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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 1.5 1994/10/17 23:49:15 cgd 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.Ft int 51.Fn sigaction "int sig" "struct sigaction *act" "struct sigaction *oact" 52.Sh DESCRIPTION 53The system defines a set of signals that may be delivered to a process. 54Signal delivery resembles the occurence of a hardware interrupt: 55the signal is blocked from further occurrence, the current process 56context is saved, and a new one is built. A process may specify a 57.Em handler 58to which a signal is delivered, or specify that a signal is to be 59.Em ignored . 60A process may also specify that a default action is to be taken 61by the system when a signal occurs. 62A signal may also be 63.Em blocked , 64in which case its delivery is postponed until it is 65.Em unblocked . 66The action to be taken on delivery is determined at the time 67of delivery. 68Normally, signal handlers execute on the current stack 69of the process. This may be changed, on a per-handler basis, 70so that signals are taken on a special 71.Em "signal stack" . 72.Pp 73Signal routines execute with the signal that caused their 74invocation 75.Em blocked , 76but other signals may yet occur. 77A global 78.Em "signal mask" 79defines the set of signals currently blocked from delivery 80to a process. The signal mask for a process is initialized 81from that of its parent (normally empty). It 82may be changed with a 83.Xr sigprocmask 2 84call, or when a signal is delivered to the process. 85.Pp 86When a signal 87condition arises for a process, the signal is added to a set of 88signals pending for the process. 89If the signal is not currently 90.Em blocked 91by the process then it is delivered to the process. 92Signals may be delivered any time a process enters the operating system 93(e.g., during a system call, page fault or trap, or clock interrupt). 94If multiple signals are ready to be delivered at the same time, 95any signals that could be caused by traps are delivered first. 96Additional signals may be processed at the same time, with each 97appearing to interrupt the handlers for the previous signals 98before their first instructions. 99The set of pending signals is returned by the 100.Xr sigpending 2 101function. 102When a caught signal 103is delivered, the current state of the process is saved, 104a new signal mask is calculated (as described below), 105and the signal handler is invoked. The call to the handler 106is arranged so that if the signal handling routine returns 107normally the process will resume execution in the context 108from before the signal's delivery. 109If the process wishes to resume in a different context, then it 110must arrange to restore the previous context itself. 111.Pp 112When a signal is delivered to a process a new signal mask is 113installed for the duration of the process' signal handler 114(or until a 115.Xr sigprocmask 116call is made). 117This mask is formed by taking the union of the current signal mask set, 118the signal to be delivered, and 119the signal mask associated with the handler to be invoked. 120.Pp 121.Fn Sigaction 122assigns an action for a specific signal. 123If 124.Fa act 125is non-zero, it 126specifies an action 127.Pf ( Dv SIG_DFL , 128.Dv SIG_IGN , 129or a handler routine) and mask 130to be used when delivering the specified signal. 131If 132.Fa oact 133is non-zero, the previous handling information for the signal 134is returned to the user. 135.Pp 136Once a signal handler is installed, it remains installed 137until another 138.Fn sigaction 139call is made, or an 140.Xr execve 2 141is performed. 142A signal-specific default action may be reset by 143setting 144.Fa sa_handler 145to 146.Dv SIG_DFL . 147The defaults are process termination, possibly with core dump; 148no action; stopping the process; or continuing the process. 149See the signal list below for each signal's default action. 150If 151.Fa sa_handler 152is 153.Dv SIG_IGN 154current and pending instances 155of the signal are ignored and discarded. 156.Pp 157Options may be specified by setting 158.Em sa_flags . 159If the 160.Dv SA_NOCLDSTOP 161bit is set when installing a catching function 162for the 163.Dv SIGCHLD 164signal, 165the 166.Dv SIGCHLD 167signal will be generated only when a child process exits, 168not when a child process stops. 169Further, if the 170.Dv SA_ONSTACK 171bit is set in 172.Em sa_flags , 173the system will deliver the signal to the process on a 174.Em "signal stack" , 175specified with 176.Xr sigstack 2 . 177.Pp 178If a signal is caught during the system calls listed below, 179the call may be forced to terminate 180with the error 181.Dv EINTR , 182or the call may be restarted. 183Restart of pending calls is requested 184by setting the 185.Dv SA_RESTART 186bit in 187.Ar sa_flags . 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 slow device (such as a terminal, 197but not a regular file) 198and during a 199.Xr wait 2 200or 201.Xr ioctl 2 . 202However, calls that have already committed are not restarted, 203but instead return a partial success (for example, a short read count). 