273 lines
9.1 KiB
Groff
273 lines
9.1 KiB
Groff
.\" Copyright (c) 1980, 1991, 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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.\" @(#)signal.3 8.3 (Berkeley) 4/19/94
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.\" $FreeBSD$
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.\"
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.Dd June 7, 2004
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.Dt SIGNAL 3
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.Os
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.Sh NAME
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.Nm signal
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.Nd simplified 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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.\" The following is Quite Ugly, but syntactically correct.
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.\" Don't try to fix it.
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.Ft void
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.Fn \*(lp*signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint"
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.Pp
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or in
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.Fx Ap s
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equivalent but easier to read typedef'd version:
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.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ;
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.Pp
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.Ft sig_t
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.Fn signal "int sig" "sig_t func"
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.Sh DESCRIPTION
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This
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.Fn signal
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facility
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is a simplified interface to the more general
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.Xr sigaction 2
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facility.
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.Pp
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Signals allow the manipulation of a process from outside its
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domain as well as allowing the process to manipulate itself or
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copies of itself (children).
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There are two general types of signals:
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those that cause termination of a process and those that do not.
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Signals which cause termination of a program might result from
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an irrecoverable error or might be the result of a user at a terminal
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typing the `interrupt' character.
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Signals are used when a process is stopped because it wishes to access
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its control terminal while in the background (see
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.Xr tty 4 ) .
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Signals are optionally generated
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when a process resumes after being stopped,
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when the status of child processes changes,
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or when input is ready at the control terminal.
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Most signals result in the termination of the process receiving them
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if no action
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is taken; some signals instead cause the process receiving them
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to be stopped, or are simply discarded if the process has not
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requested otherwise.
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Except for the
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.Dv SIGKILL
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and
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.Dv SIGSTOP
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signals, the
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.Fn signal
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function allows for a signal to be caught, to be ignored, or to generate
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an interrupt.
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These signals are defined in the file
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.In signal.h :
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.Bl -column No ".Dv SIGVTALRM" "create core image"
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.It Sy "Num" Ta Sy "Name" Ta Sy "Default Action" Ta Sy "Description"
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.It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup"
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.It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program"
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.It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program"
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.It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction"
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.It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap"
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.It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program"
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(formerly
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.Dv SIGIOT )
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.It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed"
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.It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception"
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.It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program"
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.It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error"
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.It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation"
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.It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked"
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.It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader"
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.It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired"
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.It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal"
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.It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket"
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.It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)"
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.It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard"
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.It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop"
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.It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed"
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.It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from"
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control terminal
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.It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to"
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control terminal
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.It 23 Ta Dv SIGIO Ta "discard signal" Ta 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 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see"
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.Xr setrlimit 2 )
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.It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see"
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.Xr setrlimit 2 )
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.It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see"
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.Xr setitimer 2 )
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.It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see"
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.Xr setitimer 2 )
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.It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change"
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.It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard"
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.It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1"
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.It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2"
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.It 32 Ta Dv SIGTHR Ta "terminate process" Ta "thread interrupt"
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.It 33 Ta Dv SIGLIBRT Ta "terminate process" Ta "real-time library interrupt"
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.El
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.Pp
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The
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.Fa sig
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argument specifies which signal was received.
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The
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.Fa func
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procedure allows a user to choose the action upon receipt of a signal.
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To set the default action of the signal to occur as listed above,
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.Fa func
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should be
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.Dv SIG_DFL .
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A
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.Dv SIG_DFL
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resets the default action.
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To ignore the signal
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.Fa func
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should be
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.Dv SIG_IGN .
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This will cause subsequent instances of the signal to be ignored
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and pending instances to be discarded.
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If
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.Dv SIG_IGN
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is not used,
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further occurrences of the signal are
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automatically blocked and
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.Fa func
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is called.
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.Pp
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The handled signal is unblocked when the
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function returns and
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the process continues from where it left off when the signal occurred.
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.Bf -symbolic
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Unlike previous signal facilities, the handler
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func() remains installed after a signal has been delivered.
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.Ef
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.Pp
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For some system calls, if a signal is caught while the call is
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executing and the call is prematurely terminated,
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the call is automatically restarted.
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Any handler installed with
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.Xr signal 3
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will have the
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.Dv SA_RESTART
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flag set, meaning that any restartable system call will not return on
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receipt of a signal.
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The affected system calls include
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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 low speed device
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and during a
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.Xr ioctl 2
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or
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.Xr wait 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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These semantics could be changed with
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.Xr siginterrupt 3 .
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.Pp
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When a process which has installed signal handlers forks,
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the child process inherits the signals.
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All caught signals may be reset to their default action by a call
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to the
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.Xr execve 2
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function;
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ignored signals remain ignored.
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.Pp
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If a process explicitly specifies
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.Dv SIG_IGN
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as the action for the signal
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.Dv SIGCHLD ,
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the system will not create zombie processes when children
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of the calling process exit.
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As a consequence, the system will discard the exit status
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from the child processes.
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If the calling process subsequently issues a call to
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.Xr wait 2
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or equivalent, it will block until all of the calling process's
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children terminate, and then return 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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.Pp
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See
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.Xr sigaction 2
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for a list of functions
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that are considered safe for use in signal handlers.
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.Sh RETURN VALUES
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The previous action is returned on a successful call.
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Otherwise, SIG_ERR is returned and the global variable
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.Va errno
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is set to indicate the error.
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.Sh ERRORS
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The
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.Fn signal
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function
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will fail and no action will take place if one of the
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following occur:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The
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.Fa sig
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argument
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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 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 sigaction 2 ,
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.Xr sigaltstack 2 ,
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.Xr sigprocmask 2 ,
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.Xr sigsuspend 2 ,
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.Xr 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 tty 4
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.Sh HISTORY
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The
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.Nm
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facility appeared in
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.Bx 4.0 .
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The option to avoid the creation of child zombies through ignoring
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.Dv SIGCHLD
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appeared in
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.Fx 5.0 .
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