1.\" $OpenBSD: signal.3,v 1.16 2001/11/07 18:46:58 deraadt 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.Dd April 19, 1994 35.Dt SIGNAL 3 36.Os 37.Sh NAME 38.Nm signal 39.Nd simplified software signal facilities 40.Sh SYNOPSIS 41.Fd #include <signal.h> 42.Ft void 43.\" can not do this with just .Fn yet 44.\" .br 45.\" .Po 46.Fn (*signal "int sigcatch" "void (*func)(int sigraised))) (int" 47.\" .Pc Ns \*(lp\*(rp 48.Sh DESCRIPTION 49The 50.Fn signal 51facility 52is a simplified interface to the more general 53.Xr sigaction 2 54facility. 55.Pp 56Signals allow the manipulation of a process from outside its 57domain as well as allowing the process to manipulate itself or 58copies of itself (children). 59There are two general types of signals: 60those that cause termination of a process and those that do not. 61Signals which cause termination of a program might result from 62an irrecoverable error or might be the result of a user at a terminal 63typing the 64.Dq interrupt 65character. 66.Pp 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. 79.Pp 80Except for the 81.Dv SIGKILL 82and 83.Dv SIGSTOP 84signals, the 85.Fn signal 86function allows for any signal to be caught, to be ignored, or to generate 87an interrupt. 88These signals are defined in the file 89.Aq Pa signal.h : 90.Bl -column SIGVTALARMXX "create core imagexxx" 91.It Sy "Name Default Action Description" 92.It Dv SIGHUP No " terminate process" " terminal line hangup" 93.It Dv SIGINT No " terminate process" " interrupt program" 94.It Dv SIGQUIT No " create core image" " quit program" 95.It Dv SIGILL No " create core image" " illegal instruction" 96.It Dv SIGTRAP No " create core image" " trace trap" 97.It Dv SIGABRT No " create core image" Xr abort 3 98call (formerly 99.Dv SIGIOT ) 100.It Dv SIGEMT No " create core image" " emulate instruction executed" 101.It Dv SIGFPE No " create core image" " floating-point exception" 102.It Dv SIGKILL No " terminate process" " kill program" 103.It Dv SIGBUS No " create core image" " bus error" 104.It Dv SIGSEGV No " create core image" " segmentation violation" 105.It Dv SIGSYS No " create core image" " system call given invalid argument" 106.It Dv SIGPIPE No " terminate process" " write on a pipe with no reader" 107.It Dv SIGALRM No " terminate process" " real-time timer expired" 108.It Dv SIGTERM No " terminate process" " software termination signal" 109.It Dv SIGURG No " discard signal" " urgent condition present on socket" 110.It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)" 111.It Dv SIGTSTP No " stop process" " stop signal generated from keyboard" 112.It Dv SIGCONT No " discard signal" " continue after stop" 113.It Dv SIGCHLD No " discard signal" " child status has changed" 114.It Dv SIGTTIN No " stop process" " background read attempted from control terminal" 115.It Dv SIGTTOU No " stop process" " background write attempted to control terminal" 116.It Dv SIGIO No " discard signal" Tn " I/O" 117is possible on a descriptor (see 118.Xr fcntl 2 ) 119.It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see" 120.Xr setrlimit 2 ) 121.It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see" 122.Xr setrlimit 2 ) 123.It Dv SIGVTALRM No " terminate process" " virtual time alarm (see" 124.Xr setitimer 2 ) 125.It Dv SIGPROF No " terminate process" " profiling timer alarm (see" 126.Xr setitimer 2 ) 127.It Dv SIGWINCH No " discard signal" " window size change" 128.It Dv SIGINFO No " discard signal" " status request from keyboard" 129.It Dv SIGUSR1 No " terminate process" " user-defined signal 1" 130.It Dv SIGUSR2 No " terminate process" " user-defined signal 2" 131.El 132.Pp 133The 134.Fa func 135argument is a function to be called as the action upon receipt of the signal 136.Fa sigcatch . 137The function will be called with one argument, 138.Fa sigraised , 139which is the signal raised (thus the same function, 140.Fa func , 141can be used by more than one 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 164.Fa func 165returns and 166the process continues from where it left off when the signal occurred. 167.Bf -symbolic 168Unlike previous signal facilities, the handler 169func() remains installed after a signal has been delivered. 