1.\" $NetBSD: signal.3,v 1.17 2002/02/21 11:09:07 wiz 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 39.Sh NAME 40.Nm signal 41.Nd simplified software signal facilities 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.Fd #include \*[Lt]signal.h\*[Gt] 46.Ft void 47.\" can not do this with just .Fn yet 48.br 49.Po 50.Fn *signal "int sig" "void (*func)(int)" 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. See 87.Xr signal 7 88for comprehensive list of supported signals. 89.Pp 90The 91.Fa func 92procedure allows a user to choose the action upon receipt of a signal. 93To set the default action of the signal to occur as listed above, 94.Fa func 95should be 96.Dv SIG_DFL . 97A 98.Dv SIG_DFL 99resets the default action. 100To ignore the signal 101.Fa func 102should be 103.Dv SIG_IGN . 104This will cause subsequent instances of the signal to be ignored 105and pending instances to be discarded. If 106.Dv SIG_IGN 107is not used, 108further occurrences of the signal are 109automatically blocked and 110.Fa func 111is called. 112.Pp 113The handled signal is unblocked with the 114function returns and 115the process continues from where it left off when the signal occurred. 116.Bf -symbolic 117Unlike previous signal facilities, the handler 118func() remains installed after a signal has been delivered. 119.Ef 120.Pp 121For some system calls, if a signal is caught while the call is 122executing and the call is prematurely terminated, 123the call is automatically restarted. 124(The handler is installed using the 125.Dv SA_RESTART 126flag with 127.Xr sigaction 2 ) . 128The affected system calls include 129.Xr read 2 , 130.Xr write 2 , 131.Xr sendto 2 , 132.Xr recvfrom 2 , 133.Xr sendmsg 2 134and 135.Xr recvmsg 2 136on a communications channel or a low speed device 137and during a 138.Xr ioctl 2 139or 140.Xr wait 2 . 141However, calls that have already committed are not restarted, 142but instead return a partial success (for example, a short read count). 143.Pp 144When a process which has installed signal handlers forks, 145the child process inherits the signals. 146All caught signals may be reset to their default action by a call 147to the 148.Xr execve 2 149function; 150ignored signals remain ignored. 151.Sh RETURN VALUES 152The previous action is returned on a successful call. 153Otherwise, 154.Dv SIG_ERR 155is returned and the global variable 156.Va errno 157is set to indicate the error. 158.Sh ERRORS 159.Fn signal 160will fail and no action will take place if one of the following occur: 161.Bl -tag -width Er 162.It Bq Er EINVAL 163Specified 164.Em sig 165is not a valid signal number. 166.It Bq Er EINVAL 167An attempt is made to ignore or supply a handler for 168.Dv SIGKILL 169or 170.Dv SIGSTOP . 171.El 172.Sh SEE ALSO 173.Xr kill 1 , 174.Xr kill 2 , 175.Xr ptrace 2 , 176.Xr sigaction 2 , 177.Xr sigaltstack 2 , 178.Xr sigprocmask 2 , 179.Xr sigsuspend 2 , 180.Xr setjmp 3 , 181.Xr tty 4 , 182.Xr signal 7 183.Sh HISTORY 184This 185.Fn signal 186facility appeared in 187.Bx 4.0 . 188