1.\" $NetBSD: signal.3,v 1.9 1999/03/09 19:45:09 erh Exp $ 2.\" 3.\" Copyright (c) 1980, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)signal.3 8.3 (Berkeley) 4/19/94 35.\" 36.Dd April 19, 1994 37.Dt SIGNAL 3 38.Os BSD 4 39.Sh NAME 40.Nm signal 41.Nd simplified software signal facilities 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.Fd #include <signal.h> 46.Ft void 47.\" can not do this with just .Fn yet 48.br 49.Po 50.Fn *signal "int sig" "void (*func)()" 51.Pc Ns \*(lp\*(rp 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). There are two general types of signals: 63those that cause termination of a process and those that do not. 64Signals which cause termination of a program might result from 65an irrecoverable error or might be the result of a user at a terminal 66typing the `interrupt' character. 67Signals are used when a process is stopped because it wishes to access 68its control terminal while in the background (see 69.Xr tty 4 ) . 70Signals are optionally generated 71when a process resumes after being stopped, 72when the status of child processes changes, 73or when input is ready at the control terminal. 74Most signals result in the termination of the process receiving them 75if no action 76is taken; some signals instead cause the process receiving them 77to be stopped, or are simply discarded if the process has not 78requested otherwise. 79Except for the 80.Dv SIGKILL 81and 82.Dv SIGSTOP 83signals, the 84.Fn signal 85function allows for a signal to be caught, to be ignored, or to generate 86an interrupt. 87These signals are defined in the file 88.Aq Pa signal.h : 89.Bl -column SIGVTALARMXX "create core imagexxx" 90.It Sy " Name " " Default Action " " Description" 91.It Dv SIGHUP No " terminate process" " terminal line hangup" 92.It Dv SIGINT No " terminate process" " interrupt program" 93.It Dv SIGQUIT No " create core image" " quit program" 94.It Dv SIGILL No " create core image" " illegal instruction" 95.It Dv SIGTRAP No " create core image" " trace trap" 96.It Dv SIGABRT No " create core image" Xr abort 3 97call (formerly 98.Dv SIGIOT ) 99.It Dv SIGEMT No " create core image" " emulate instruction executed" 100.It Dv SIGFPE No " create core image" " floating-point exception" 101.It Dv SIGKILL No " terminate process" " kill program" 102.It Dv SIGBUS No " create core image" " bus error" 103.It Dv SIGSEGV No " create core image" " segmentation violation" 104.It Dv SIGSYS No " create core image" " system call given invalid argument" 105.It Dv SIGPIPE No " terminate process" " write on a pipe with no reader" 106.It Dv SIGALRM No " terminate process" " real-time timer expired" 107.It Dv SIGTERM No " terminate process" " software termination signal" 108.It Dv SIGURG No " discard signal" " urgent condition present on socket" 109.It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)" 110.It Dv SIGTSTP No " stop process" " stop signal generated from keyboard" 111.It Dv SIGCONT No " discard signal" " continue after stop" 112.It Dv SIGCHLD No " discard signal" " child status has changed" 113.It Dv SIGTTIN No " stop process" " background read attempted from control terminal" 114.It Dv SIGTTOU No " stop process" " background write attempted to control terminal" 115.It Dv SIGIO No " discard signal" Tn " I/O" 116is possible on a descriptor (see 117.Xr fcntl 2 ) 118.It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see" 119.Xr setrlimit 2 ) 120.It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see" 121.Xr setrlimit 2 ) 122.It Dv SIGVTALRM No " terminate process" " virtual time alarm (see" 123.Xr setitimer 2 ) 124.It Dv SIGPROF No " terminate process" " profiling timer alarm (see" 125.Xr setitimer 2 ) 126.It Dv SIGWINCH No " discard signal" " Window size change" 127.It Dv SIGINFO No " discard signal" " status request from keyboard" 128.It Dv SIGUSR1 No " terminate process" " User defined signal 1" 129.It Dv SIGUSR2 No " terminate process" " User defined signal 2" 130.El 131.Pp 132The 133.Fa func 134procedure allows a user to choose the action upon receipt of a signal. 135To set the default action of the signal to occur as listed above, 136.Fa func 137should be 138.Dv SIG_DFL . 139A 140.Dv SIG_DFL 141resets the default action. 142To ignore the signal 143.Fa func 144should be 145.Dv SIG_IGN . 146This will cause subsequent instances of the signal to be ignored 147and pending instances to be discarded. If 148.Dv SIG_IGN 149is not used, 150further occurrences of the signal are 151automatically blocked and 152.Fa func 153is called. 154.Pp 155The handled signal is unblocked with the 156function returns and 157the process continues from where it left off when the signal occurred. 158.Bf -symbolic 159Unlike previous signal facilities, the handler 160func() remains installed after a signal has been delivered. 161.Ef 162.Pp 163For some system calls, if a signal is caught while the call is 164executing and the call is prematurely terminated, 165the call is automatically restarted. 166(The handler is installed using the 167.Dv SA_RESTART 168flag with 169.Xr sigaction 2 ) . 170The affected system calls include 171.Xr read 2 , 172.Xr write 2 , 173.Xr sendto 2 , 174.Xr recvfrom 2 , 175.Xr sendmsg 2 176and 177.Xr recvmsg 2 178on a communications channel or a low speed device 179and during a 180.Xr ioctl 2 181or 182.Xr wait 2 . 183However, calls that have already committed are not restarted, 184but instead return a partial success (for example, a short read count). 185.Pp 186When a process which has installed signal handlers forks, 187the child process inherits the signals. 188All caught signals may be reset to their default action by a call 189to the 190.Xr execve 2 191function; 192ignored signals remain ignored. 193.Sh RETURN VALUES 194The previous action is returned on a successful call. 195Otherwise, \-1 is returned and the global variable 196.Va errno 197is set to indicate the error. 198.Sh ERRORS 199.Fn signal 200will fail and no action will take place if one of the following occur: 201.Bl -tag -width Er 202.It Bq Er EINVAL 203.Em Sig 204is not a valid signal number. 205.It Bq Er EINVAL 206An attempt is made to ignore or supply a handler for 207.Dv SIGKILL 208or 209.Dv SIGSTOP . 210.El 211.Sh SEE ALSO 212.Xr kill 1 , 213.Xr ptrace 2 , 214.Xr kill 2 , 215.Xr sigaction 2 , 216.Xr sigaltstack 2 , 217.Xr sigprocmask 2 , 218.Xr sigsuspend 2 , 219.Xr setjmp 3 , 220.Xr tty 4 221.Sh HISTORY 222This 223.Fn signal 224facility appeared in 225.Bx 4.0 . 226