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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.\" XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX 42.\" The prototype for signal(3) cannot be cleanly marked up in -mdoc 43.\" without the following lower-level tweak. 44.nr in-synopsis-section 0 45.Pp 46.Ft "void \*(lp*" Ns 47.Fo signal 48.Fa "int sig" 49.Fa "void \*(lp*func\*(rp\*(lpint\*(rp" 50.Fc Ns 51.Ft "\*(rp\*(lpint\*(rp" ; 52.Pp 53.nr in-synopsis-section 1 54.\" XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX 55or in 56.Fx Ap s 57equivalent but easier to read typedef'd version: 58.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ; 59.Pp 60.Ft sig_t 61.Fn signal "int sig" "sig_t func" 62.Sh DESCRIPTION 63This 64.Fn signal 65facility 66is a simplified interface to the more general 67.Xr sigaction 2 68facility. 69.Pp 70Signals allow the manipulation of a process from outside its 71domain as well as allowing the process to manipulate itself or 72copies of itself (children). 73There are two general types of signals: 74those that cause termination of a process and those that do not. 75Signals which cause termination of a program might result from 76an irrecoverable error or might be the result of a user at a terminal 77typing the `interrupt' character. 78Signals are used when a process is stopped because it wishes to access 79its control terminal while in the background (see 80.Xr tty 4 ) . 81Signals are optionally generated 82when a process resumes after being stopped, 83when the status of child processes changes, 84or when input is ready at the control terminal. 85Most signals result in the termination of the process receiving them 86if no action 87is taken; some signals instead cause the process receiving them 88to be stopped, or are simply discarded if the process has not 89requested otherwise. 90Except for the 91.Dv SIGKILL 92and 93.Dv SIGSTOP 94signals, the 95.Fn signal 96function allows for a signal to be caught, to be ignored, or to generate 97an interrupt. 98These signals are defined in the file 99.In signal.h : 100.Bl -column No ".Dv SIGVTALRM" "create core image" 101.It Sy "No Name Default Action Description" 102.It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup" 103.It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program" 104.It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program" 105.It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction" 106.It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap" 107.It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program" 108(formerly 109.Dv SIGIOT ) 110.It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed" 111.It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception" 112.It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program" 113.It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error" 114.It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation" 115.It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked" 116.It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader" 117.It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired" 118.It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal" 119.It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket" 120.It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)" 121.It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard" 122.It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop" 123.It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed" 124.It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from" 125control terminal 126.It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to" 127control terminal 128.It 23 Ta Dv SIGIO Ta "discard signal" Ta Tn "I/O" 129is possible on a descriptor (see 130.Xr fcntl 2 ) 131.It 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see" 132.Xr setrlimit 2 ) 133.It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see" 134.Xr setrlimit 2 ) 135.It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see" 136.Xr setitimer 2 ) 137.It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see" 138.Xr setitimer 2 ) 139.It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change" 140.It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard" 141.It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1" 142.It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2" 143.It 32 Ta Dv SIGTHR Ta "terminate process" Ta "thread interrupt" 144.El 145.Pp 146The 147.Fa sig 148argument specifies which signal was received. 149The 150.Fa func 151procedure allows a user to choose the action upon receipt of a signal. 152To set the default action of the signal to occur as listed above, 153.Fa func 154should be 155.Dv SIG_DFL . 156A 157.Dv SIG_DFL 158resets the default action. 159To ignore the signal 160.Fa func 161should be 162.Dv SIG_IGN . 163This will cause subsequent instances of the signal to be ignored 164and pending instances to be discarded. 165If 166.Dv SIG_IGN 167is not used, 168further occurrences of the signal are 169automatically blocked and 170.Fa func 171is called. 172.Pp 173The handled signal is unblocked when the 174function returns and 175the process continues from where it left off when the signal occurred. 176.Bf -symbolic 177Unlike previous signal facilities, the handler 178func() remains installed after a signal has been delivered. 179.Ef 180.Pp 181For some system calls, if a signal is caught while the call is 182executing and the call is prematurely terminated, 183the call is automatically restarted. 184(The handler is installed using the 185.Dv SA_RESTART 186flag with 187.Xr sigaction 2 . ) 188The affected system calls include 189.Xr read 2 , 190.Xr write 2 , 191.Xr sendto 2 , 192.Xr recvfrom 2 , 193.Xr sendmsg 2 194and 195.Xr recvmsg 2 196on a communications channel or a low speed device 197and during a 198.Xr ioctl 2 199or 200.Xr wait 2 . 201However, calls that have already committed are not restarted, 202but instead return a partial success (for example, a short read count). 203These semantics could be changed with 204.Xr siginterrupt 3 . 205.Pp 206When a process which has installed signal handlers forks, 207the child process inherits the signals. 208All caught signals may be reset to their default action by a call 209to the 210.Xr execve 2 211function; 212ignored signals remain ignored. 213.Pp 214If a process explicitly specifies 215.Dv SIG_IGN 216as the action for the signal 217.Dv SIGCHLD , 218the system will not create zombie processes when children 219of the calling process exit. 220As a consequence, the system will discard the exit status 221from the child processes. 222If the calling process subsequently issues a call to 223.Xr wait 2 224or equivalent, it will block until all of the calling process's 225children terminate, and then return a value of \-1 with 226.Va errno 227set to 228.Er ECHILD . 229.Pp 230See 231.Xr sigaction 2 232for a list of functions 233that are considered safe for use in signal handlers. 234.Sh RETURN VALUES 235The previous action is returned on a successful call. 236Otherwise, SIG_ERR is returned and the global variable 237.Va errno 238is set to indicate the error. 239.Sh ERRORS 240The 241.Fn signal 242function 243will fail and no action will take place if one of the 244following occur: 245.Bl -tag -width Er 246.It Bq Er EINVAL 247The 248.Fa sig 249argument 250is not a valid signal number. 251.It Bq Er EINVAL 252An attempt is made to ignore or supply a handler for 253.Dv SIGKILL 254or 255.Dv SIGSTOP . 256.El 257.Sh SEE ALSO 258.Xr kill 1 , 259.Xr kill 2 , 260.Xr ptrace 2 , 261.Xr sigaction 2 , 262.Xr sigaltstack 2 , 263.Xr sigprocmask 2 , 264.Xr sigsuspend 2 , 265.Xr wait 2 , 266.Xr fpsetmask 3 , 267.Xr setjmp 3 , 268.Xr siginterrupt 3 , 269.Xr tty 4 270.Sh HISTORY 271The 272.Nm 273facility appeared in 274.Bx 4.0 . 275The option to avoid the creation of child zombies through ignoring 276.Dv SIGCHLD 277appeared in 278.Fx 5.0 . 279