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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.Dd December 1, 2017 29.Dt SIGNAL 3 30.Os 31.Sh NAME 32.Nm signal 33.Nd simplified software signal facilities 34.Sh LIBRARY 35.Lb libc 36.Sh SYNOPSIS 37.In signal.h 38.\" The following is Quite Ugly, but syntactically correct. 39.\" Don't try to fix it. 40.Ft void 41.Fn \*(lp*signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint" 42.Pp 43or in 44.Fx Ap s 45equivalent but easier to read typedef'd version: 46.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ; 47.Pp 48.Ft sig_t 49.Fn signal "int sig" "sig_t func" 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. 86These signals are defined in the file 87.In signal.h : 88.Bl -column No ".Dv SIGVTALRM" "create core image" 89.It Sy "Num" Ta Sy "Name" Ta Sy "Default Action" Ta Sy "Description" 90.It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup" 91.It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program" 92.It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program" 93.It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction" 94.It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap" 95.It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program" 96(formerly 97.Dv SIGIOT ) 98.It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed" 99.It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception" 100.It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program" 101.It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error" 102.It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation" 103.It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked" 104.It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader" 105.It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired" 106.It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal" 107.It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket" 108.It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)" 109.It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard" 110.It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop" 111.It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed" 112.It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from" 113control terminal 114.It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to" 115control terminal 116.It 23 Ta Dv SIGIO Ta "discard signal" Ta Tn "I/O" 117is possible on a descriptor (see 118.Xr fcntl 2 ) 119.It 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see" 120.Xr setrlimit 2 ) 121.It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see" 122.Xr setrlimit 2 ) 123.It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see" 124.Xr setitimer 2 ) 125.It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see" 126.Xr setitimer 2 ) 127.It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change" 128.It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard" 129.It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1" 130.It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2" 131.It 32 Ta Dv SIGTHR Ta "terminate process" Ta "thread interrupt" 132.It 33 Ta Dv SIGLIBRT Ta "terminate process" Ta "real-time library interrupt" 133.El 134.Pp 135The 136.Fa sig 137argument specifies which signal was received. 138The 139.Fa func 140procedure allows a user to choose the action upon receipt of a signal. 141To set the default action of the signal to occur as listed above, 142.Fa func 143should be 144.Dv SIG_DFL . 145A 146.Dv SIG_DFL 147resets the default action. 148To ignore the signal 149.Fa func 150should be 151.Dv SIG_IGN . 152This will cause subsequent instances of the signal to be ignored 153and pending instances to be discarded. 154If 155.Dv SIG_IGN 156is not used, 157further occurrences of the signal are 158automatically blocked and 159.Fa func 160is called. 161.Pp 162The handled signal is unblocked when the 163function returns and 164the process continues from where it left off when the signal occurred. 165.Bf -symbolic 166Unlike previous signal facilities, the handler 167func() remains installed after a signal has been delivered. 168.Ef 169.Pp 170For some system calls, if a signal is caught while the call is 171executing and the call is prematurely terminated, 172the call is automatically restarted. 173Any handler installed with 174.Xr signal 3 175will have the 176.Dv SA_RESTART 177flag set, meaning that any restartable system call will not return on 178receipt of a signal. 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). 194These semantics could be changed with 195.Xr siginterrupt 3 . 196.Pp 197When a process which has installed signal handlers forks, 198the child process inherits the signals. 199All caught signals may be reset to their default action by a call 200to the 201.Xr execve 2 202function; 203ignored signals remain ignored. 204.Pp 205If a process explicitly specifies 206.Dv SIG_IGN 207as the action for the signal 208.Dv SIGCHLD , 209the system will not create zombie processes when children 210of the calling process exit. 211As a consequence, the system will discard the exit status 212from the child processes. 213If the calling process subsequently issues a call to 214.Xr wait 2 215or equivalent, it will block until all of the calling process's 216children terminate, and then return a value of \-1 with 217.Va errno 218set to 219.Er ECHILD . 220.Pp 221See 222.Xr sigaction 2 223for a list of functions 224that are considered safe for use in signal handlers. 225.Sh RETURN VALUES 226The previous action is returned on a successful call. 227Otherwise, SIG_ERR is returned and the global variable 228.Va errno 229is set to indicate the error. 230.Sh ERRORS 231The 232.Fn signal 233function 234will fail and no action will take place if one of the 235following occur: 236.Bl -tag -width Er 237.It Bq Er EINVAL 238The 239.Fa sig 240argument 241is not a valid signal number. 242.It Bq Er EINVAL 243An attempt is made to ignore or supply a handler for 244.Dv SIGKILL 245or 246.Dv SIGSTOP . 247.El 248.Sh SEE ALSO 249.Xr kill 1 , 250.Xr kill 2 , 251.Xr ptrace 2 , 252.Xr sigaction 2 , 253.Xr sigaltstack 2 , 254.Xr sigprocmask 2 , 255.Xr sigsuspend 2 , 256.Xr wait 2 , 257.Xr fpsetmask 3 , 258.Xr setjmp 3 , 259.Xr siginterrupt 3 , 260.Xr tty 4 261.Sh HISTORY 262The 263.Fn signal 264function appeared in 265.At v4 . 266The current 267.Nm 268facility appeared in 269.Bx 4.0 . 270The option to avoid the creation of child zombies through ignoring 271.Dv SIGCHLD 272appeared in 273.Fx 5.0 . 274