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.\" 31.Dd April 19, 1994 32.Dt SIGNAL 3 33.Os 34.Sh NAME 35.Nm signal 36.Nd simplified software signal facilities 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In signal.h 41.\" The following is Quite Ugly, but syntactically correct. Don't try to 42.\" fix it. 43.Ft void 44.Fn (*signal "int sig" "void (*func)(int)))(int" 45.Pp 46or in 47.Dx Ns 's 48equivalent but easier to read typedef'd version: 49.Ft typedef "void (*sig_t) (int)" ; 50.Ft sig_t 51.Fn signal "int sig" "sig_t func" 52.Sh DESCRIPTION 53This 54.Fn signal 55facility 56is a simplified interface to the more general 57.Xr sigaction 2 58facility. 59.Pp 60Signals allow the manipulation of a process from outside its 61domain as well as allowing the process to manipulate itself or 62copies of itself (children). 63There are two general types of signals: 64those that cause termination of a process and those that do not. 65Signals which cause termination of a program might result from 66an irrecoverable error or might be the result of a user at a terminal 67typing the `interrupt' character. 68Signals are used when a process is stopped because it wishes to access 69its control terminal while in the background (see 70.Xr tty 4 ) . 71Signals are optionally generated 72when a process resumes after being stopped, 73when the status of child processes changes, 74or when input is ready at the control terminal. 75Most signals result in the termination of the process receiving them 76if no action 77is taken; some signals instead cause the process receiving them 78to be stopped, or are simply discarded if the process has not 79requested otherwise. 80Except for the 81.Dv SIGKILL 82and 83.Dv SIGSTOP 84signals, the 85.Fn signal 86function allows for a signal to be caught, to be ignored, or to generate 87an interrupt. 88These signals are defined in the file 89.In signal.h : 90.Bl -column SIGCKPTEXITXX "create core imagexxx" 91.It Sy NAME Ta Sy Default Action Ta Sy Description 92.It Dv SIGHUP Ta terminate process Ta terminal line hangup 93.It Dv SIGINT Ta terminate process Ta interrupt program 94.It Dv SIGQUIT Ta create core image Ta quit program 95.It Dv SIGILL Ta create core image Ta illegal instruction 96.It Dv SIGTRAP Ta create core image Ta trace trap 97.It Dv SIGABRT Ta create core image Ta abort program (formerly Dv SIGIOT ) 98.It Dv SIGEMT Ta create core image Ta emulate instruction executed 99.It Dv SIGFPE Ta create core image Ta floating-point exception 100.It Dv SIGKILL Ta terminate process Ta kill program 101.It Dv SIGBUS Ta create core image Ta bus error 102.It Dv SIGSEGV Ta create core image Ta segmentation violation 103.It Dv SIGSYS Ta create core image Ta non-existent system call invoked 104.It Dv SIGPIPE Ta terminate process Ta write on a pipe with no reader 105.It Dv SIGALRM Ta terminate process Ta real-time timer expired 106.It Dv SIGTERM Ta terminate process Ta software termination signal 107.It Dv SIGURG Ta discard signal Ta urgent condition present on socket 108.It Dv SIGSTOP Ta stop process Ta stop (cannot be caught or ignored) 109.It Dv SIGTSTP Ta stop process Ta stop signal generated from keyboard 110.It Dv SIGCONT Ta discard signal Ta continue after stop 111.It Dv SIGCHLD Ta discard signal Ta child status has changed 112.It Dv SIGTTIN Ta stop process Ta background read attempted from control terminal 113.It Dv SIGTTOU Ta stop process Ta background write attempted to control terminal 114.It Dv SIGIO Ta discard signal Ta I/O is possible on a descriptor (see Xr fcntl 2 ) 115.It Dv SIGXCPU