1.\" $NetBSD: sigaction.2,v 1.34 2003/05/11 19:22:18 kleink Exp $ 2.\" 3.\" Copyright (c) 1980, 1990, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)sigaction.2 8.2 (Berkeley) 4/3/94 35.\" 36.Dd May 11, 2003 37.Dt SIGACTION 2 38.Os 39.Sh NAME 40.Nm sigaction 41.Nd software signal facilities 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In signal.h 46.Ft int 47.Fn sigaction "int sig" "const struct sigaction *act" "struct sigaction *oact" 48.Sh DESCRIPTION 49The system defines a set of signals that may be delivered to a process. 50Signal delivery resembles the occurrence of a hardware interrupt: 51the signal is blocked from further occurrence, the current process 52context is saved, and a new one is built. 53A process may specify a 54.Em handler 55to which a signal is delivered, or specify that a signal is to be 56.Em ignored . 57A process may also specify that a default action is to be taken 58by the system when a signal occurs. 59A signal may also be 60.Em blocked , 61in which case its delivery is postponed until it is 62.Em unblocked . 63The action to be taken on delivery is determined at the time of delivery. 64Normally, signal handlers execute on the current stack of the process. 65This may be changed, on a per-handler basis, so that signals are 66taken on a special 67.Em "signal stack" . 68.Pp 69Signal routines execute with the signal that caused their 70invocation 71.Em blocked , 72but other signals may yet occur. 73A global 74.Em "signal mask" 75defines the set of signals currently blocked from delivery 76to a process. 77The signal mask for a process is initialized from that of its parent 78(normally empty). 79It may be changed with a 80.Xr sigprocmask 2 81call, or when a signal is delivered to the process. 82Signal masks are represented using the 83.Em sigset_t 84type; the 85.Xr sigsetops 3 86interface is used to modify such data. 87.Pp 88When a signal 89condition arises for a process, the signal is added to a set of 90signals pending for the process. 91If the signal is not currently 92.Em blocked 93by the process then it is delivered to the process. 94Signals may be delivered any time a process enters the operating system 95(e.g., during a system call, page fault or trap, or clock interrupt). 96If multiple signals are ready to be delivered at the same time, 97any signals that could be caused by traps are delivered first. 98Additional signals may be processed at the same time, with each 99appearing to interrupt the handlers for the previous signals 100before their first instructions. 101The set of pending signals is returned by the 102.Xr sigpending 2 103function. 104When a caught signal 105is delivered, the current state of the process is saved, 106a new signal mask is calculated (as described below), 107and the signal handler is invoked. 108The call to the handler is arranged so that if the signal handling 109routine returns normally the process will resume execution in the 110context from before the signal's delivery. 111If the process wishes to resume in a different context, then it 112must arrange to restore the previous context itself. 113.Pp 114.Em "struct sigaction" 115includes the following members: 116.Bd -literal -offset indent 117void (*sa_handler)(int) 118sigset_t sa_mask 119int sa_flags 120.Ed 121.Pp 122When a signal is delivered to a process a new signal mask is 123installed for the duration of the process' signal handler 124(or until a 125.Xr sigprocmask 2 126call is made). 127This mask is formed by taking the union of the current signal mask, 128the signal to be delivered, and 129the signal mask associated with the handler to be invoked, 130.Em sa_mask . 131.Pp 132.Fn sigaction 133assigns an action for a specific signal. 134If 135.Fa act 136is non-zero, it 137specifies an action 138.Pf ( Dv SIG_DFL , 139.Dv SIG_IGN , 140or a handler routine) and mask 141to be used when delivering the specified signal. 142If 143.Fa oact 144is non-zero, the previous handling information for the signal 145is returned to the user. 146.Pp 147Once a signal handler is installed, it remains installed 148until another 149.Fn sigaction 150call is made, or an 151.Xr execve 2 152is performed. 153A signal-specific default action may be reset by 154setting 155.Fa sa_handler 156to 157.Dv SIG_DFL . 158Alternately, if the 159.Dv SA_RESETHAND 160bit is set the default action will be reinstated when the signal 161is first posted. 162The defaults are process termination, possibly with core dump; 163no action; stopping the process; or continuing the process. 164See the signal list below for each signal's default action. 