669 lines
17 KiB
Groff
669 lines
17 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 June 7, 2004
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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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.Bd -literal
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struct sigaction {
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union {
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void (*__sa_handler)(int);
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void (*__sa_sigaction)(int, struct __siginfo *, 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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#define sa_handler __sigaction_u.__sa_handler
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#define sa_sigaction __sigaction_u.__sa_sigaction
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.Ed
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.Ft int
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.Fo sigaction
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.Fa "int sig"
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.Fa "const struct sigaction * restrict act"
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.Fa "struct sigaction * restrict oact"
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.Fc
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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.
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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
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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.
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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.
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The signal mask for a process is initialized
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from that of its parent (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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system call.
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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
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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 2
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system 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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The
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.Fn sigaction
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system call
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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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.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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system 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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.Va 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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.Va 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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.Va 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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.Va 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.
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If the calling process subsequently issues 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
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.Va errno
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set to
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.Er ECHILD .
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The same effect of avoiding zombie creation can also be achieved by setting
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.Va sa_handler
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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 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_RESTART
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See paragraph below.
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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 by the
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.Va sa_sigaction
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member of
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.Vt "struct sigaction"
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and should match the 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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.Va 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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The following is a list of all signals
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with names as in the include file
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.In 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" Ta 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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.Va 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 ,
|
|
.Fn setuid ,
|
|
.Fn sigaction ,
|
|
.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 ,
|
|
.Fn sigsuspend ,
|
|
.Fn sleep ,
|
|
.Fn stat ,
|
|
.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 ,
|
|
.Fn tcgetpgrp ,
|
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.Fn tcsendbreak ,
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|
.Fn tcsetattr ,
|
|
.Fn tcsetpgrp ,
|
|
.Fn time ,
|
|
.Fn times ,
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|
.Fn umask ,
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|
.Fn uname ,
|
|
.Fn unlink ,
|
|
.Fn utime ,
|
|
.Fn wait ,
|
|
.Fn waitpid ,
|
|
.Fn write .
|
|
.Pp
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|
Realtime Interfaces:
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|
.Pp
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|
.Fn aio_error ,
|
|
.Fn clock_gettime ,
|
|
.Fn sigpause ,
|
|
.Fn timer_getoverrun ,
|
|
.Fn aio_return ,
|
|
.Fn fdatasync ,
|
|
.Fn sigqueue ,
|
|
.Fn timer_gettime ,
|
|
.Fn aio_suspend ,
|
|
.Fn sem_post ,
|
|
.Fn sigset ,
|
|
.Fn timer_settime .
|
|
.Pp
|
|
.Tn ANSI C
|
|
Interfaces:
|
|
.Pp
|
|
.Fn strcpy ,
|
|
.Fn strcat ,
|
|
.Fn strncpy ,
|
|
.Fn strncat ,
|
|
and perhaps some others.
|
|
.Pp
|
|
Extension Interfaces:
|
|
.Pp
|
|
.Fn strlcpy ,
|
|
.Fn strlcat .
|
|
.Pp
|
|
All functions not in the above lists are considered to be unsafe
|
|
with respect to signals.
|
|
That is to say, the behaviour of such
|
|
functions when called from a signal handler is undefined.
|
|
In general though, signal handlers should do little more than set a
|
|
flag; most other actions are not safe.
|
|
.Pp
|
|
Also, it is good practice to make a copy of the global variable
|
|
.Va errno
|
|
and restore it before returning from the signal handler.
|
|
This protects against the side effect of
|
|
.Va errno
|
|
being set by functions called from inside the signal handler.
|
|
.Sh RETURN VALUES
|
|
.Rv -std sigaction
|
|
.Sh EXAMPLES
|
|
There are three possible prototypes the handler may match:
|
|
.Bl -tag -offset indent -width short
|
|
.It Tn ANSI C :
|
|
.Ft void
|
|
.Fn handler int ;
|
|
.It Traditional BSD style:
|
|
.Ft void
|
|
.Fn handler int "int code" "struct sigcontext *scp" ;
|
|
.It Tn POSIX Dv SA_SIGINFO :
|
|
.Ft void
|
|
.Fn handler int "siginfo_t *info" "ucontext_t *uap" ;
|
|
.El
|
|
.Pp
|
|
The handler function should match the
|
|
.Dv SA_SIGINFO
|
|
prototype if the
|
|
.Dv SA_SIGINFO
|
|
bit is set in
|
|
.Va sa_flags .
|
|
It then should be pointed to by the
|
|
.Va sa_sigaction
|
|
member of
|
|
.Vt "struct sigaction" .
|
|
Note that you should not assign
|
|
.Dv SIG_DFL
|
|
or
|
|
.Dv SIG_IGN
|
|
this way.
|
|
.Pp
|
|
If the
|
|
.Dv SA_SIGINFO
|
|
flag is not set, the handler function should match
|
|
either the
|
|
.Tn ANSI C
|
|
or traditional
|
|
.Bx
|
|
prototype and be pointed to by
|
|
the
|
|
.Va sa_handler
|
|
member of
|
|
.Vt "struct sigaction" .
