1.\" $NetBSD: signal.3,v 1.6 1996/03/26 02:05:55 jtc 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 SYNOPSIS 43.Fd #include <signal.h> 44.Ft void 45.\" can not do this with just .Fn yet 46.br 47.Po 48.Fn *signal "int sig" "void (*func)()" 49.Pc Ns \*(lp\*(rp 50.Sh DESCRIPTION 51This 52.Fn signal 53facility 54is a simplified interface to the more general 55.Xr sigaction 2 56facility. 57.Pp 58Signals allow the manipulation of a process from outside its 59domain as well as allowing the process to manipulate itself or 60copies of itself (children). There are two general types of signals: 61those that cause termination of a process and those that do not. 62Signals which cause termination of a program might result from 63an irrecoverable error or might be the result of a user at a terminal 64typing the `interrupt' character. 65Signals are used when a process is stopped because it wishes to access 66its control terminal while in the background (see 67.Xr tty 4 ) . 68Signals are optionally generated 69when a process resumes after being stopped, 70when the status of child processes changes, 71or when input is ready at the control terminal. 72Most signals result in the termination of the process receiving them 73if no action 74is taken; some signals instead cause the process receiving them 75to be stopped, or are simply discarded if the process has not 76requested otherwise. 77Except for the 78.Dv SIGKILL 79and 80.Dv SIGSTOP 81signals, the 82.Fn signal 83function allows for a signal to be caught, to be ignored, or to generate 84an interrupt. 85These signals are defined in the file 86.Aq Pa signal.h : 87.Bl -column SIGVTALARMXX "create core imagexxx" 88.It Sy " Name " " Default Action " " Description" 89.It Dv SIGHUP No " terminate process" " terminal line hangup" 90.It Dv SIGINT No " terminate process" " interrupt program" 91.It Dv SIGQUIT No " create core image" " quit program" 92.It Dv SIGILL No " create core image" " illegal instruction" 93.It Dv SIGTRAP No " create core image" " trace trap" 94.It Dv SIGABRT No " create core image" Xr abort 2 95call (formerly 96.Dv SIGIOT ) 97.It Dv SIGEMT No " create core image" " emulate instruction executed" 98.It Dv SIGFPE No " create core image" " floating-point exception" 99.It Dv SIGKILL No " terminate process" " kill program" 100.It Dv SIGBUS No " create core image" " bus error" 101.It Dv SIGSEGV No " create core image" " segmentation violation" 102.It Dv SIGSYS No " create core image" " system call given invalid argument" 103.It Dv SIGPIPE No " terminate process" " write on a pipe with no reader" 104.It Dv SIGALRM No " terminate process" " real-time timer expired" 105.It Dv SIGTERM No " terminate process" " software termination signal" 106.It Dv SIGURG No " discard signal" " urgent condition present on socket" 107.It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)" 108.It Dv SIGTSTP No " stop process" " stop signal generated from keyboard" 109.It Dv SIGCONT No " discard signal" " continue after stop" 110.It Dv SIGCHLD No " discard signal" " child status has changed" 111.It Dv SIGTTIN No " stop process" " background read attempted from control terminal" 112.It Dv SIGTTOU No " stop process" " background write attempted to control terminal" 113.It Dv SIGIO No " discard signal" Tn " I/O" 114is possible on a descriptor (see 115.Xr fcntl 2 ) 116.It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see" 117.Xr setrlimit 2 ) 118.It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see" 119.Xr setrlimit 2 ) 120.It Dv SIGVTALRM No " terminate process" " virtual time alarm (see" 121.Xr setitimer 2 ) 122.It Dv SIGPROF No " terminate process" " profiling timer alarm (see" 123.Xr setitimer 2 ) 124.It Dv SIGWINCH No " discard signal" " Window size change" 125.It Dv SIGINFO No " discard signal" " status request from keyboard" 126.It Dv SIGUSR1 No " terminate process" " User defined signal 1" 127.It Dv SIGUSR2 No " terminate process" " User defined signal 2" 128.El 129.Pp 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. If 146.Dv SIG_IGN 147is not used, 148further occurrences of the signal are 149automatically blocked and 150.Fa func 151is called. 152.Pp 153The handled signal is unblocked with the 154function returns and 155the process continues from where it left off when the signal occurred. 156.Bf -symbolic 157Unlike previous signal facilities, the handler 158func() remains installed after a signal has been delivered. 159.Ef 160.Pp 161For some system calls, if a signal is caught while the call is 162executing and the call is prematurely terminated, 163the call is automatically restarted. 164(The handler is installed using the 165.Dv SA_RESTART 166flag with 167.Xr sigaction 2 . ) 168The affected system calls include 169.Xr read 2 , 170.Xr write 2 , 171.Xr sendto 2 , 172.Xr recvfrom 2 , 173.Xr sendmsg 2 174and 175.Xr recvmsg 2 176on a communications channel or a low speed device 177and during a 178.Xr ioctl 2 179or 180.Xr wait 2 . 181However, calls that have already committed are not restarted, 182but instead return a partial success (for example, a short read count). 183.Pp 184When a process which has installed signal handlers forks, 185the child process inherits the signals. 186All caught signals may be reset to their default action by a call 187to the 188.Xr execve 2 189function; 190ignored signals remain ignored. 191.Sh RETURN VALUES 192The previous action is returned on a successful call. 193Otherwise, \-1 is returned and the global variable 194.Va errno 195is set to indicate the error. 196.Sh ERRORS 197.Xr Signal 198will fail and no action will take place if one of the 199following occur: 200.Bl -tag -width Er 201.It Bq Er EINVAL 202.Em Sig 203is not a valid signal number. 204.It Bq Er EINVAL 205An attempt is made to ignore or supply a handler for 206.Dv SIGKILL 207or 208.Ev SIGSTOP . 209.Sh SEE ALSO 210.Xr kill 1 , 211.Xr ptrace 2 , 212.Xr kill 2 , 213.Xr sigaction 2 , 214.Xr sigaltstack 2 , 215.Xr sigprocmask 2 , 216.Xr sigsuspend 2 , 217.Xr setjmp 3 , 218.Xr tty 4 219.Sh HISTORY 220This 221.Nm signal 222facility appeared in 223.Bx 4.0 . 224