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. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)signal.3 8.3 (Berkeley) 4/19/94 33.\" $FreeBSD: src/lib/libc/gen/signal.3,v 1.17.2.9 2003/03/13 18:05:37 trhodes Exp $ 34.\" $DragonFly: src/lib/libc/gen/signal.3,v 1.6 2007/09/08 20:50:49 swildner Exp $ 35.\" 36.Dd April 19, 1994 37.Dt SIGNAL 3 38.Os 39.Sh NAME 40.Nm signal 41.Nd simplified software signal facilities 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In signal.h 46.\" The following is Quite Ugly, but syntactically correct. Don't try to 47.\" fix it. 48.Ft void \*(lp* 49.Fn signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint" 50.Pp 51or in 52.Dx Ns 's 53equivalent but easier to read typedef'd version: 54.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ; 55.Ft sig_t 56.Fn signal "int sig" "sig_t func" 57.Sh DESCRIPTION 58This 59.Fn signal 60facility 61is a simplified interface to the more general 62.Xr sigaction 2 63facility. 64.Pp 65Signals allow the manipulation of a process from outside its 66domain as well as allowing the process to manipulate itself or 67copies of itself (children). 68There are two general types of signals: 69those that cause termination of a process and those that do not. 70Signals which cause termination of a program might result from 71an irrecoverable error or might be the result of a user at a terminal 72typing the `interrupt' character. 73Signals are used when a process is stopped because it wishes to access 74its control terminal while in the background (see 75.Xr tty 4 ) . 76Signals are optionally generated 77when a process resumes after being stopped, 78when the status of child processes changes, 79or when input is ready at the control terminal. 80Most signals result in the termination of the process receiving them 81if no action 82is taken; some signals instead cause the process receiving them 83to be stopped, or are simply discarded if the process has not 84requested otherwise. 85Except for the 86.Dv SIGKILL 87and 88.Dv SIGSTOP 89signals, the 90.Fn signal 91function allows for a signal to be caught, to be ignored, or to generate 92an interrupt. 93These signals are defined in the file 94.In signal.h : 95.Bl -column ".Dv SIGCKPTEXIT" "create core imagexxx" 96.It Sy "Name Default Action Description" 97.It Dv SIGHUP Ta "terminate process" Ta "terminal line hangup" 98.It Dv SIGINT Ta "terminate process" Ta "interrupt program" 99.It Dv SIGQUIT Ta "create core image" Ta "quit program" 100.It Dv SIGILL Ta "create core image" Ta "illegal instruction" 101.It Dv SIGTRAP Ta "create core image" Ta "trace trap" 102.It Dv SIGABRT Ta "create core image" Ta "abort program" 103(formerly 104.Dv SIGIOT ) 105.It Dv SIGEMT Ta "create core image" Ta "emulate instruction executed" 106.It Dv SIGFPE Ta "create core image" Ta "floating-point exception" 107.It Dv SIGKILL Ta "terminate process" Ta "kill program" 108.It Dv SIGBUS Ta "create core image" Ta "bus error" 109.It Dv SIGSEGV Ta "create core image" Ta "segmentation violation" 110.It Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked" 111.It Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader" 112.It Dv SIGALRM Ta "terminate process" Ta "real-time timer expired" 113.It Dv SIGTERM Ta "terminate process" Ta "software termination signal" 114.It Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket" 115.It Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)" 116.It Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard" 117.It Dv SIGCONT Ta "discard signal" Ta "continue after stop" 118.It Dv SIGCHLD Ta "discard signal" Ta "child status has changed" 119.It Dv SIGTTIN Ta "stop process" Ta "background read attempted from control terminal" 120.It Dv SIGTTOU Ta "stop process" Ta "background write attempted to control terminal" 121.It Dv SIGIO Ta "discard signal" Ta Tn "I/O" 122is possible on a descriptor (see 123.Xr fcntl 2 ) 124.It Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see" 125.Xr setrlimit 2 ) 126.It Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see" 127.Xr setrlimit 2 ) 128.It Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see" 129.Xr setitimer 2 ) 130.It Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see" 131.Xr setitimer 2 ) 132.It Dv SIGWINCH Ta "discard signal" Ta "Window size change" 133.It Dv SIGINFO Ta "discard signal" Ta "status request from keyboard" 134.It Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1" 135.It Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2" 136.It Dv SIGCKPT Ta "checkpoint process" Ta "Checkpoint" 137.It Dv SIGCKPTEXIT Ta "terminate process" Ta "Checkpoint and exit" 138.El 139.Pp 140The 141.Fa sig 142argument specifies which signal was received. 143The 144.Fa func 145procedure allows a user to choose the action upon receipt of a signal. 146To set the default action of the signal to occur as listed above, 147.Fa func 148should be 149.Dv SIG_DFL . 150A 151.Dv SIG_DFL 152resets the default action. 153To ignore the signal 154.Fa func 155should be 156.Dv SIG_IGN . 157This will cause subsequent instances of the signal to be ignored 158and pending instances to be discarded. 159If 160.Dv SIG_IGN 161is not used, 162further occurrences of the signal are 163automatically blocked and 164.Fa func 165is called. 166.Pp 167The handled signal is unblocked when the 168function returns and 169the process continues from where it left off when the signal occurred. 170.Bf -symbolic 171Unlike previous signal facilities, the handler 172func() remains installed after a signal has been delivered. 173.Ef 174.Pp 175For some system calls, if a signal is caught while the call is 176executing and the call is prematurely terminated, 177the call is automatically restarted. 178(The handler is installed using the 179.Dv SA_RESTART 180flag with 181.Xr sigaction 2 . ) 182The affected system calls include 183.Xr read 2 , 184.Xr write 2 , 185.Xr sendto 2 , 186.Xr recvfrom 2 , 187.Xr sendmsg 2 188and 189.Xr recvmsg 2 190on a communications channel or a low speed device 191and during a 192.Xr ioctl 2 193or 194.Xr wait 2 . 195However, calls that have already committed are not restarted, 196but instead return a partial success (for example, a short read count). 197These semantics could be changed with 198.Xr siginterrupt 3 . 199.Pp 200When a process which has installed signal handlers forks, 201the child process inherits the signals. 202All caught signals may be reset to their default action by a call 203to the 204.Xr execve 2 205function; 206ignored signals remain ignored. 207.Pp 208See 209.Xr sigaction 2 210for a list of functions 211that are considered safe for use in signal handlers. 212.Sh RETURN VALUES 213The previous action is returned on a successful call. 214Otherwise, 215.Dv SIG_ERR 216is returned and the global variable 217.Va errno 218is set to indicate the error. 219.Sh ERRORS 220The 221.Fn signal 222function 223will fail and no action will take place if one of the 224following occur: 225.Bl -tag -width Er 226.It Bq Er EINVAL 227The 228.Fa sig 229argument 230is not a valid signal number. 231.It Bq Er EINVAL 232An attempt is made to ignore or supply a handler for 233.Dv SIGKILL 234or 235.Dv SIGSTOP . 236.El 237.Sh SEE ALSO 238.Xr kill 1 , 239.Xr kill 2 , 240.Xr ptrace 2 , 241.Xr sigaction 2 , 242.Xr sigaltstack 2 , 243.Xr sigprocmask 2 , 244.Xr sigsuspend 2 , 245.Xr fpsetmask 3 , 246.Xr setjmp 3 , 247.Xr siginterrupt 3 , 248.Xr tty 4 249.Sh HISTORY 250This 251.Fn signal 252facility appeared in 253.Bx 4.0 . 254