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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.\" $FreeBSD$ 30.\" 31.Dd June 7, 2004 32.Dt SIGNAL 3 33.Os 34.Sh NAME 35.Nm signal 36.Nd simplified software signal facilities 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In signal.h 41.\" The following is Quite Ugly, but syntactically correct. 42.\" Don't try to fix it. 43.Ft void 44.Fn \*(lp*signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint" 45.Pp 46or in 47.Fx Ap s 48equivalent but easier to read typedef'd version: 49.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ; 50.Pp 51.Ft sig_t 52.Fn signal "int sig" "sig_t func" 53.Sh DESCRIPTION 54This 55.Fn signal 56facility 57is a simplified interface to the more general 58.Xr sigaction 2 59facility. 60.Pp 61Signals allow the manipulation of a process from outside its 62domain as well as allowing the process to manipulate itself or 63copies of itself (children). 64There 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.In signal.h : 91.Bl -column No ".Dv SIGVTALRM" "create core image" 92.It Sy "Num" Ta Sy "Name" Ta Sy "Default Action" Ta Sy "Description" 93.It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup" 94.It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program" 95.It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program" 96.It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction" 97.It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap" 98.It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program" 99(formerly 100.Dv SIGIOT ) 101.It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed" 102.It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception" 103.It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program" 104.It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error" 105.It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation" 106.It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked" 107.It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader" 108.It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired" 109.It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal" 110.It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket" 111.It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)" 112.It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard" 113.It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop" 114.It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed" 115.It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from" 116control terminal 117.It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to" 118control terminal 119.It 23 Ta Dv SIGIO Ta "discard signal" Ta Tn "I/O" 120is possible on a descriptor (see 121.Xr fcntl 2 ) 122.It 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see" 123.Xr setrlimit 2 ) 124.It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see" 125.Xr setrlimit 2 ) 126.It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see" 127.Xr setitimer 2 ) 128.It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see" 129.Xr setitimer 2 ) 130.It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change" 131.It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard" 132.It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1" 133.It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2" 134.It 32 Ta Dv SIGTHR Ta "terminate process" Ta "thread interrupt" 135.It 33 Ta Dv SIGLIBRT Ta "terminate process" Ta "real-time library interrupt" 136.El 137.Pp 138The 139.Fa sig 140argument specifies which signal was received. 141The 142.Fa func 143procedure allows a user to choose the action upon receipt of a signal. 144To set the default action of the signal to occur as listed above, 145.Fa func 146should be 147.Dv SIG_DFL . 148A 149.Dv SIG_DFL 150resets the default action. 151To ignore the signal 152.Fa func 153should be 154.Dv SIG_IGN . 155This will cause subsequent instances of the signal to be ignored 156and pending instances to be discarded. 157If 158.Dv SIG_IGN 159is not used, 160further occurrences of the signal are 161automatically blocked and 162.Fa func 163is called. 164.Pp 165The handled signal is unblocked when the 166function returns and 167the process continues from where it left off when the signal occurred. 168.Bf -symbolic 169Unlike previous signal facilities, the handler 170func() remains installed after a signal has been delivered. 171.Ef 172.Pp 173For some system calls, if a signal is caught while the call is 174executing and the call is prematurely terminated, 175the call is automatically restarted. 176Any handler installed with 177.Xr signal 3 178will have the 179.Dv SA_RESTART 180flag set, meaning that any restartable system call will not return on 181receipt of a signal. 182The affected system calls include 183.Xr read 2 , 184.Xr write 2 , 185.Xr sendto 2 , 186.Xr recvfrom 2 , 187.Xr sendmsg 2 188and 189.Xr recvmsg 2 190on a communications channel or a low speed device 191and during a 192.Xr ioctl 2 193or 194.Xr wait 2 . 195However, calls that have already committed are not restarted, 196but instead return a partial success (for example, a short read count). 197These semantics could be changed with 198.Xr siginterrupt 3 . 199.Pp 200When a process which has installed signal handlers forks, 201the child process inherits the signals. 202All caught signals may be reset to their default action by a call 203to the 204.Xr execve 2 205function; 206ignored signals remain ignored. 207.Pp 208If a process explicitly specifies 209.Dv SIG_IGN 210as the action for the signal 211.Dv SIGCHLD , 212the system will not create zombie processes when children 213of the calling process exit. 214As a consequence, the system will discard the exit status 215from the child processes. 216If the calling process subsequently issues a call to 217.Xr wait 2 218or equivalent, it will block until all of the calling process's 219children terminate, and then return a value of \-1 with 220.Va errno 221set to 222.Er ECHILD . 223.Pp 224See 225.Xr sigaction 2 226for a list of functions 227that are considered safe for use in signal handlers. 228.Sh RETURN VALUES 229The previous action is returned on a successful call. 230Otherwise, SIG_ERR is returned and the global variable 231.Va errno 232is set to indicate the error. 233.Sh ERRORS 234The 235.Fn signal 236function 237will fail and no action will take place if one of the 238following occur: 239.Bl -tag -width Er 240.It Bq Er EINVAL 241The 242.Fa sig 243argument 244is not a valid signal number. 245.It Bq Er EINVAL 246An attempt is made to ignore or supply a handler for 247.Dv SIGKILL 248or 249.Dv SIGSTOP . 250.El 251.Sh SEE ALSO 252.Xr kill 1 , 253.Xr kill 2 , 254.Xr ptrace 2 , 255.Xr sigaction 2 , 256.Xr sigaltstack 2 , 257.Xr sigprocmask 2 , 258.Xr sigsuspend 2 , 259.Xr wait 2 , 260.Xr fpsetmask 3 , 261.Xr setjmp 3 , 262.Xr siginterrupt 3 , 263.Xr tty 4 264.Sh HISTORY 265The 266.Nm 267facility appeared in 268.Bx 4.0 . 269The option to avoid the creation of child zombies through ignoring 270.Dv SIGCHLD 271appeared in 272.Fx 5.0 . 273