204.Pp 205After a 206.Xr fork 2 207or 208.Xr vfork 2 209all signals, the signal mask, the signal stack, 210and the restart/interrupt flags are inherited by the child. 211.Pp 212.Xr Execve 2 213reinstates the default 214action for all signals which were caught and 215resets all signals to be caught on the user stack. 216Ignored signals remain ignored; 217the signal mask remains the same; 218signals that restart pending system calls continue to do so. 219.Pp 220The following is a list of all signals 221with names as in the include file 222.Aq Pa signal.h : 223.Bl -column SIGVTALARMXX "create core imagexxx" 224.It Sy " NAME " " Default Action " " Description" 225.It Dv SIGHUP No " terminate process" " terminal line hangup" 226.It Dv SIGINT No " terminate process" " interrupt program" 227.It Dv SIGQUIT No " create core image" " quit program" 228.It Dv SIGILL No " create core image" " illegal instruction" 229.It Dv SIGTRAP No " create core image" " trace trap" 230.It Dv SIGABRT No " create core image" Xr abort 2 231call (formerly 232.Dv SIGIOT ) 233.It Dv SIGEMT No " create core image" " emulate instruction executed" 234.It Dv SIGFPE No " create core image" " floating-point exception" 235.It Dv SIGKILL No " terminate process" " kill program" 236.It Dv SIGBUS No " create core image" " bus error" 237.It Dv SIGSEGV No " create core image" " segmentation violation" 238.It Dv SIGSYS No " create core image" " system call given invalid argument" 239.It Dv SIGPIPE No " terminate process" " write on a pipe with no reader" 240.It Dv SIGALRM No " terminate process" " real-time timer expired" 241.It Dv SIGTERM No " terminate process" " software termination signal" 242.It Dv SIGURG No " discard signal" " urgent condition present on socket" 243.It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)" 244.It Dv SIGTSTP No " stop process" " stop signal generated from keyboard" 245.It Dv SIGCONT No " discard signal" " continue after stop" 246.It Dv SIGCHLD No " discard signal" " child status has changed" 247.It Dv SIGTTIN No " stop process" " background read attempted from control terminal" 248.It Dv SIGTTOU No " stop process" " background write attempted to control terminal" 249.It Dv SIGIO No " discard signal" Tn " I/O" 250is possible on a descriptor (see 251.Xr fcntl 2 ) 252.It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see" 253.Xr setrlimit 2 ) 254.It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see" 255.Xr setrlimit 2 ) 256.It Dv SIGVTALRM No " terminate process" " virtual time alarm (see" 257.Xr setitimer 2 ) 258.It Dv SIGPROF No " terminate process" " profiling timer alarm (see" 259.Xr setitimer 2 ) 260.It Dv SIGWINCH No " discard signal" " Window size change" 261.It Dv SIGINFO No " discard signal" " status request from keyboard" 262.It Dv SIGUSR1 No " terminate process" " User defined signal 1" 263.It Dv SIGUSR2 No " terminate process" " User defined signal 2" 264.El 265.Sh NOTE 266The mask specified in 267.Fa act 268is not allowed to block 269.Dv SIGKILL 270or 271.Dv SIGSTOP 272This is done silently by the system. 273.Sh RETURN VALUES 274A 0 value indicated that the call succeeded. A \-1 return value 275indicates an error occurred and 276.Va errno 277is set to indicated the reason. 278.Sh ERROR 279.Fn Sigaction 280will fail and no new signal handler will be installed if one 281of the following occurs: 282.Tw Er 283.Tl Bq Er EFAULT 284Either 285.Fa act 286or 287.Fa oact 288points to memory that is not a valid part of the process 289address space. 290.Tl Bq Er EINVAL 291.Fa Sig 292is not a valid signal number. 293.Tl Bq Er EINVAL 294An attempt is made to ignore or supply a handler for 295.Em SIGKIL 296or 297.Dv SIGSTOP 298.Tl 299.Sh STANDARD 300The 301.Nm sigaction 302function is defined by 303.St -p1003.1-88 . 304The 305.Dv SA_ONSTACK 306and 307.Dv SA_RESTART 308flags are Berkeley extensions, 309as are the signals, 310.Dv SIGTRAP , 311.Dv SIGEMT , 312.Dv SIGBUS , 313.Dv SIGSYS , 314.Dv SIGURG , 315.Dv SIGIO , 316.Dv SIGXCPU , 317.Dv SIGXFSZ , 318.Dv SIGVTALRM , 319.Dv SIGPROF , 320.Dv SIGWINCH , 321and 322.Dv SIGINFO . 323Most of those signals are available on most 324.Tn BSD Ns \-derived 325systems. 326.Sh SEE ALSO 327.Xr kill 1 , 328.Xr ptrace 2 , 329.Xr kill 2 , 330.Xr sigaction 2 , 331.Xr sigprocmask 2 , 332.Xr sigsetops 2 , 333.Xr sigsuspend 2 , 334.Xr sigblock 2 , 335.Xr sigsetmask 2 , 336.Xr sigpause 2 , 337.Xr sigstack 2 , 338.Xr sigvec 3 , 339.Xr setjmp 3 , 340.Xr siginterrupt 3 , 341.Xr tty 4 342.Sh EXAMPLE 343On a 344.Tn VAX\-11, 345the handler routine can be declared: 346.Bd -literal -offset indent 347void handler(sig, code, scp) 348int sig, code; 349struct sigcontext *scp; 350.Ed 351.Pp 352Here 353.Fa sig 354is the signal number, into which the hardware faults and traps are 355mapped as defined below. 356.Em Code 357is a parameter that is either a constant 358as given below or the code provided by 359the hardware (Compatibility mode faults are distinguished from the 360other 361.Dv SIGILL 362traps by having 363.Dv PSL_CM 364set in the psl). 365.Fa Scp 366is a pointer to the 367.Fa sigcontext 368structure (defined in 369.Aq Pa signal.h ) , 370used to restore the context from before the signal. 371