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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.\" from: @(#)sigaction.2 6.3 (Berkeley) 7/23/91 33.\" $Id: sigaction.2,v 1.3 1993/10/15 00:05:58 jtc Exp $ 34.\" 35.Dd July 23, 1991 36.Dt SIGACTION 2 37.Os 38.Sh NAME 39.Nm sigaction 40.Nd software signal facilities 41.Sh SYNOPSIS 42.Fd #include <signal.h> 43.Bd -literal 44struct sigaction { 45 void (*sa_handler)(); 46 sigset_t sa_mask; 47 int sa_flags; 48}; 49.Ed 50.Fn sigaction "int sig" "struct sigaction *act" "struct sigaction *oact" 51.Sh DESCRIPTION 52The system defines a set of signals that may be delivered to a process. 53Signal delivery resembles the occurence of a hardware interrupt: 54the signal is blocked from further occurrence, the current process 55context is saved, and a new one is built. A process may specify a 56.Em handler 57to which a signal is delivered, or specify that a signal is to be 58.Em ignored . 59A process may also specify that a default action is to be taken 60by the system when a signal occurs. 61A signal may also be 62.Em blocked , 63in which case its delivery is postponed until it is 64.Em unblocked . 65The action to be taken on delivery is determined at the time 66of delivery. 67Normally, signal handlers execute on the current stack 68of the process. This may be changed, on a per-handler basis, 69so that signals are taken on a special 70.Em "signal stack" . 71.Pp 72Signal routines execute with the signal that caused their 73invocation 74.Em blocked , 75but other signals may yet occur. 76A global 77.Em "signal mask" 78defines the set of signals currently blocked from delivery 79to a process. The signal mask for a process is initialized 80from that of its parent (normally empty). It 81may be changed with a 82.Xr sigprocmask 2 83call, or when a signal is delivered to the process. 84.Pp 85When a signal 86condition arises for a process, the signal is added to a set of 87signals pending for the process. 88If the signal is not currently 89.Em blocked 90by the process then it is delivered to the process. 91Signals may be delivered any time a process enters the operating system 92(e.g., during a system call, page fault or trap, or clock interrupt). 93If multiple signals are ready to be delivered at the same time, 94any signals that could be caused by traps are delivered first. 95Additional signals may be processed at the same time, with each 96appearing to interrupt the handlers for the previous signals 97before their first instructions. 98The set of pending signals is returned by the 99.Xr sigpending 2 100function. 101When a caught signal 102is delivered, the current state of the process is saved, 103a new signal mask is calculated (as described below), 104and the signal handler is invoked. The call to the handler 105is arranged so that if the signal handling routine returns 106normally the process will resume execution in the context 107from before the signal's delivery. 108If the process wishes to resume in a different context, then it 109must arrange to restore the previous context itself. 110.Pp 111When a signal is delivered to a process a new signal mask is 112installed for the duration of the process' signal handler 113(or until a 114.Xr sigprocmask 115call is made). 116This mask is formed by taking the union of the current signal mask set, 117the signal to be delivered, and 118the signal mask associated with the handler to be invoked. 119.Pp 120.Fn Sigaction 121assigns an action for a specific signal. 122If 123.Fa act 124is non-zero, it 125specifies an action 126.Pf ( Dv SIG_DFL , 127.Dv SIG_IGN , 128or a handler routine) and mask 129to be used when delivering the specified signal. 130If 131.Fa oact 132is non-zero, the previous handling information for the signal 133is returned to the user. 134.Pp 135Once a signal handler is installed, it remains installed 136until another 137.Fn sigaction 138call is made, or an 139.Xr execve 2 140is performed. 141A signal-specific default action may be reset by 142setting 143.Fa sa_handler 144to 145.Dv SIG_DFL . 146The defaults are process termination, possibly with core dump; 147no action; stopping the process; or continuing the process. 148See the signal list below for each signal's default action. 149If 150.Fa sa_handler 151is 152.Dv SIG_IGN 153current and pending instances 154of the signal are ignored and discarded. 155.Pp 156Options may be specified by setting 157.Em sa_flags . 158If the 159.Dv SA_NOCLDSTOP 160bit is set when installing a catching function 161for the 162.Dv SIGCHLD 163signal, 164the 165.Dv SIGCHLD 166signal will be generated only when a child process exits, 167not when a child process stops. 168Further, if the 169.Dv SA_ONSTACK 170bit is set in 171.Em sa_flags , 172the system will deliver the signal to the process on a 173.Em "signal stack" , 174specified with 175.Xr sigstack 2 . 176.Pp 177If a signal is caught during the system calls listed below, 178the call may be forced to terminate 179with the error 180.Dv EINTR , 181or the call may be restarted. 182Restart of pending calls is requested 183by setting the 184.Dv SA_RESTART 185bit in 186.Ar sa_flags . 187The affected system calls include 188.Xr read 2 , 189.Xr write 2 , 190.Xr sendto 2 , 191.Xr recvfrom 2 , 192.Xr sendmsg 2 193and 194.Xr recvmsg 2 195on a communications channel or a slow device (such as a terminal, 196but not a regular file) 197and during a 198.Xr wait 2 199or 200.Xr ioctl 2 . 