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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.\" @(#)signal.3 8.3 (Berkeley) 4/19/94 29.\" 30.Dd December 1, 2017 31.Dt SIGNAL 3 32.Os 33.Sh NAME 34.Nm signal 35.Nd simplified software signal facilities 36.Sh LIBRARY 37.Lb libc 38.Sh SYNOPSIS 39.In signal.h 40.\" The following is Quite Ugly, but syntactically correct. 41.\" Don't try to fix it. 42.Ft void 43.Fn \*(lp*signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint" 44.Pp 45or in 46.Fx Ap s 47equivalent but easier to read typedef'd version: 48.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ; 49.Pp 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). 63There are two general types of signals: 64those that cause termination of a process and those that do not. 65Signals which cause termination of a program might result from 66an irrecoverable error or might be the result of a user at a terminal 67typing the `interrupt' character. 68Signals are used when a process is stopped because it wishes to access 69its control terminal while in the background (see 70.Xr tty 4 ) . 71Signals are optionally generated 72when a process resumes after being stopped, 73when the status of child processes changes, 74or when input is ready at the control terminal. 75Most signals result in the termination of the process receiving them 76if no action 77is taken; some signals instead cause the process receiving them 78to be stopped, or are simply discarded if the process has not 79requested otherwise. 80Except for the 81.Dv SIGKILL 82and 83.Dv SIGSTOP 84signals, the 85.Fn signal 86function allows for a signal to be caught, to be ignored, or to generate 87an interrupt. 88These signals are defined in the file 89.In signal.h : 90.Bl -column No ".Dv SIGVTALRM" "create core image" 91.It Sy "Num" Ta Sy "Name" Ta Sy "Default Action" Ta Sy "Description" 92.It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup" 93.It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program" 94.It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program" 95.It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction" 96.It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap" 97.It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program" 98(formerly 99.Dv SIGIOT ) 100.It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed" 101.It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception" 102.It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program" 103.It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error" 104.It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation" 105.It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked" 106.It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader" 107.It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired" 108.It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal" 109.It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket" 110.It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)" 111.It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard" 112.It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop" 113.It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed" 114.It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from" 115control terminal 116.It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to" 117control terminal 118.It 23 Ta Dv SIGIO Ta "discard signal" Ta Tn "I/O" 119is possible on a descriptor (see 120.Xr fcntl 2 ) 121.It 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see" 122.Xr setrlimit 2 ) 123.It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see" 124.Xr setrlimit 2 ) 125.It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see" 126.Xr setitimer 2 ) 127.It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see" 128.Xr setitimer 2 ) 129.It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change" 130.It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard" 131.It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1" 132.It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2" 133.It 32 Ta Dv SIGTHR Ta "terminate process" Ta "thread interrupt" 134.It 33 Ta Dv SIGLIBRT Ta "terminate process" Ta "real-time library interrupt" 135.El 136.Pp 137The 138.Fa sig 139argument specifies which signal was received. 140The 141.Fa func 142procedure allows a user to choose the action upon receipt of a signal. 143To set the default action of the signal to occur as listed above, 144.Fa func 145should be 146.Dv SIG_DFL . 147A 148.Dv SIG_DFL 149resets the default action. 150To ignore the signal 151.Fa func 152should be 153.Dv SIG_IGN . 154This will cause subsequent instances of the signal to be ignored 155and pending instances to be discarded. 156If 157.Dv SIG_IGN 158is not used, 159further occurrences of the signal are 160automatically blocked and 161.Fa func 162is called. 163.Pp 164The handled signal is unblocked when the 165function returns 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. 175Any handler installed with 176.Xr signal 3 177will have the 178.Dv SA_RESTART 179flag set, meaning that any restartable system call will not return on 180receipt of a signal. 181The affected system calls include 182.Xr read 2 , 183.Xr write 2 , 184.Xr sendto 2 , 185.Xr recvfrom 2 , 186.Xr sendmsg 2 187and 188.Xr recvmsg 2 189on a communications channel or a low speed device 190and during a 191.Xr ioctl 2 192or 193.Xr wait 2 . 194However, calls that have already committed are not restarted, 195but instead return a partial success (for example, a short read count). 196These semantics could be changed with 197.Xr siginterrupt 3 . 198.Pp 199When a process which has installed signal handlers forks, 200the child process inherits the signals. 201All caught signals may be reset to their default action by a call 202to the 203.Xr execve 2 204function; 205ignored signals remain ignored. 206.Pp 207If a process explicitly specifies 208.Dv SIG_IGN 209as the action for the signal 210.Dv SIGCHLD , 211the system will not create zombie processes when children 212of the calling process exit. 213As a consequence, the system will discard the exit status 214from the child processes. 215If the calling process subsequently issues a call to 216.Xr wait 2 217or equivalent, it will block until all of the calling process's 218children terminate, and then return a value of \-1 with 219.Va errno 220set to 221.Er ECHILD . 222.Pp 223See 224.Xr sigaction 2 225for a list of functions 226that are considered safe for use in signal handlers. 227.Sh RETURN VALUES 228The previous action is returned on a successful call. 229Otherwise, SIG_ERR is returned and the global variable 230.Va errno 231is set to indicate the error. 232.Sh ERRORS 233The 234.Fn signal 235function 236will fail and no action will take place if one of the 237following occur: 238.Bl -tag -width Er 239.It Bq Er EINVAL 240The 241.Fa sig 242argument 243is not a valid signal number. 244.It Bq Er EINVAL 245An attempt is made to ignore or supply a handler for 246.Dv SIGKILL 247or 248.Dv SIGSTOP . 249.El 250.Sh SEE ALSO 251.Xr kill 1 , 252.Xr kill 2 , 253.Xr ptrace 2 , 254.Xr sigaction 2 , 255.Xr sigaltstack 2 , 256.Xr sigprocmask 2 , 257.Xr sigsuspend 2 , 258.Xr wait 2 , 259.Xr fpsetmask 3 , 260.Xr setjmp 3 , 261.Xr siginterrupt 3 , 262.Xr tty 4 263.Sh HISTORY 264The 265.Fn signal 266function appeared in 267.At v4 . 268The current 269.Nm 270facility appeared in 271.Bx 4.0 . 272The option to avoid the creation of child zombies through ignoring 273.Dv SIGCHLD 274appeared in 275.Fx 5.0 . 276