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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.\" 34.Dd April 19, 1994 35.Dt SIGNAL 3 36.Os BSD 4 37.Sh NAME 38.Nm signal 39.Nd simplified software signal facilities 40.Sh SYNOPSIS 41.Fd #include <signal.h> 42.Ft void \*(lp* 43.Fn signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint" 44 45or in FreeBSD's equivalent but easier to read typedef'd version: 46.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" 47.Ft sig_t 48.Fn signal "int sig" "sig_t func" 49.Sh DESCRIPTION 50This 51.Fn signal 52facility 53is a simplified interface to the more general 54.Xr sigaction 2 55facility. 56.Pp 57Signals allow the manipulation of a process from outside its 58domain as well as allowing the process to manipulate itself or 59copies of itself (children). There are two general types of signals: 60those that cause termination of a process and those that do not. 61Signals which cause termination of a program might result from 62an irrecoverable error or might be the result of a user at a terminal 63typing the `interrupt' character. 64Signals are used when a process is stopped because it wishes to access 65its control terminal while in the background (see 66.Xr tty 4 ) . 67Signals are optionally generated 68when a process resumes after being stopped, 69when the status of child processes changes, 70or when input is ready at the control terminal. 71Most signals result in the termination of the process receiving them 72if no action 73is taken; some signals instead cause the process receiving them 74to be stopped, or are simply discarded if the process has not 75requested otherwise. 76Except for the 77.Dv SIGKILL 78and 79.Dv SIGSTOP 80signals, the 81.Fn signal 82function allows for a signal to be caught, to be ignored, or to generate 83an interrupt. 84These signals are defined in the file 85.Aq Pa signal.h : 86.Bl -column SIGVTALARMXX "create core imagexxx" 87.It Sy " Name " " Default Action " " Description" 88.It Dv SIGHUP No " terminate process" " terminal line hangup" 89.It Dv SIGINT No " terminate process" " interrupt program" 90.It Dv SIGQUIT No " create core image" " quit program" 91.It Dv SIGILL No " create core image" " illegal instruction" 92.It Dv SIGTRAP No " create core image" " trace trap" 93.It Dv SIGABRT No " create core image" Xr abort 2 94call (formerly 95.Dv SIGIOT ) 96.It Dv SIGEMT No " create core image" " emulate instruction executed" 97.It Dv SIGFPE No " create core image" " floating-point exception" 98.It Dv SIGKILL No " terminate process" " kill program" 99.It Dv SIGBUS No " create core image" " bus error" 100.It Dv SIGSEGV No " create core image" " segmentation violation" 101.It Dv SIGSYS No " create core image" " non-existent system call invoked" 102.It Dv SIGPIPE No " terminate process" " write on a pipe with no reader" 103.It Dv SIGALRM No " terminate process" " real-time timer expired" 104.It Dv SIGTERM No " terminate process" " software termination signal" 105.It Dv SIGURG No " discard signal" " urgent condition present on socket" 106.It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)" 107.It Dv SIGTSTP No " stop process" " stop signal generated from keyboard" 108.It Dv SIGCONT No " discard signal" " continue after stop" 109.It Dv SIGCHLD No " discard signal" " child status has changed" 110.It Dv SIGTTIN No " stop process" " background read attempted from control terminal" 111.It Dv SIGTTOU No " stop process" " background write attempted to control terminal" 112.It Dv SIGIO No " discard signal" Tn " I/O" 113is possible on a descriptor (see 114.Xr fcntl 2 ) 115.It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see" 116.Xr setrlimit 2 ) 117.It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see" 118.Xr setrlimit 2 ) 119.It Dv SIGVTALRM No " terminate process" " virtual time alarm (see" 120.Xr setitimer 2 ) 121.It Dv SIGPROF No " terminate process" " profiling timer alarm (see" 122.Xr setitimer 2 ) 123.It Dv SIGWINCH No " discard signal" " Window size change" 124.It Dv SIGINFO No " discard signal" " status request from keyboard" 125.It Dv SIGUSR1 No " terminate process" " User defined signal 1" 126.It Dv SIGUSR2 No " terminate process" " User defined signal 2" 127.El 128.Pp 129The 130.Fa sig 131parameter specifies which signal was received. 132The 133.Fa func 134procedure allows a user to choose the action upon receipt of a signal. 135To set the default action of the signal to occur as listed above, 136.Fa func 137should be 138.Dv SIG_DFL . 139A 140.Dv SIG_DFL 141resets the default action. 142To ignore the signal 143.Fa func 144should be 145.Dv SIG_IGN . 146This will cause subsequent instances of the signal to be ignored 147and pending instances to be discarded. If 148.Dv SIG_IGN 149is not used, 150further occurrences of the signal are 151automatically blocked and 152.Fa func 153is called. 154.Pp 155The handled signal is unblocked with the 156function returns and 157the process continues from where it left off when the signal occurred. 158.Bf -symbolic 159Unlike previous signal facilities, the handler 160func() remains installed after a signal has been delivered. 161.Ef 162.Pp 163For some system calls, if a signal is caught while the call is 164executing and the call is prematurely terminated, 165the call is automatically restarted. 166(The handler is installed using the 167.Dv SA_RESTART 168flag with 169.Xr sigaction 2 . ) 170The affected system calls include 171.Xr read 2 , 172.Xr write 2 , 173.Xr sendto 2 , 174.Xr recvfrom 2 , 175.Xr sendmsg 2 176and 177.Xr recvmsg 2 178on a communications channel or a low speed device 179and during a 180.Xr ioctl 2 181or 182.Xr wait 2 . 183However, calls that have already committed are not restarted, 184but instead return a partial success (for example, a short read count). 185.Pp 186When a process which has installed signal handlers forks, 187the child process inherits the signals. 188All caught signals may be reset to their default action by a call 189to the 190.Xr execve 2 191function; 192ignored signals remain ignored. 193.Sh RETURN VALUES 194The previous action is returned on a successful call. 195Otherwise, \-1 is returned and the global variable 196.Va errno 197is set to indicate the error. 198.Sh ERRORS 199.Fn Signal 200will fail and no action will take place if one of the 201following occur: 202.Bl -tag -width [EINVAL] 203.It Bq Er EINVAL 204.Em Sig 205is not a valid signal number. 206.It Bq Er EINVAL 207An attempt is made to ignore or supply a handler for 208.Dv SIGKILL 209or 210.Ev SIGSTOP . 211.Sh SEE ALSO 212.Xr kill 1 , 213.Xr ptrace 2 , 214.Xr kill 2 , 215.Xr sigaction 2 , 216.Xr sigaltstack 2 , 217.Xr sigprocmask 2 , 218.Xr sigsuspend 2 , 219.Xr setjmp 3 , 220.Xr tty 4 221.Sh HISTORY 222This 223.Nm signal 224facility appeared in 225.Bx 4.0 . 226