c84cab5626
sigvec(2) references have been updated to sigaction(2), sigsetmask(2) and sigblock(2) to sigprocmask(2), sigpause(2) to sigsuspend(2). Some legacy man pages still refer to them, that is OK.
667 lines
17 KiB
Groff
667 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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.\" 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 18, 2010
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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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void (*sa_handler)(int);
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void (*sa_sigaction)(int, siginfo_t *, void *);
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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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.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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The above declaration of
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.Vt "struct sigaction"
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is not literal.
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It is provided only to list the accessible members.
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See
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.In sys/signal.h
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for the actual definition.
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In particular, the storage occupied by sa_handler and sa_sigaction overlaps,
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and an application can not use both simultaneously.
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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 ,
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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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.Tn ANSI C
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Interfaces:
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.Pp
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.Fn strcpy ,
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.Fn strcat ,
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.Fn strncpy ,
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.Fn strncat ,
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and perhaps some others.
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.Pp
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Extension Interfaces:
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.Pp
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.Fn strlcpy ,
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.Fn strlcat .
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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.
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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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In general though, signal handlers should do little more than set a
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flag; most other actions are not safe.
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.Pp
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Also, it is good practice to make a copy of the global variable
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.Va errno
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and restore it before returning from the signal handler.
|
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This protects against the side effect of
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.Va errno
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being set by functions called from inside the signal handler.
|
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.Sh RETURN VALUES
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.Rv -std sigaction
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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 Tn ANSI C :
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.Ft void
|
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.Fn handler int ;
|
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.It Traditional BSD style:
|
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.Ft void
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.Fn handler int "int code" "struct sigcontext *scp" ;
|
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.It Tn POSIX Dv SA_SIGINFO :
|
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.Ft void
|
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.Fn handler int "siginfo_t *info" "ucontext_t *uap" ;
|
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.El
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.Pp
|
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The handler function should match the
|
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.Dv SA_SIGINFO
|
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prototype if the
|
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.Dv SA_SIGINFO
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bit is set in
|
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.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
|
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.Dv SIG_DFL
|
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or
|
|
.Dv SIG_IGN
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this way.
|
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.Pp
|
|
If the
|
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.Dv SA_SIGINFO
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flag is not set, the handler function should match
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either the
|
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.Tn ANSI C
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or traditional
|
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.Bx
|
|
prototype and be pointed to by
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the
|
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.Va sa_handler
|
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member of
|
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.Vt "struct sigaction" .
|
|
In practice,
|
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.Fx
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always sends the three arguments of the latter and since the
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.Tn ANSI C
|
|
prototype is a subset, both will work.
|
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The
|
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.Va sa_handler
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member declaration in
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.Fx
|
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include files is that of
|
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.Tn ANSI C
|
|
(as required by
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|
.Tn POSIX ) ,
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|
so a function pointer of a
|
|
.Bx Ns -style
|
|
function needs to be casted to
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|
compile without warning.
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|
The traditional
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|
.Bx
|
|
style is not portable and since its capabilities
|
|
are a full subset of a
|
|
.Dv SA_SIGINFO
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|
handler,
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|
its use is deprecated.
|
|
.Pp
|
|
The
|
|
.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
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|
.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 SEE ALSO
|
|
.Xr kill 1 ,
|
|
.Xr kill 2 ,
|
|
.Xr ptrace 2 ,
|
|
.Xr sigaltstack 2 ,
|
|
.Xr sigpending 2 ,
|
|
.Xr sigprocmask 2 ,
|
|
.Xr sigsuspend 2 ,
|
|
.Xr wait 2 ,
|
|
.Xr fpsetmask 3 ,
|
|
.Xr setjmp 3 ,
|
|
.Xr siginfo 3 ,
|
|
.Xr siginterrupt 3 ,
|
|
.Xr sigsetops 3 ,
|
|
.Xr ucontext 3 ,
|
|
.Xr tty 4
|
|
.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 .
|