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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*\*(rp 43.Fn \*(lpsignal "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" " system call given invalid argument" 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 func 131procedure allows a user to choose the action upon receipt of a signal. 132To set the default action of the signal to occur as listed above, 133.Fa func 134should be 135.Dv SIG_DFL . 136A 137.Dv SIG_DFL 138resets the default action. 139To ignore the signal 140.Fa func 141should be 142.Dv SIG_IGN . 143This will cause subsequent instances of the signal to be ignored 144and pending instances to be discarded. If 145.Dv SIG_IGN 146is not used, 147further occurrences of the signal are 148automatically blocked and 149.Fa func 150is called. 151.Pp 152The handled signal is unblocked with the 153function returns and 154the process continues from where it left off when the signal occurred. 155.Bf -symbolic 156Unlike previous signal facilities, the handler 157func() remains installed after a signal has been delivered. 158.Ef 159.Pp 160For some system calls, if a signal is caught while the call is 161executing and the call is prematurely terminated, 162the call is automatically restarted. 163(The handler is installed using the 164.Dv SA_RESTART 165flag with 166.Xr sigaction 2 . ) 167The affected system calls include 168.Xr read 2 , 169.Xr write 2 , 170.Xr sendto 2 , 171.Xr recvfrom 2 , 172.Xr sendmsg 2 173and 174.Xr recvmsg 2 175on a communications channel or a low speed device 176and during a 177.Xr ioctl 2 178or 179.Xr wait 2 . 180However, calls that have already committed are not restarted, 181but instead return a partial success (for example, a short read count). 182.Pp 183When a process which has installed signal handlers forks, 184the child process inherits the signals. 185All caught signals may be reset to their default action by a call 186to the 187.Xr execve 2 188function; 189ignored signals remain ignored. 190.Sh RETURN VALUES 191The previous action is returned on a successful call. 192Otherwise, \-1 is returned and the global variable 193.Va errno 194is set to indicate the error. 195.Sh ERRORS 196.Xr Signal 197will fail and no action will take place if one of the 198following occur: 199.Bl -tag -width [EINVAL] 200.It Bq Er EINVAL 201.Em Sig 202is not a valid signal number. 203.It Bq Er EINVAL 204An attempt is made to ignore or supply a handler for 205.Dv SIGKILL 206or 207.Ev SIGSTOP . 208.Sh SEE ALSO 209.Xr kill 1 , 210.Xr ptrace 2 , 211.Xr kill 2 , 212.Xr sigaction 2 , 213.Xr sigaltstack 2 , 214.Xr sigprocmask 2 , 215.Xr sigsuspend 2 , 216.Xr setjmp 3 , 217.Xr tty 4 218.Sh HISTORY 219This 220.Nm signal 221facility appeared in 222.Bx 4.0 . 223