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