1.\" $NetBSD: signal.3,v 1.20 2002/10/01 16:48:35 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.\" 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.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). 61There are two general types of signals: 62those that cause termination of a process and those that do not. 63Signals which cause termination of a program might result from 64an irrecoverable error or might be the result of a user at a terminal 65typing the `interrupt' character. 66Signals are used when a process is stopped because it wishes to access 67its control terminal while in the background (see 68.Xr tty 4 ) . 69Signals are optionally generated 70when a process resumes after being stopped, 71when the status of child processes changes, 72or when input is ready at the control terminal. 73Most signals result in the termination of the process receiving them 74if no action 75is taken; some signals instead cause the process receiving them 76to be stopped, or are simply discarded if the process has not 77requested otherwise. 78Except for the 79.Dv SIGKILL 80and 81.Dv SIGSTOP 82signals, the 83.Fn signal 84function allows for a signal to be caught, to be ignored, or to generate 85an interrupt. 86See 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. 106If 107.Dv SIG_IGN 108is not used, 109further occurrences of the signal are 110automatically blocked and 111.Fa func 112is called. 113.Pp 114The handled signal is unblocked with the 115function returns and 116the process continues from where it left off when the signal occurred. 117.Bf -symbolic 118Unlike previous signal facilities, the handler 119func() remains installed after a signal has been delivered. 120.Ef 121.Pp 122For some system calls, if a signal is caught while the call is 123executing and the call is prematurely terminated, 124the call is automatically restarted. 125(The handler is installed using the 126.Dv SA_RESTART 127flag with 128.Xr sigaction 2 ) . 129The affected system calls include 130.Xr read 2 , 131.Xr write 2 , 132.Xr sendto 2 , 133.Xr recvfrom 2 , 134.Xr sendmsg 2 135and 136.Xr recvmsg 2 137on a communications channel or a low speed device 138and during a 139.Xr ioctl 2 140or 141.Xr wait 2 . 142However, calls that have already committed are not restarted, 143but instead return a partial success (for example, a short read count). 144.Pp 145When a process which has installed signal handlers forks, 146the child process inherits the signals. 147All caught signals may be reset to their default action by a call 148to the 149.Xr execve 2 150function; 151ignored signals remain ignored. 152.Sh RETURN VALUES 153The previous action is returned on a successful call. 154Otherwise, 155.Dv SIG_ERR 156is returned and the global variable 157.Va errno 158is set to indicate the error. 159.Sh ERRORS 160.Fn signal 161will fail and no action will take place if one of the following occur: 162.Bl -tag -width Er 163.It Bq Er EINVAL 164Specified 165.Em sig 166is not a valid signal number. 167.It Bq Er EINVAL 168An attempt is made to ignore or supply a handler for 169.Dv SIGKILL 170or 171.Dv SIGSTOP . 172.El 173.Sh SEE ALSO 174.Xr kill 1 , 175.Xr kill 2 , 176.Xr ptrace 2 , 177.Xr sigaction 2 , 178.Xr sigaltstack 2 , 179.Xr sigprocmask 2 , 180.Xr sigsuspend 2 , 181.Xr psignal 3 , 182.Xr setjmp 3 , 183.Xr strsignal 3 , 184.Xr tty 4 , 185.Xr signal 7 186.Sh HISTORY 187This 188.Fn signal 189facility appeared in 190.Bx 4.0 . 191