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 LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.Fd #include <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 50or in FreeBSD's equivalent but easier to read typedef'd version: 51.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" 52.Ft sig_t 53.Fn signal "int sig" "sig_t func" 54.Sh DESCRIPTION 55This 56.Fn signal 57facility 58is a simplified interface to the more general 59.Xr sigaction 2 60facility. 61.Pp 62Signals allow the manipulation of a process from outside its 63domain as well as allowing the process to manipulate itself or 64copies of itself (children). There are two general types of signals: 65those that cause termination of a process and those that do not. 66Signals which cause termination of a program might result from 67an irrecoverable error or might be the result of a user at a terminal 68typing the `interrupt' character. 69Signals are used when a process is stopped because it wishes to access 70its control terminal while in the background (see 71.Xr tty 4 ) . 72Signals are optionally generated 73when a process resumes after being stopped, 74when the status of child processes changes, 75or when input is ready at the control terminal. 76Most signals result in the termination of the process receiving them 77if no action 78is taken; some signals instead cause the process receiving them 79to be stopped, or are simply discarded if the process has not 80requested otherwise. 81Except for the 82.Dv SIGKILL 83and 84.Dv SIGSTOP 85signals, the 86.Fn signal 87function allows for a signal to be caught, to be ignored, or to generate 88an interrupt. 89These signals are defined in the file 90.Aq Pa signal.h : 91.Bl -column SIGVTALARMXX "create core imagexxx" 92.It Sy Name Default Action Description 93.It Dv SIGHUP Ta "terminate process" Ta "terminal line hangup" 94.It Dv SIGINT Ta "terminate process" Ta "interrupt program" 95.It Dv SIGQUIT Ta "create core image" Ta "quit program" 96.It Dv SIGILL Ta "create core image" Ta "illegal instruction" 97.It Dv SIGTRAP Ta "create core image" Ta "trace trap" 98.It Dv SIGABRT Ta "create core image" Ta Xr abort 2 99call (formerly 100.Dv SIGIOT ) 101.It Dv SIGEMT Ta "create core image" Ta "emulate instruction executed" 102.It Dv SIGFPE Ta "create core image" Ta "floating-point exception" 103.It Dv SIGKILL Ta "terminate process" Ta "kill program" 104.It Dv SIGBUS Ta "create core image" Ta "bus error" 105.It Dv SIGSEGV Ta "create core image" Ta "segmentation violation" 106.It Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked" 107.It Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader" 108.It Dv SIGALRM Ta "terminate process" Ta "real-time timer expired" 109.It Dv SIGTERM Ta "terminate process" Ta "software termination signal" 110.It Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket" 111.It Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)" 112.It Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard" 113.It Dv SIGCONT Ta "discard signal" Ta "continue after stop" 114.It Dv SIGCHLD Ta "discard signal" Ta "child status has changed" 115.It Dv SIGTTIN Ta "stop process" Ta "background read attempted from control terminal" 116.It Dv SIGTTOU Ta "stop process" Ta "background write attempted to control terminal" 117.It Dv SIGIO Ta "discard signal" Ta Tn "I/O" 118is possible on a descriptor (see 119.Xr fcntl 2 ) 120.It Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see" 121.Xr setrlimit 2 ) 122.It Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see" 123.Xr setrlimit 2 ) 124.It Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see" 125.Xr setitimer 2 ) 126.It Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see" 127.Xr setitimer 2 ) 128.It Dv SIGWINCH Ta "discard signal" Ta "Window size change" 129.It Dv SIGINFO Ta "discard signal" Ta "status request from keyboard" 130.It Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1" 131.It Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2" 132.El 133.Pp 134The 135.Fa sig 136parameter specifies which signal was received. 137The 138.Fa func 139procedure allows a user to choose the action upon receipt of a signal. 140To set the default action of the signal to occur as listed above, 141.Fa func 142should be 143.Dv SIG_DFL . 144A 145.Dv SIG_DFL 146resets the default action. 147To ignore the signal 148.Fa func 149should be 150.Dv SIG_IGN . 151This will cause subsequent instances of the signal to be ignored 152and pending instances to be discarded. 153If 154.Dv SIG_IGN 155is not used, 156further occurrences of the signal are 157automatically blocked and 158.Fa func 159is called. 160.Pp 161The handled signal is unblocked when the 162function returns and 163the process continues from where it left off when the signal occurred. 164.Bf -symbolic 165Unlike previous signal facilities, the handler 166func() remains installed after a signal has been delivered. 167.Ef 168.Pp 169For some system calls, if a signal is caught while the call is 170executing and the call is prematurely terminated, 171the call is automatically restarted. 172(The handler is installed using the 173.Dv SA_RESTART 174flag with 175.Xr sigaction 2 . ) 176The affected system calls include 177.Xr read 2 , 178.Xr write 2 , 179.Xr sendto 2 , 180.Xr recvfrom 2 , 181.Xr sendmsg 2 182and 183.Xr recvmsg 2 184on a communications channel or a low speed device 185and during a 186.Xr ioctl 2 187or 188.Xr wait 2 . 189However, calls that have already committed are not restarted, 190but instead return a partial success (for example, a short read count). 191This semantics could be changed with 192.Xr siginterrupt 3 . 193.Pp 194When a process which has installed signal handlers forks, 195the child process inherits the signals. 196All caught signals may be reset to their default action by a call 197to the 198.Xr execve 2 199function; 200ignored signals remain ignored. 201.Sh RETURN VALUES 202The previous action is returned on a successful call. 203Otherwise, SIG_ERR is returned and the global variable 204.Va errno 205is set to indicate the error. 206.Sh ERRORS 207.Fn Signal 208will fail and no action will take place if one of the 209following occur: 210.Bl -tag -width [EINVAL] 211.It Bq Er EINVAL 212.Em Sig 213is not a valid signal number. 214.It Bq Er EINVAL 215An attempt is made to ignore or supply a handler for 216.Dv SIGKILL 217or 218.Ev SIGSTOP . 219.Sh SEE ALSO 220.Xr kill 1 , 221.Xr kill 2 , 222.Xr ptrace 2 , 223.Xr sigaction 2 , 224.Xr sigaltstack 2 , 225.Xr sigprocmask 2 , 226.Xr sigsuspend 2 , 227.Xr fpsetmask 3 , 228.Xr setjmp 3 , 229.Xr siginterrupt 3 , 230.Xr tty 4 231.Sh HISTORY 232This 233.Nm signal 234facility appeared in 235.Bx 4.0 . 236