
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.It Dv SIGCHLD No " discard signal" " child status has changed"
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.It Dv SIGTTIN No " stop process" " background read attempted from control terminal"
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.It Dv SIGTTOU No " stop process" " background write attempted to control terminal"
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.It Dv SIGIO No " discard signal" Tn " I/O"
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is possible on a descriptor (see
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.Xr fcntl 2 )
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.It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see"
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.Xr setrlimit 2 )
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.It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see"
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.Xr setrlimit 2 )
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.It Dv SIGVTALRM No " terminate process" " virtual time alarm (see"
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.Xr setitimer 2 )
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.It Dv SIGPROF No " terminate process" " profiling timer alarm (see"
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.Xr setitimer 2 )
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.It Dv SIGWINCH No " discard signal" " Window size change"
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.It Dv SIGINFO No " discard signal" " status request from keyboard"
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.It Dv SIGUSR1 No " terminate process" " User defined signal 1"
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.It Dv SIGUSR2 No " terminate process" " User defined signal 2"
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.El
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.Sh NOTE
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The
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.Fa sa_mask
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field 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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Any attempt to do so will be silently ignored.
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.Pp
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The following functions are either reentrant or not interruptible
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by signals and are async-signal safe.
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Therefore applications may
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invoke them, without restriction, from signal-catching functions:
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.Pp
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Base Interfaces:
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.Pp
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.Fn _exit ,
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.Fn access ,
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.Fn alarm ,
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.Fn cfgetispeed ,
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.Fn cfgetospeed ,
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.Fn cfsetispeed ,
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.Fn cfsetospeed ,
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.Fn chdir ,
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.Fn chmod ,
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.Fn chown ,
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.Fn close ,
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.Fn creat ,
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.Fn dup ,
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.Fn dup2 ,
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.Fn execle ,
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.Fn execve ,
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.Fn fcntl ,
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.Fn fork ,
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.Fn fpathconf ,
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.Fn fstat ,
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.Fn fsync ,
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.Fn getegid ,
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.Fn geteuid ,
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.Fn getgid ,
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.Fn getgroups ,
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.Fn getpgrp ,
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.Fn getpid ,
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.Fn getppid ,
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.Fn getuid ,
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.Fn kill ,
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.Fn link ,
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.Fn lseek ,
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.Fn mkdir ,
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.Fn mkfifo ,
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.Fn open ,
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.Fn pathconf ,
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.Fn pause ,
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.Fn pipe ,
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.Fn raise ,
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.Fn read ,
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.Fn rename ,
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.Fn rmdir ,
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.Fn setgid ,
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.Fn setpgid ,
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.Fn setsid ,
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.Fn setuid ,
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.Fn sigaction ,
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.Fn sigaddset ,
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.Fn sigdelset ,
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.Fn sigemptyset ,
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.Fn sigfillset ,
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.Fn sigismember ,
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.Fn signal ,
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.Fn sigpending ,
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.Fn sigprocmask ,
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.Fn sigsuspend ,
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.Fn sleep ,
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.Fn stat ,
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.Fn sysconf ,
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.Fn tcdrain ,
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.Fn tcflow ,
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.Fn tcflush ,
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.Fn tcgetattr ,
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.Fn tcgetpgrp ,
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.Fn tcsendbreak ,
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.Fn tcsetattr ,
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.Fn tcsetpgrp ,
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.Fn time ,
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.Fn times ,
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.Fn umask ,
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.Fn uname ,
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.Fn unlink ,
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.Fn utime ,
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.Fn wait ,
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.Fn waitpid ,
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.Fn write .
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.Pp
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Realtime Interfaces:
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.Pp
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.Fn aio_error ,
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.Fn clock_gettime ,
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.Fn sigpause ,
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.Fn timer_getoverrun ,
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.Fn aio_return ,
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.Fn fdatasync ,
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.Fn sigqueue ,
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.Fn timer_gettime ,
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.Fn aio_suspend ,
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.Fn sem_post ,
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.Fn sigset ,
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.Fn timer_settime .
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.Pp
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All functions not in the above lists are considered to be unsafe
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with respect to signals. That is to say, the behaviour of such
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functions when called from a signal handler is undefined.
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.Sh RETURN VALUES
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A 0 value indicated that the call succeeded. A \-1 return value
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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 EXAMPLES
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There are three possible prototypes the handler may match:
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.Bl -tag -offset indent -width short
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.It ANSI C:
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.Fd
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void handler(int);
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.It Traditional BSD style:
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.Fd
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void handler(int, int code, struct sigcontext *scp);
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.It POSIX SA_SIGINFO:
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.Fd
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void handler(int, siginfo_t *info, void *context);
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.El
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.Pp
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The handler function should match the SA_SIGINFO prototype if the
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SA_SIGINFO bit is set in flags.
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It then should be pointed to by the
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.Dv sa_sigaction
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member of
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.Dv struct sigaction .
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Note that you should not assign SIG_DFL or SIG_IGN this way.
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.Pp
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If the SA_SIGINFO flag is not set, the handler function should match
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either the ANSI C or traditional BSD prototype and be pointed to by
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the
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.Dv sa_handler
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member of
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.Dv struct sigaction .
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In pratice,
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.Fx
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always sends the three arguments of the latter and since the ANSI C
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prototype is a subset, both will work.
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The
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.Dv sa_handler
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member declaration in
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.Fx
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include files is that of ANSI C (as required by POSIX),
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so a function pointer of a BSD-style function needs to be casted to
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compile without warning.
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The traditional BSD style is not portable and since its capabilities
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are a full subset of a SA_SIGINFO handler,
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its use is deprecated.
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.Pp
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The
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.Fa sig
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argument is the signal number, one of the
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.Dv SIG...
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values from <signal.h>.
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.Pp
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The
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.Fa code
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argument of the BSD-style handler and the
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.Dv si_code
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member of the
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.Dv info
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argument to a SA_SIGINFO handler contain a numeric code explaning the
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cause of the signal, usually one of the
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.Dv SI_...
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values from
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<sys/signal.h> or codes specific to a signal, i.e. one of the
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.Dv FPE_...
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values for SIGFPE.
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.Pp
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The
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.Fa scp
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argument to a BSD-style handler points to an instance of struct
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sigcontext.
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.Pp
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The
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.Fa context
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argument to a POSIX SA_SIGINFO handler points to an instance of
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ucontext_t.
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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 call is expected to conform 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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Those signals are available on most
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.Tn BSD Ns \-derived
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systems.
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The
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.Dv SA_NODEFER
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and
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.Dv SA_RESETHAND
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flags are intended for backwards compatibility with other operating
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systems. The
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.Dv SA_NOCLDSTOP ,
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and
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.Dv SA_NOCLDWAIT
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.\" and
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.\" SA_SIGINFO
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flags are featuring options commonly found in other operating 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 sigblock 2 ,
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.Xr sigpause 2 ,
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.Xr sigpending 2 ,
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.Xr sigprocmask 2 ,
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.Xr sigsetmask 2 ,
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.Xr sigsuspend 2 ,
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.Xr sigvec 2 ,
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.Xr wait 2 ,
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.Xr fpsetmask 3 ,
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.Xr setjmp 3 ,
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.Xr siginterrupt 3 ,
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.Xr sigsetops 3 ,
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.Xr tty 4
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