|
|
In practice,
|
|
.Fx
|
|
always sends the three arguments of the latter and since the
|
|
.Tn ANSI C
|
|
prototype is a subset, both will work.
|
|
The
|
|
.Va sa_handler
|
|
member declaration in
|
|
.Fx
|
|
include files is that of
|
|
.Tn ANSI C
|
|
(as required by
|
|
.Tn POSIX ) ,
|
|
so a function pointer of a
|
|
.Bx Ns -style
|
|
function needs to be casted to
|
|
compile without warning.
|
|
The traditional
|
|
.Bx
|
|
style is not portable and since its capabilities
|
|
are a full subset of a
|
|
.Dv SA_SIGINFO
|
|
handler,
|
|
its use is deprecated.
|
|
.Pp
|
|
The
|
|
.Fa sig
|
|
argument is the signal number, one of the
|
|
.Dv SIG...
|
|
values from
|
|
.In signal.h .
|
|
.Pp
|
|
The
|
|
.Fa code
|
|
argument of the
|
|
.Bx Ns -style
|
|
handler and the
|
|
.Va si_code
|
|
member of the
|
|
.Fa info
|
|
argument to a
|
|
.Dv SA_SIGINFO
|
|
handler contain a numeric code explaining the
|
|
cause of the signal, usually one of the
|
|
.Dv SI_...
|
|
values from
|
|
.In sys/signal.h
|
|
or codes specific to a signal, i.e., one of the
|
|
.Dv FPE_...
|
|
values for
|
|
.Dv SIGFPE .
|
|
.Pp
|
|
The
|
|
.Fa scp
|
|
argument to a
|
|
.Bx Ns -style
|
|
handler points to an instance of
|
|
.Vt "struct sigcontext" .
|
|
.Pp
|
|
The
|
|
.Fa uap
|
|
argument to a
|
|
.Tn POSIX
|
|
.Dv SA_SIGINFO
|
|
handler points to an instance of
|
|
ucontext_t.
|
|
.Sh ERRORS
|
|
The
|
|
.Fn sigaction
|
|
system call
|
|
will fail and no new signal handler will be installed if one
|
|
of the following occurs:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EFAULT
|
|
Either
|
|
.Fa act
|
|
or
|
|
.Fa oact
|
|
points to memory that is not a valid part of the process
|
|
address space.
|
|
.It Bq Er EINVAL
|
|
The
|
|
.Fa sig
|
|
argument
|
|
is not a valid signal number.
|
|
.It Bq Er EINVAL
|
|
An attempt is made to ignore or supply a handler for
|
|
.Dv SIGKILL
|
|
or
|
|
.Dv SIGSTOP .
|
|
.El
|
|
.Sh STANDARDS
|
|
The
|
|
.Fn sigaction
|
|
system call is expected to conform to
|
|
.St -p1003.1-90 .
|
|
The
|
|
.Dv SA_ONSTACK
|
|
and
|
|
.Dv SA_RESTART
|
|
flags are Berkeley extensions,
|
|
as are the signals,
|
|
.Dv SIGTRAP ,
|
|
.Dv SIGEMT ,
|
|
.Dv SIGBUS ,
|
|
.Dv SIGSYS ,
|
|
.Dv SIGURG ,
|
|
.Dv SIGIO ,
|
|
.Dv SIGXCPU ,
|
|
.Dv SIGXFSZ ,
|
|
.Dv SIGVTALRM ,
|
|
.Dv SIGPROF ,
|
|
.Dv SIGWINCH ,
|
|
and
|
|
.Dv SIGINFO .
|
|
Those signals are available on most
|
|
.Bx Ns \-derived
|
|
systems.
|
|
The
|
|
.Dv SA_NODEFER
|
|
and
|
|
.Dv SA_RESETHAND
|
|
flags are intended for backwards compatibility with other operating
|
|
systems.
|
|
The
|
|
.Dv SA_NOCLDSTOP ,
|
|
and
|
|
.Dv SA_NOCLDWAIT
|
|
.\" and
|
|
.\" SA_SIGINFO
|
|
flags are featuring options commonly found in other operating systems.
|
|
The flags are approved by
|
|
.St -susv2 ,
|
|
along with the option to avoid zombie creation by ignoring
|
|
.Dv SIGCHLD .
|
|
.Sh SEE ALSO
|
|
.Xr kill 1 ,
|
|
.Xr kill 2 ,
|
|
.Xr ptrace 2 ,
|
|
.Xr sigaltstack 2 ,
|
|
.Xr sigblock 2 ,
|
|
.Xr sigpause 2 ,
|
|
.Xr sigpending 2 ,
|
|
.Xr sigprocmask 2 ,
|
|
.Xr sigsetmask 2 ,
|
|
.Xr sigsuspend 2 ,
|
|
.Xr sigvec 2 ,
|
|
.Xr wait 2 ,
|
|
.Xr fpsetmask 3 ,
|
|
.Xr setjmp 3 ,
|
|
.Xr siginterrupt 3 ,
|
|
.Xr sigsetops 3 ,
|
|
.Xr ucontext 3 ,
|
|
.Xr tty 4
|