1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)signal.3 8.3 (Berkeley) 4/19/94 29.\" $FreeBSD: src/lib/libc/gen/signal.3,v 1.17.2.9 2003/03/13 18:05:37 trhodes Exp $ 30.\" $DragonFly: src/lib/libc/gen/signal.3,v 1.6 2007/09/08 20:50:49 swildner Exp $ 31.\" 32.Dd April 19, 1994 33.Dt SIGNAL 3 34.Os 35.Sh NAME 36.Nm signal 37.Nd simplified software signal facilities 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In signal.h 42.\" The following is Quite Ugly, but syntactically correct. Don't try to 43.\" fix it. 44.Ft void \*(lp* 45.Fn signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint" 46.Pp 47or in 48.Dx Ns 's 49equivalent but easier to read typedef'd version: 50.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ; 51.Ft sig_t 52.Fn signal "int sig" "sig_t func" 53.Sh DESCRIPTION 54This 55.Fn signal 56facility 57is a simplified interface to the more general 58.Xr sigaction 2 59facility. 60.Pp 61Signals allow the manipulation of a process from outside its 62domain as well as allowing the process to manipulate itself or 63copies of itself (children). 64There are two general types of signals: 65those that cause termination of a process and those that do not. 66Signals which cause termination of a program might result from 67an irrecoverable error or might be the result of a user at a terminal 68typing the `interrupt' character. 69Signals are used when a process is stopped because it wishes to access 70its control terminal while in the background (see 71.Xr tty 4 ) . 72Signals are optionally generated 73when a process resumes after being stopped, 74when the status of child processes changes, 75or when input is ready at the control terminal. 76Most signals result in the termination of the process receiving them 77if no action 78is taken; some signals instead cause the process receiving them 79to be stopped, or are simply discarded if the process has not 80requested otherwise. 81Except for the 82.Dv SIGKILL 83and 84.Dv SIGSTOP 85signals, the 86.Fn signal 87function allows for a signal to be caught, to be ignored, or to generate 88an interrupt. 89These signals are defined in the file 90.In signal.h : 91.Bl -column ".Dv SIGCKPTEXIT" "create core imagexxx" 92.It Sy "Name Default Action Description" 93.It Dv SIGHUP Ta "terminate process" Ta "terminal line hangup" 94.It Dv SIGINT Ta "terminate process" Ta "interrupt program" 95.It Dv SIGQUIT Ta "create core image" Ta "quit program" 96.It Dv SIGILL Ta "create core image" Ta "illegal instruction" 97.It Dv SIGTRAP Ta "create core image" Ta "trace trap" 98.It Dv SIGABRT Ta "create core image" Ta "abort program" 99(formerly 100.Dv SIGIOT ) 101.It Dv SIGEMT Ta "create core image" Ta "emulate instruction executed" 102.It Dv SIGFPE Ta "create core image" Ta "floating-point exception" 103.It Dv SIGKILL Ta "terminate process" Ta "kill program" 104.It Dv SIGBUS Ta "create core image" Ta "bus error" 105.It Dv SIGSEGV Ta "create core image" Ta "segmentation violation" 106.It Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked" 107.It Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader" 108.It Dv SIGALRM Ta "terminate process" Ta "real-time timer expired" 109.It Dv SIGTERM Ta "terminate process" Ta "software termination signal" 110.It Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket" 111.It Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)" 112.It Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard" 113.It Dv SIGCONT Ta "discard signal" Ta "continue after stop" 114.It Dv SIGCHLD Ta "discard signal" Ta "child status has changed" 115.It Dv SIGTTIN Ta "stop process" Ta "background read attempted from control terminal" 116.It Dv SIGTTOU Ta "stop process" Ta "background write attempted to control terminal" 117.It Dv SIGIO Ta "discard signal" Ta Tn "I/O" 118is possible on a descriptor (see 119.Xr fcntl 2 ) 120.It Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see" 121.Xr setrlimit 2 ) 122.It Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see" 123.Xr setrlimit 2 ) 124.It Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see" 125.Xr setitimer 2 ) 126.It Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see" 127.Xr setitimer 2 ) 128.It Dv SIGWINCH Ta "discard signal" Ta "Window size change" 129.It Dv SIGINFO Ta "discard signal" Ta "status request from keyboard" 130.It Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1" 131.It Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2" 132.It Dv SIGCKPT Ta "checkpoint process" Ta "Checkpoint" 133.It Dv SIGCKPTEXIT Ta "terminate process" Ta "Checkpoint and exit" 134.El 135.Pp 136The 137.Fa sig 138argument specifies which signal was received. 139The 140.Fa func 141procedure allows a user to choose the action upon receipt of a signal. 142To set the default action of the signal to occur as listed above, 143.Fa func 144should be 145.Dv SIG_DFL . 146A 147.Dv SIG_DFL 148resets the default action. 149To ignore the signal 150.Fa func 151should be 152.Dv SIG_IGN . 153This will cause subsequent instances of the signal to be ignored 154and pending instances to be discarded. 155If 156.Dv SIG_IGN 157is not used, 158further occurrences of the signal are 159automatically blocked and 160.Fa func 161is called. 162.Pp 163The handled signal is unblocked when the 164function returns and 165the process continues from where it left off when the signal occurred. 166.Bf -symbolic 167Unlike previous signal facilities, the handler 168func() remains installed after a signal has been delivered. 169.Ef 170.Pp 171For some system calls, if a signal is caught while the call is 172executing and the call is prematurely terminated, 173the call is automatically restarted. 174(The handler is installed using the 175.Dv SA_RESTART 176flag with 177.Xr sigaction 2 . ) 178The affected system calls include 179.Xr read 2 , 180.Xr write 2 , 181.Xr sendto 2 , 182.Xr recvfrom 2 , 183.Xr sendmsg 2 184and 185.Xr recvmsg 2 186on a communications channel or a low speed device 187and during a 188.Xr ioctl 2 189or 190.Xr wait 2 . 191However, calls that have already committed are not restarted, 192but instead return a partial success (for example, a short read count). 193These semantics could be changed with 194.Xr siginterrupt 3 . 195.Pp 196When a process which has installed signal handlers forks, 197the child process inherits the signals. 198All caught signals may be reset to their default action by a call 199to the 200.Xr execve 2 201function; 202ignored signals remain ignored. 203.Pp 204See 205.Xr sigaction 2 206for a list of functions 207that are considered safe for use in signal handlers. 208.Sh RETURN VALUES 209The previous action is returned on a successful call. 210Otherwise, 211.Dv SIG_ERR 212is returned and the global variable 213.Va errno 214is set to indicate the error. 215.Sh ERRORS 216The 217.Fn signal 218function 219will fail and no action will take place if one of the 220following occur: 221.Bl -tag -width Er 222.It Bq Er EINVAL 223The 224.Fa sig 225argument 226is not a valid signal number. 227.It Bq Er EINVAL 228An attempt is made to ignore or supply a handler for 229.Dv SIGKILL 230or 231.Dv SIGSTOP . 232.El 233.Sh SEE ALSO 234.Xr kill 1 , 235.Xr kill 2 , 236.Xr ptrace 2 , 237.Xr sigaction 2 , 238.Xr sigaltstack 2 , 239.Xr sigprocmask 2 , 240.Xr sigsuspend 2 , 241.Xr fpsetmask 3 , 242.Xr setjmp 3 , 243.Xr siginterrupt 3 , 244.Xr tty 4 245.Sh HISTORY 246This 247.Fn signal 248facility appeared in 249.Bx 4.0 . 250