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 BSD 4 38.Sh NAME 39.Nm signal 40.Nd simplified software signal facilities 41.Sh SYNOPSIS 42.Fd #include <signal.h> 43.\" The following is Quite Ugly, but syntactically correct. Don't try to 44.\" fix it. 45.Ft void \*(lp* 46.Fn signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint" 47 48or in FreeBSD's equivalent but easier to read typedef'd version: 49.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" 50.Ft sig_t 51.Fn signal "int sig" "sig_t func" 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 Ta "terminate process" Ta "terminal line hangup" 92.It Dv SIGINT Ta "terminate process" Ta "interrupt program" 93.It Dv SIGQUIT Ta "create core image" Ta "quit program" 94.It Dv SIGILL Ta "create core image" Ta "illegal instruction" 95.It Dv SIGTRAP Ta "create core image" Ta "trace trap" 96.It Dv SIGABRT Ta "create core image" Ta Xr abort 2 97call (formerly 98.Dv SIGIOT ) 99.It Dv SIGEMT Ta "create core image" Ta "emulate instruction executed" 100.It Dv SIGFPE Ta "create core image" Ta "floating-point exception" 101.It Dv SIGKILL Ta "terminate process" Ta "kill program" 102.It Dv SIGBUS Ta "create core image" Ta "bus error" 103.It Dv SIGSEGV Ta "create core image" Ta "segmentation violation" 104.It Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked" 105.It Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader" 106.It Dv SIGALRM Ta "terminate process" Ta "real-time timer expired" 107.It Dv SIGTERM Ta "terminate process" Ta "software termination signal" 108.It Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket" 109.It Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)" 110.It Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard" 111.It Dv SIGCONT Ta "discard signal" Ta "continue after stop" 112.It Dv SIGCHLD Ta "discard signal" Ta "child status has changed" 113.It Dv SIGTTIN Ta "stop process" Ta "background read attempted from control terminal" 114.It Dv SIGTTOU Ta "stop process" Ta "background write attempted to control terminal" 115.It Dv SIGIO Ta "discard signal" Ta Tn "I/O" 116is possible on a descriptor (see 117.Xr fcntl 2 ) 118.It Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see" 119.Xr setrlimit 2 ) 120.It Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see" 121.Xr setrlimit 2 ) 122.It Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see" 123.Xr setitimer 2 ) 124.It Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see" 125.Xr setitimer 2 ) 126.It Dv SIGWINCH Ta "discard signal" Ta "Window size change" 127.It Dv SIGINFO Ta "discard signal" Ta "status request from keyboard" 128.It Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1" 129.It Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2" 130.El 131.Pp 132The 133.Fa sig 134parameter specifies which signal was received. 135The 136.Fa func 137procedure allows a user to choose the action upon receipt of a signal. 138To set the default action of the signal to occur as listed above, 139.Fa func 140should be 141.Dv SIG_DFL . 142A 143.Dv SIG_DFL 144resets the default action. 145To ignore the signal 146.Fa func 147should be 148.Dv SIG_IGN . 149This will cause subsequent instances of the signal to be ignored 150and pending instances to be discarded. If 151.Dv SIG_IGN 152is not used, 153further occurrences of the signal are 154automatically blocked and 155.Fa func 156is called. 157.Pp 158The handled signal is unblocked when the 159function returns and 160the process continues from where it left off when the signal occurred. 161.Bf -symbolic 162Unlike previous signal facilities, the handler 163func() remains installed after a signal has been delivered. 164.Ef 165.Pp 166For some system calls, if a signal is caught while the call is 167executing and the call is prematurely terminated, 168the call is automatically restarted. 169(The handler is installed using the 170.Dv SA_RESTART 171flag with 172.Xr sigaction 2 . ) 173The affected system calls include 174.Xr read 2 , 175.Xr write 2 , 176.Xr sendto 2 , 177.Xr recvfrom 2 , 178.Xr sendmsg 2 179and 180.Xr recvmsg 2 181on a communications channel or a low speed device 182and during a 183.Xr ioctl 2 184or 185.Xr wait 2 . 186However, calls that have already committed are not restarted, 187but instead return a partial success (for example, a short read count). 188This semantics could be changed with 189.Xr siginterrupt 3 . 190.Pp 191When a process which has installed signal handlers forks, 192the child process inherits the signals. 193All caught signals may be reset to their default action by a call 194to the 195.Xr execve 2 196function; 197ignored signals remain ignored. 198.Sh RETURN VALUES 199The previous action is returned on a successful call. 200Otherwise, SIG_ERR is returned and the global variable 201.Va errno 202is set to indicate the error. 203.Sh ERRORS 204.Fn Signal 205will fail and no action will take place if one of the 206following occur: 207.Bl -tag -width [EINVAL] 208.It Bq Er EINVAL 209.Em Sig 210is not a valid signal number. 211.It Bq Er EINVAL 212An attempt is made to ignore or supply a handler for 213.Dv SIGKILL 214or 215.Ev SIGSTOP . 216.Sh SEE ALSO 217.Xr kill 1 , 218.Xr kill 2 , 219.Xr ptrace 2 , 220.Xr sigaction 2 , 221.Xr sigaltstack 2 , 222.Xr sigprocmask 2 , 223.Xr sigsuspend 2 , 224.Xr fpsetmask 3 , 225.Xr setjmp 3 , 226.Xr siginterrupt 3 , 227.Xr tty 4 228.Sh HISTORY 229This 230.Nm signal 231facility appeared in 232.Bx 4.0 . 233