201However, calls that have already committed are not restarted, 202but instead return a partial success (for example, a short read count). 203.Pp 204After a 205.Xr fork 2 206or 207.Xr vfork 2 208all signals, the signal mask, the signal stack, 209and the restart/interrupt flags are inherited by the child. 210.Pp 211.Xr Execve 2 212reinstates the default 213action for all signals which were caught and 214resets all signals to be caught on the user stack. 215Ignored signals remain ignored; 216the signal mask remains the same; 217signals that restart pending system calls continue to do so. 218.Pp 219The following is a list of all signals 220with names as in the include file 221.Aq Pa signal.h : 222.Bl -column SIGVTALARMXX "create core imagexxx" 223.It Sy " NAME " " Default Action " " Description" 224.It Dv SIGHUP No " terminate process" " terminal line hangup" 225.It Dv SIGINT No " terminate process" " interrupt program" 226.It Dv SIGQUIT No " create core image" " quit program" 227.It Dv SIGILL No " create core image" " illegal instruction" 228.It Dv SIGTRAP No " create core image" " trace trap" 229.It Dv SIGABRT No " create core image" Xr abort 2 230call (formerly 231.Dv SIGIOT ) 232.It Dv SIGEMT No " create core image" " emulate instruction executed" 233.It Dv SIGFPE No " create core image" " floating-point exception" 234.It Dv SIGKILL No " terminate process" " kill program" 235.It Dv SIGBUS No " create core image" " bus error" 236.It Dv SIGSEGV No " create core image" " segmentation violation" 237.It Dv SIGSYS No " create core image" " system call given invalid argument" 238.It Dv SIGPIPE No " terminate process" " write on a pipe with no reader" 239.It Dv SIGALRM No " terminate process" " real-time timer expired" 240.It Dv SIGTERM No " terminate process" " software termination signal" 241.It Dv SIGURG No " discard signal" " urgent condition present on socket" 242.It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)" 243.It Dv SIGTSTP No " stop process" " stop signal generated from keyboard" 244.It Dv SIGCONT No " discard signal" " continue after stop" 245.It Dv SIGCHLD No " discard signal" " child status has changed" 246.It Dv SIGTTIN No " stop process" " background read attempted from control terminal" 247.It Dv SIGTTOU No " stop process" " background write attempted to control terminal" 248.It Dv SIGIO No " discard signal" Tn " I/O" 249is possible on a descriptor (see 250.Xr fcntl 2 ) 251.It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see" 252.Xr setrlimit 2 ) 253.It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see" 254.Xr setrlimit 2 ) 255.It Dv SIGVTALRM No " terminate process" " virtual time alarm (see" 256.Xr setitimer 2 ) 257.It Dv SIGPROF No " terminate process" " profiling timer alarm (see" 258.Xr setitimer 2 ) 259.It Dv SIGWINCH No " discard signal" " Window size change" 260.It Dv SIGINFO No " discard signal" " status request from keyboard" 261.It Dv SIGUSR1 No " terminate process" " User defined signal 1" 262.It Dv SIGUSR2 No " terminate process" " User defined signal 2" 263.El 264.Sh NOTE 265The mask specified in 266.Fa act 267is not allowed to block 268.Dv SIGKILL 269or 270.Dv SIGSTOP 271This is done silently by the system. 272.Sh RETURN VALUES 273A 0 value indicated that the call succeeded. A \-1 return value 274indicates an error occurred and 275.Va errno 276is set to indicated the reason. 277.Sh ERROR 278.Fn Sigaction 279will fail and no new signal handler will be installed if one 280of the following occurs: 281.Tw Er 282.Tl Bq Er EFAULT 283Either 284.Fa act 285or 286.Fa oact 287points to memory that is not a valid part of the process 288address space. 289.Tl Bq Er EINVAL 290.Fa Sig 291is not a valid signal number. 292.Tl Bq Er EINVAL 293An attempt is made to ignore or supply a handler for 294.Em SIGKIL 295or 296.Dv SIGSTOP 297.Tl 298.Sh STANDARD 299The 300.Nm sigaction 301function is defined by 302.St -p1003.1-88 . 303The 304.Dv SA_ONSTACK 305and 306.Dv SA_RESTART 307flags are Berkeley extensions, 308as are the signals, 309.Dv SIGTRAP , 310.Dv SIGEMT , 311.Dv SIGBUS , 312.Dv SIGSYS , 313.Dv SIGURG , 314.Dv SIGIO , 315.Dv SIGXCPU , 316.Dv SIGXFSZ , 317.Dv SIGVTALRM , 318.Dv SIGPROF , 319.Dv SIGWINCH , 320and 321.Dv SIGINFO . 322Most of those signals are available on most 323.Tn BSD Ns \-derived 324systems. 325.Sh SEE ALSO 326.Xr kill 1 , 327.Xr ptrace 2 , 328.Xr kill 2 , 329.Xr sigaction 2 , 330.Xr sigprocmask 2 , 331.Xr sigsetops 2 , 332.Xr sigsuspend 2 , 333.Xr sigblock 2 , 334.Xr sigsetmask 2 , 335.Xr sigpause 2 , 336.Xr sigstack 2 , 337.Xr sigvec 2 , 338.Xr setjmp 3 , 339.Xr siginterrupt 3 , 340.Xr tty 4 341.Sh EXAMPLE 342On a 343.Tn VAX\-11, 344the handler routine can be declared: 345.Bd -literal -offset indent 346void handler(sig, code, scp) 347int sig, code; 348struct sigcontext *scp; 349.Ed 350.Pp 351Here 352.Fa sig 353is the signal number, into which the hardware faults and traps are 354mapped as defined below. 355.Em Code 356is a parameter that is either a constant 357as given below or the code provided by 358the hardware (Compatibility mode faults are distinguished from the 359other 360.Dv SIGILL 361traps by having 362.Dv PSL_CM 363set in the psl). 364.Fa Scp 365is a pointer to the 366.Fa sigcontext 367structure (defined in 368.Aq Pa signal.h ) , 369used to restore the context from before the signal. 370