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. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)signal.3 8.3 (Berkeley) 4/19/94 33.\" $FreeBSD$ 34.\" 35.Dd April 19, 1994 36.Dt SIGNAL 3 37.Os 38.Sh NAME 39.Nm signal 40.Nd simplified software signal facilities 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In signal.h 45.\" The following is Quite Ugly, but syntactically correct. Don't try to 46.\" fix it. 47.Ft void \*(lp* 48.Fn signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint" 49.Pp 50or in 51.Fx Ns 's 52equivalent but easier to read typedef'd version: 53.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ; 54.Ft sig_t 55.Fn signal "int sig" "sig_t func" 56.Sh DESCRIPTION 57This 58.Fn signal 59facility 60is a simplified interface to the more general 61.Xr sigaction 2 62facility. 63.Pp 64Signals allow the manipulation of a process from outside its 65domain as well as allowing the process to manipulate itself or 66copies of itself (children). There are two general types of signals: 67those that cause termination of a process and those that do not. 68Signals which cause termination of a program might result from 69an irrecoverable error or might be the result of a user at a terminal 70typing the `interrupt' character. 71Signals are used when a process is stopped because it wishes to access 72its control terminal while in the background (see 73.Xr tty 4 ) . 74Signals are optionally generated 75when a process resumes after being stopped, 76when the status of child processes changes, 77or when input is ready at the control terminal. 78Most signals result in the termination of the process receiving them 79if no action 80is taken; some signals instead cause the process receiving them 81to be stopped, or are simply discarded if the process has not 82requested otherwise. 83Except for the 84.Dv SIGKILL 85and 86.Dv SIGSTOP 87signals, the 88.Fn signal 89function allows for a signal to be caught, to be ignored, or to generate 90an interrupt. 91These signals are defined in the file 92.Aq Pa signal.h : 93.Bl -column SIGVTALARMXX "create core imagexxx" 94.It Sy "Name Default Action Description" 95.It Dv SIGHUP Ta "terminate process" Ta "terminal line hangup" 96.It Dv SIGINT Ta "terminate process" Ta "interrupt program" 97.It Dv SIGQUIT Ta "create core image" Ta "quit program" 98.It Dv SIGILL Ta "create core image" Ta "illegal instruction" 99.It Dv SIGTRAP Ta "create core image" Ta "trace trap" 100.It Dv SIGABRT Ta "create core image" Ta "abort program" 101(formerly 102.Dv SIGIOT ) 103.It Dv SIGEMT Ta "create core image" Ta "emulate instruction executed" 104.It Dv SIGFPE Ta "create core image" Ta "floating-point exception" 105.It Dv SIGKILL Ta "terminate process" Ta "kill program" 106.It Dv SIGBUS Ta "create core image" Ta "bus error" 107.It Dv SIGSEGV Ta "create core image" Ta "segmentation violation" 108.It Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked" 109.It Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader" 110.It Dv SIGALRM Ta "terminate process" Ta "real-time timer expired" 111.It Dv SIGTERM Ta "terminate process" Ta "software termination signal" 112.It Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket" 113.It Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)" 114.It Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard" 115.It Dv SIGCONT Ta "discard signal" Ta "continue after stop" 116.It Dv SIGCHLD Ta "discard signal" Ta "child status has changed" 117.It Dv SIGTTIN Ta "stop process" Ta "background read attempted from control terminal" 118.It Dv SIGTTOU Ta "stop process" Ta "background write attempted to control terminal" 119.It Dv SIGIO Ta "discard signal" Ta Tn "I/O" 120is possible on a descriptor (see 121.Xr fcntl 2 ) 122.It Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see" 123.Xr setrlimit 2 ) 124.It Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see" 125.Xr setrlimit 2 ) 126.It Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see" 127.Xr setitimer 2 ) 128.It Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see" 129.Xr setitimer 2 ) 130.It Dv SIGWINCH Ta "discard signal" Ta "Window size change" 131.It Dv SIGINFO Ta "discard signal" Ta "status request from keyboard" 132.It Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1" 133.It Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2" 134.El 135.Pp 136The 137.Fa sig 138argument specifies which signal was received. 139The 140.Fa func 141procedure allows a user to choose the action upon receipt of a signal. 142To set the default action of the signal to occur as listed above, 143.Fa func 144should be 145.Dv SIG_DFL . 146A 147.Dv SIG_DFL 148resets the default action. 149To ignore the signal 150.Fa func 151should be 152.Dv SIG_IGN . 153This will cause subsequent instances of the signal to be ignored 154and pending instances to be discarded. 155If 156.Dv SIG_IGN 157is not used, 158further occurrences of the signal are 159automatically blocked and 160.Fa func 161is called. 162.Pp 163The handled signal is unblocked when the 164function returns and 165the process continues from where it left off when the signal occurred. 166.Bf -symbolic 167Unlike previous signal facilities, the handler 168func() remains installed after a signal has been delivered. 169.Ef 170.Pp 171For some system calls, if a signal is caught while the call is 172executing and the call is prematurely terminated, 173the call is automatically restarted. 174(The handler is installed using the 175.Dv SA_RESTART 176flag with 177.Xr sigaction 2 . ) 178The affected system calls include 179.Xr read 2 , 180.Xr write 2 , 181.Xr sendto 2 , 182.Xr recvfrom 2 , 183.Xr sendmsg 2 184and 185.Xr recvmsg 2 186on a communications channel or a low speed device 187and during a 188.Xr ioctl 2 189or 190.Xr wait 2 . 191However, calls that have already committed are not restarted, 192but instead return a partial success (for example, a short read count). 193These semantics could be changed with 194.Xr siginterrupt 3 . 195.Pp 196When a process which has installed signal handlers forks, 197the child process inherits the signals. 198All caught signals may be reset to their default action by a call 199to the 200.Xr execve 2 201function; 202ignored signals remain ignored. 203.Pp 204See 205.Xr sigaction 2 206for a list of functions 207that are considered safe for use in signal handlers. 208.Sh RETURN VALUES 209The previous action is returned on a successful call. 210Otherwise, SIG_ERR is returned and the global variable 211.Va errno 212is set to indicate the error. 213.Sh ERRORS 214The 215.Fn signal 216function 217will fail and no action will take place if one of the 218following occur: 219.Bl -tag -width Er 220.It Bq Er EINVAL 221The 222.Fa sig 223argument 224is not a valid signal number. 225.It Bq Er EINVAL 226An attempt is made to ignore or supply a handler for 227.Dv SIGKILL 228or 229.Ev SIGSTOP . 230.El 231.Sh SEE ALSO 232.Xr kill 1 , 233.Xr kill 2 , 234.Xr ptrace 2 , 235.Xr sigaction 2 , 236.Xr sigaltstack 2 , 237.Xr sigprocmask 2 , 238.Xr sigsuspend 2 , 239.Xr fpsetmask 3 , 240.Xr setjmp 3 , 241.Xr siginterrupt 3 , 242.Xr tty 4 243.Sh HISTORY 244This 245.Fn signal 246facility appeared in 247.Bx 4.0 . 248