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 1.5 2006/05/26 19:39:36 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). There are two general types of signals: 68those that cause termination of a process and those that do not. 69Signals which cause termination of a program might result from 70an irrecoverable error or might be the result of a user at a terminal 71typing the `interrupt' character. 72Signals are used when a process is stopped because it wishes to access 73its control terminal while in the background (see 74.Xr tty 4 ) . 75Signals are optionally generated 76when a process resumes after being stopped, 77when the status of child processes changes, 78or when input is ready at the control terminal. 79Most signals result in the termination of the process receiving them 80if no action 81is taken; some signals instead cause the process receiving them 82to be stopped, or are simply discarded if the process has not 83requested otherwise. 84Except for the 85.Dv SIGKILL 86and 87.Dv SIGSTOP 88signals, the 89.Fn signal 90function allows for a signal to be caught, to be ignored, or to generate 91an interrupt. 92These signals are defined in the file 93.In signal.h : 94.Bl -column SIGVTALARMXX "create core imagexxx" 95.It Sy "Name Default Action Description" 96.It Dv SIGHUP Ta "terminate process" Ta "terminal line hangup" 97.It Dv SIGINT Ta "terminate process" Ta "interrupt program" 98.It Dv SIGQUIT Ta "create core image" Ta "quit program" 99.It Dv SIGILL Ta "create core image" Ta "illegal instruction" 100.It Dv SIGTRAP Ta "create core image" Ta "trace trap" 101.It Dv SIGABRT Ta "create core image" Ta "abort program" 102(formerly 103.Dv SIGIOT ) 104.It Dv SIGEMT Ta "create core image" Ta "emulate instruction executed" 105.It Dv SIGFPE Ta "create core image" Ta "floating-point exception" 106.It Dv SIGKILL Ta "terminate process" Ta "kill program" 107.It Dv SIGBUS Ta "create core image" Ta "bus error" 108.It Dv SIGSEGV Ta "create core image" Ta "segmentation violation" 109.It Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked" 110.It Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader" 111.It Dv SIGALRM Ta "terminate process" Ta "real-time timer expired" 112.It Dv SIGTERM Ta "terminate process" Ta "software termination signal" 113.It Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket" 114.It Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)" 115.It Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard" 116.It Dv SIGCONT Ta "discard signal" Ta "continue after stop" 117.It Dv SIGCHLD Ta "discard signal" Ta "child status has changed" 118.It Dv SIGTTIN Ta "stop process" Ta "background read attempted from control terminal" 119.It Dv SIGTTOU Ta "stop process" Ta "background write attempted to control terminal" 120.It Dv SIGIO Ta "discard signal" Ta Tn "I/O" 121is possible on a descriptor (see 122.Xr fcntl 2 ) 123.It Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see" 124.Xr setrlimit 2 ) 125.It Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see" 126.Xr setrlimit 2 ) 127.It Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see" 128.Xr setitimer 2 ) 129.It Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see" 130.Xr setitimer 2 ) 131.It Dv SIGWINCH Ta "discard signal" Ta "Window size change" 132.It Dv SIGINFO Ta "discard signal" Ta "status request from keyboard" 133.It Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1" 134.It Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2" 135.It Dv SIGCKPT Ta "checkpoint process" Ta "Checkpoint" 136.It Dv SIGCKPTEXIT Ta "terminate process" Ta "Checkpoint and exit" 137.El 138.Pp 139The 140.Fa sig 141argument specifies which signal was received. 142The 143.Fa func 144procedure allows a user to choose the action upon receipt of a signal. 145To set the default action of the signal to occur as listed above, 146.Fa func 147should be 148.Dv SIG_DFL . 149A 150.Dv SIG_DFL 151resets the default action. 152To ignore the signal 153.Fa func 154should be 155.Dv SIG_IGN . 156This will cause subsequent instances of the signal to be ignored 157and pending instances to be discarded. 158If 159.Dv SIG_IGN 160is not used, 161further occurrences of the signal are 162automatically blocked and 163.Fa func 164is called. 165.Pp 166The handled signal is unblocked when the 167function returns and 168the process continues from where it left off when the signal occurred. 169.Bf -symbolic 170Unlike previous signal facilities, the handler 171func() remains installed after a signal has been delivered. 172.Ef 173.Pp 174For some system calls, if a signal is caught while the call is 175executing and the call is prematurely terminated, 176the call is automatically restarted. 177(The handler is installed using the 178.Dv SA_RESTART 179flag with 180.Xr sigaction 2 . ) 181The affected system calls include 182.Xr read 2 , 183.Xr write 2 , 184.Xr sendto 2 , 185.Xr recvfrom 2 , 186.Xr sendmsg 2 187and 188.Xr recvmsg 2 189on a communications channel or a low speed device 190and during a 191.Xr ioctl 2 192or 193.Xr wait 2 . 194However, calls that have already committed are not restarted, 195but instead return a partial success (for example, a short read count). 196These semantics could be changed with 197.Xr siginterrupt 3 . 198.Pp 199When a process which has installed signal handlers forks, 200the child process inherits the signals. 201All caught signals may be reset to their default action by a call 202to the 203.Xr execve 2 204function; 205ignored signals remain ignored. 206.Pp 207See 208.Xr sigaction 2 209for a list of functions 210that are considered safe for use in signal handlers. 211.Sh RETURN VALUES 212The previous action is returned on a successful call. 213Otherwise, SIG_ERR is returned and the global variable 214.Va errno 215is set to indicate the error. 216.Sh ERRORS 217The 218.Fn signal 219function 220will fail and no action will take place if one of the 221following occur: 222.Bl -tag -width Er 223.It Bq Er EINVAL 224The 225.Fa sig 226argument 227is not a valid signal number. 228.It Bq Er EINVAL 229An attempt is made to ignore or supply a handler for 230.Dv SIGKILL 231or 232.Ev SIGSTOP . 233.El 234.Sh SEE ALSO 235.Xr kill 1 , 236.Xr kill 2 , 237.Xr ptrace 2 , 238.Xr sigaction 2 , 239.Xr sigaltstack 2 , 240.Xr sigprocmask 2 , 241.Xr sigsuspend 2 , 242.Xr fpsetmask 3 , 243.Xr setjmp 3 , 244.Xr siginterrupt 3 , 245.Xr tty 4 246.Sh HISTORY 247This 248.Fn signal 249facility appeared in 250.Bx 4.0 . 251