165If 166.Fa sa_handler 167is set to 168.Dv SIG_DFL , 169the default action for the signal is to discard the signal, 170and if a signal is pending, 171the pending signal is discarded even if the signal is masked. 172If 173.Fa sa_handler 174is set to 175.Dv SIG_IGN , 176current and pending instances 177of the signal are ignored and discarded. 178.Pp 179Options may be specified by setting 180.Em sa_flags . 181If the 182.Dv SA_NOCLDSTOP 183bit is set when installing a catching function 184for the 185.Dv SIGCHLD 186signal, 187the 188.Dv SIGCHLD 189signal will be generated only when a child process exits, 190not when a child process stops. 191Further, if the 192.Dv SA_ONSTACK 193bit is set in 194.Em sa_flags , 195the system will deliver the signal to the process on a 196.Em "signal stack" , 197specified with 198.Xr sigaltstack 2 . 199Finally, if the 200.Dv SA_NOCLDWAIT 201bit is set in 202.Em sa_flags , 203the system will not create a zombie when the child exits, but the child 204process will be automatically waited for. 205.Pp 206If a signal is caught during the system calls listed below, 207the call may be forced to terminate 208with the error 209.Er EINTR , 210the call may return with a data transfer shorter than requested, 211or the call may be restarted. 212Restarting of pending calls is requested 213by setting the 214.Dv SA_RESTART 215bit in 216.Ar sa_flags . 217The affected system calls include 218.Xr open 2 , 219.Xr read 2 , 220.Xr write 2 , 221.Xr sendto 2 , 222.Xr recvfrom 2 , 223.Xr sendmsg 2 224and 225.Xr recvmsg 2 226on a communications channel or a slow device (such as a terminal, 227but not a regular file) 228and during a 229.Xr wait 2 230or 231.Xr ioctl 2 . 232However, calls that have already committed are not restarted, 233but instead return a partial success (for example, a short read count). 234.Pp 235After a 236.Xr fork 2 237or 238.Xr vfork 2 239all signals, the signal mask, the signal stack, 240and the restart/interrupt flags are inherited by the child. 241.Pp 242The 243.Xr execve 2 244system call reinstates the default 245action for all signals which were caught and 246resets all signals to be caught on the user stack. 247Ignored signals remain ignored; 248the signal mask remains the same; 249signals that restart pending system calls continue to do so. 250.Pp 251See 252.Xr signal 7 253for comprehensive list of supported signals. 254.Sh NOTES 255The mask specified in 256.Fa act 257is not allowed to block 258.Dv SIGKILL 259or 260.Dv SIGSTOP . 261This is enforced silently by the system. 262.Sh RETURN VALUES 263A 0 value indicates that the call succeeded. 264A \-1 return value indicates an error occurred and 265.Va errno 266is set to indicate the reason. 267.Sh EXAMPLES 268The handler routine can be declared: 269.Bd -literal -offset indent 270void 271handler(sig, code, scp) 272 int sig, code; 273 struct sigcontext *scp; 274.Ed 275.Pp 276Here 277.Fa sig 278is the signal number, into which the hardware faults and traps are 279mapped. 280.Fa code 281is a parameter that is either a constant 282or the code provided by the hardware. 283.Fa scp 284is a pointer to the 285.Fa sigcontext 286structure (defined in 287.Aq Pa signal.h ) , 288used to restore the context from before the signal. 289.Pp 290For POSIX compliance, the 291.Fa sa_handler 292is declared to be (void (*)(int)) and the above handler will need to be 293casted to that type. 294Future versions of 295.Nx 296will replace the 297.Fa sigcontext 298interface with the 299.Fa siginfo 300interface. 301.Sh ERRORS 302.Fn sigaction 303will fail and no new signal handler will be installed if one 304of the following occurs: 305.Bl -tag -width Er 306.It Bq Er EFAULT 307Either 308.Fa act 309or 310.Fa oact 311points to memory that is not a valid part of the process 312address space. 313.It Bq Er EINVAL 314.Fa sig 315is not a valid signal number. 316.It Bq Er EINVAL 317An attempt is made to ignore or supply a handler for 318.Dv SIGKILL 319or 320.Dv SIGSTOP . 321.It Bq Er EINVAL 322The 323.Em sa_flags 324word contains bits other than 325.Dv SA_ONSTACK , 326.Dv SA_RESTART , 327.Dv SA_RESETHAND , 328.Dv SA_NODEFER , 329.Dv SA_SIGINFO , 330.Dv SA_NOCLDWAIT , 331and 332.Dv SA_NOCLDSTOP . 333.El 334.Sh SEE ALSO 335.Xr kill 1 , 336.Xr kill 2 , 337.Xr ptrace 2 , 338.Xr sigaltstack 2 , 339.Xr sigprocmask 2 , 340.Xr sigsuspend 2 , 341.Xr setjmp 3 , 342.Xr sigsetops 3 , 343.Xr tty 4 , 344.Xr signal 7 345.Sh STANDARDS 346The 347.Fn sigaction 348function conforms to 349.St -p1003.1-90 . 350The 351.Dv SA_ONSTACK 352and 353.Dv SA_RESTART 354flags are Berkeley extensions, available on most 355.Bx Ns \-derived 356systems. 357