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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 June 7, 2004 36.Dt SIGNAL 3 37.Os 38.Sh NAME 39.Nm signal 40.Nd simplified software signal facilities 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In signal.h 45.\" The following is Quite Ugly, but syntactically correct. 46.\" Don't try to 47.\" fix it. 48.Ft void \*(lp* 49.Fn signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint" 50.Pp 51or in 52.Fx Ns 's 53equivalent but easier to read typedef'd version: 54.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ; 55.Ft sig_t 56.Fn signal "int sig" "sig_t func" 57.Sh DESCRIPTION 58This 59.Fn signal 60facility 61is a simplified interface to the more general 62.Xr sigaction 2 63facility. 64.Pp 65Signals allow the manipulation of a process from outside its 66domain as well as allowing the process to manipulate itself or 67copies of itself (children). 68There are two general types of signals: 69those that cause termination of a process and those that do not. 70Signals which cause termination of a program might result from 71an irrecoverable error or might be the result of a user at a terminal 72typing the `interrupt' character. 73Signals are used when a process is stopped because it wishes to access 74its control terminal while in the background (see 75.Xr tty 4 ) . 76Signals are optionally generated 77when a process resumes after being stopped, 78when the status of child processes changes, 79or when input is ready at the control terminal. 80Most signals result in the termination of the process receiving them 81if no action 82is taken; some signals instead cause the process receiving them 83to be stopped, or are simply discarded if the process has not 84requested otherwise. 85Except for the 86.Dv SIGKILL 87and 88.Dv SIGSTOP 89signals, the 90.Fn signal 91function allows for a signal to be caught, to be ignored, or to generate 92an interrupt. 93These signals are defined in the file 94.In signal.h : 95.Bl -column No ".Dv SIGVTALRM" "create core image" 96.It Sy "No Name Default Action Description" 97.It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup" 98.It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program" 99.It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program" 100.It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction" 101.It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap" 102.It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program" 103(formerly 104.Dv SIGIOT ) 105.It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed" 106.It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception" 107.It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program" 108.It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error" 109.It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation" 110.It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked" 111.It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader" 112.It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired" 113.It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal" 114.It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket" 115.It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)" 116.It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard" 117.It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop" 118.It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed" 119.It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from" 120control terminal 121.It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to" 122control terminal 123.It 23 Ta Dv SIGIO Ta "discard signal" Ta Tn "I/O" 124is possible on a descriptor (see 125.Xr fcntl 2 ) 126.It 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see" 127.Xr setrlimit 2 ) 128.It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see" 129.Xr setrlimit 2 ) 130.It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see" 131.Xr setitimer 2 ) 132.It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see" 133.Xr setitimer 2 ) 134.It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change" 135.It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard" 136.It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1" 137.It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2" 138.It 32 Ta Dv SIGTHR Ta "terminate process" Ta "thread interrupt" 139.El 140.Pp 141The 142.Fa sig 143argument specifies which signal was received. 144The 145.Fa func 146procedure allows a user to choose the action upon receipt of a signal. 147To set the default action of the signal to occur as listed above, 148.Fa func 149should be 150.Dv SIG_DFL . 151A 152.Dv SIG_DFL 153resets the default action. 154To ignore the signal 155.Fa func 156should be 157.Dv SIG_IGN . 158This will cause subsequent instances of the signal to be ignored 159and pending instances to be discarded. 160If 161.Dv SIG_IGN 162is not used, 163further occurrences of the signal are 164automatically blocked and 165.Fa func 166is called. 167.Pp 168The handled signal is unblocked when the 169function returns and 170the process continues from where it left off when the signal occurred. 171.Bf -symbolic 172Unlike previous signal facilities, the handler 173func() remains installed after a signal has been delivered. 174.Ef 175.Pp 176For some system calls, if a signal is caught while the call is 177executing and the call is prematurely terminated, 178the call is automatically restarted. 179(The handler is installed using the 180.Dv SA_RESTART 181flag with 182.Xr sigaction 2 . ) 183The affected system calls include 184.Xr read 2 , 185.Xr write 2 , 186.Xr sendto 2 , 187.Xr recvfrom 2 , 188.Xr sendmsg 2 189and 190.Xr recvmsg 2 191on a communications channel or a low speed device 192and during a 193.Xr ioctl 2 194or 195.Xr wait 2 . 196However, calls that have already committed are not restarted, 197but instead return a partial success (for example, a short read count). 198These semantics could be changed with 199.Xr siginterrupt 3 . 200.Pp 201When a process which has installed signal handlers forks, 202the child process inherits the signals. 203All caught signals may be reset to their default action by a call 204to the 205.Xr execve 2 206function; 207ignored signals remain ignored. 208.Pp 209If a process explicitly specifies 210.Dv SIG_IGN 211as the action for the signal 212.Dv SIGCHLD , 213the system will not create zombie processes when children 214of the calling process exit. 215As a consequence, the system will discard the exit status 216from the child processes. 217If the calling process subsequently issues a call to 218.Xr wait 2 219or equivalent, it will block until all of the calling process's 220children terminate, and then return a value of \-1 with 221.Va errno 222set to 223.Er ECHILD . 224.Pp 225See 226.Xr sigaction 2 227for a list of functions 228that are considered safe for use in signal handlers. 229.Sh RETURN VALUES 230The previous action is returned on a successful call. 231Otherwise, SIG_ERR is returned and the global variable 232.Va errno 233is set to indicate the error. 234.Sh ERRORS 235The 236.Fn signal 237function 238will fail and no action will take place if one of the 239following occur: 240.Bl -tag -width Er 241.It Bq Er EINVAL 242The 243.Fa sig 244argument 245is not a valid signal number. 246.It Bq Er EINVAL 247An attempt is made to ignore or supply a handler for 248.Dv SIGKILL 249or 250.Ev SIGSTOP . 251.El 252.Sh SEE ALSO 253.Xr kill 1 , 254.Xr kill 2 , 255.Xr ptrace 2 , 256.Xr sigaction 2 , 257.Xr sigaltstack 2 , 258.Xr sigprocmask 2 , 259.Xr sigsuspend 2 , 260.Xr wait 2 , 261.Xr fpsetmask 3 , 262.Xr setjmp 3 , 263.Xr siginterrupt 3 , 264.Xr tty 4 265.Sh HISTORY 266This 267.Fn signal 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