170.Ef 171.Pp 172For some system calls, if a signal is caught while the call is 173executing and the call is prematurely terminated, 174the call is automatically restarted. 175(The handler is installed using the 176.Dv SA_RESTART 177flag with 178.Xr sigaction 2 . ) 179The affected system calls include 180.Xr read 2 , 181.Xr write 2 , 182.Xr sendto 2 , 183.Xr recvfrom 2 , 184.Xr sendmsg 2 , 185and 186.Xr recvmsg 2 187on a communications channel or a low-speed device 188and during a 189.Xr ioctl 2 190or 191.Xr wait 2 . 192However, calls that have already committed are not restarted, 193but instead return a partial success (for example, a short read count). 194.Pp 195When a process which has installed signal handlers forks, 196the child process inherits the signals. 197All caught signals may be reset to their default action by a call 198to the 199.Xr execve 2 200function; 201ignored signals remain ignored. 202.Pp 203The following functions are either reentrant or not interruptible 204by signals and are async-signal safe. 205Therefore applications may 206invoke them, without restriction, from signal-catching functions: 207.Pp 208Base Interfaces: 209.Pp 210.Fn _exit , 211.Fn access , 212.Fn alarm , 213.Fn cfgetispeed , 214.Fn cfgetospeed , 215.Fn cfsetispeed , 216.Fn cfsetospeed , 217.Fn chdir , 218.Fn chmod , 219.Fn chown , 220.Fn close , 221.Fn creat , 222.Fn dup , 223.Fn dup2 , 224.Fn execle , 225.Fn execve , 226.Fn fcntl , 227.Fn fork , 228.Fn fpathconf , 229.Fn fstat , 230.Fn fsync , 231.Fn getegid , 232.Fn geteuid , 233.Fn getgid , 234.Fn getgroups , 235.Fn getpgrp , 236.Fn getpid , 237.Fn getppid , 238.Fn getuid , 239.Fn kill , 240.Fn link , 241.Fn lseek , 242.Fn mkdir , 243.Fn mkfifo , 244.Fn open , 245.Fn pathconf , 246.Fn pause , 247.Fn pipe , 248.Fn raise , 249.Fn read , 250.Fn rename , 251.Fn rmdir , 252.Fn setgid , 253.Fn setpgid , 254.Fn setsid , 255.Fn setuid , 256.Fn sigaction , 257.Fn sigaddset , 258.Fn sigdelset , 259.Fn sigemptyset , 260.Fn sigfillset , 261.Fn sigismember , 262.Fn signal , 263.Fn sigpending , 264.Fn sigprocmask , 265.Fn sigsuspend , 266.Fn sleep , 267.Fn stat , 268.Fn sysconf , 269.Fn tcdrain , 270.Fn tcflow , 271.Fn tcflush , 272.Fn tcgetattr , 273.Fn tcgetpgrp , 274.Fn tcsendbreak , 275.Fn tcsetattr , 276.Fn tcsetpgrp , 277.Fn time , 278.Fn times , 279.Fn umask , 280.Fn uname , 281.Fn unlink , 282.Fn utime , 283.Fn wait , 284.Fn waitpid , 285.Fn write . 286.Pp 287Realtime Interfaces: 288.Pp 289.Fn aio_error , 290.Fn clock_gettime , 291.Fn sigpause , 292.Fn timer_getoverrun , 293.Fn aio_return , 294.Fn fdatasync , 295.Fn sigqueue , 296.Fn timer_gettime , 297.Fn aio_suspend , 298.Fn sem_post , 299.Fn sigset , 300.Fn timer_settime . 301.Pp 302ANSI C Interfaces: 303.Pp 304.Fn strcpy , 305.Fn strcat , 306.Fn strncpy , 307.Fn strncat , 308and perhaps some others. 309.Pp 310Extension Interfaces: 311.Pp 312.Fn strlcpy , 313.Fn strlcat , 314.Fn syslog_r . 315.Pp 316All functions not in the above lists are considered to be unsafe 317with respect to signals. 318That is to say, the behaviour of such functions when called from a 319signal handler is undefined. 320In general though, signal handlers should do little more than set a 321flag; most other actions are not safe. 322.Pp 323As well, inside the signal handler it is also considered more safe to 324make a copy the global variable 325.Va errno 326and restore it before returning from the signal handler. 327.Sh RETURN VALUES 328The previous action is returned on a successful call. 329Otherwise, 330.Fa SIG_ERR 331is returned and the global variable 332.Va errno 333is set to indicate the error. 334.Sh ERRORS 335.Fn signal 336will fail and no action will take place if one of the 337following occur: 338.Bl -tag -width Er 339.It Bq Er EINVAL 340A specified signal 341is not a valid signal number. 342.It Bq Er EINVAL 343An attempt is made to ignore or supply a handler for 344.Dv SIGKILL 345or 346.Ev SIGSTOP . 347.El 348.Sh SEE ALSO 349.Xr kill 1 , 350.Xr kill 2 , 351.Xr ptrace 2 , 352.Xr sigaction 2 , 353.Xr sigaltstack 2 , 354.Xr sigprocmask 2 , 355.Xr sigsuspend 2 , 356.Xr setjmp 3 , 357.Xr tty 4 358.Sh HISTORY 359This 360.Fn signal 361facility appeared in 362.Bx 4.0 . 363