Ta terminate process Ta cpu time limit exceeded (see Xr setrlimit 2 ) 116.It Dv SIGXFSZ Ta terminate process Ta file size limit exceeded (see Xr setrlimit 2 ) 117.It Dv SIGVTALRM Ta terminate process Ta virtual time alarm (see Xr setitimer 2 ) 118.It Dv SIGPROF Ta terminate process Ta profiling timer alarm (see Xr setitimer 2 ) 119.It Dv SIGWINCH Ta discard signal Ta window size change 120.It Dv SIGINFO Ta discard signal Ta status request from keyboard 121.It Dv SIGUSR1 Ta terminate process Ta user defined signal 1 122.It Dv SIGUSR2 Ta terminate process Ta user defined signal 2 123.It Dv SIGCKPT Ta checkpoint process Ta checkpoint 124.It Dv SIGCKPTEXIT Ta terminate process Ta checkpoint and exit 125.El 126.Pp 127The 128.Fa sig 129argument specifies which signal was received. 130The 131.Fa func 132procedure allows a user to choose the action upon receipt of a signal. 133To set the default action of the signal to occur as listed above, 134.Fa func 135should be 136.Dv SIG_DFL . 137A 138.Dv SIG_DFL 139resets the default action. 140To ignore the signal 141.Fa func 142should be 143.Dv SIG_IGN . 144This will cause subsequent instances of the signal to be ignored 145and pending instances to be discarded. 146If 147.Dv SIG_IGN 148is not used, 149further occurrences of the signal are 150automatically blocked and 151.Fa func 152is called. 153.Pp 154The handled signal is unblocked when the 155function returns and 156the process continues from where it left off when the signal occurred. 157.Bf -symbolic 158Unlike previous signal facilities, the handler 159func() remains installed after a signal has been delivered. 160.Ef 161.Pp 162For some system calls, if a signal is caught while the call is 163executing and the call is prematurely terminated, 164the call is automatically restarted. 165(The handler is installed using the 166.Dv SA_RESTART 167flag with 168.Xr sigaction 2 . ) 169The affected system calls include 170.Xr read 2 , 171.Xr write 2 , 172.Xr sendto 2 , 173.Xr recvfrom 2 , 174.Xr sendmsg 2 175and 176.Xr recvmsg 2 177on a communications channel or a low speed device 178and during a 179.Xr ioctl 2 180or 181.Xr wait 2 . 182However, calls that have already committed are not restarted, 183but instead return a partial success (for example, a short read count). 184These semantics could be changed with 185.Xr siginterrupt 3 . 186.Pp 187When a process which has installed signal handlers forks, 188the child process inherits the signals. 189All caught signals may be reset to their default action by a call 190to the 191.Xr execve 2 192function; 193ignored signals remain ignored. 194.Pp 195See 196.Xr sigaction 2 197for a list of functions 198that are considered safe for use in signal handlers. 199.Sh RETURN VALUES 200The previous action is returned on a successful call. 201Otherwise, 202.Dv SIG_ERR 203is returned and the global variable 204.Va errno 205is set to indicate the error. 206.Sh ERRORS 207The 208.Fn signal 209function 210will fail and no action will take place if one of the 211following occur: 212.Bl -tag -width Er 213.It Bq Er EINVAL 214The 215.Fa sig 216argument 217is not a valid signal number. 218.It Bq Er EINVAL 219An attempt is made to ignore or supply a handler for 220.Dv SIGKILL 221or 222.Dv SIGSTOP . 223.El 224.Sh SEE ALSO 225.Xr kill 1 , 226.Xr kill 2 , 227.Xr ptrace 2 , 228.Xr sigaction 2 , 229.Xr sigaltstack 2 , 230.Xr sigprocmask 2 , 231.Xr sigsuspend 2 , 232.Xr fpsetmask 3 , 233.Xr setjmp 3 , 234.Xr siginterrupt 3 , 235.Xr tty 4 236.Sh HISTORY 237This 238.Fn signal 239facility appeared in 240.Bx 4.0 . 241