1.\" $NetBSD: sigaction.2,v 1.28 2002/07/15 07:48:57 jdolecek 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 November 1, 1997 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.Fd #include \*[Lt]signal.h\*[Gt] 46.Bd -literal 47struct sigaction { 48 void (*sa_handler)(int); 49 sigset_t sa_mask; 50 int sa_flags; 51}; 52.Ed 53.Ft int 54.Fn sigaction "int sig" "const struct sigaction *act" "struct sigaction *oact" 55.Sh DESCRIPTION 56The system defines a set of signals that may be delivered to a process. 57Signal delivery resembles the occurrence of a hardware interrupt: 58the signal is blocked from further occurrence, the current process 59context is saved, and a new one is built. 60A process may specify a 61.Em handler 62to which a signal is delivered, or specify that a signal is to be 63.Em ignored . 64A process may also specify that a default action is to be taken 65by the system when a signal occurs. 66A signal may also be 67.Em blocked , 68in which case its delivery is postponed until it is 69.Em unblocked . 70The action to be taken on delivery is determined at the time of delivery. 71Normally, signal handlers execute on the current stack of the process. 72This may be changed, on a per-handler basis, so that signals are 73taken on a special 74.Em "signal stack" . 75.Pp 76Signal routines execute with the signal that caused their 77invocation 78.Em blocked , 79but other signals may yet occur. 80A global 81.Em "signal mask" 82defines the set of signals currently blocked from delivery 83to a process. 84The signal mask for a process is initialized from that of its parent 85(normally empty). 86It may be changed with a 87.Xr sigprocmask 2 88call, or when a signal is delivered to the process. 89.Pp 90When a signal 91condition arises for a process, the signal is added to a set of 92signals pending for the process. 93If the signal is not currently 94.Em blocked 95by the process then it is delivered to the process. 96Signals may be delivered any time a process enters the operating system 97(e.g., during a system call, page fault or trap, or clock interrupt). 98If multiple signals are ready to be delivered at the same time, 99any signals that could be caused by traps are delivered first. 100Additional signals may be processed at the same time, with each 101appearing to interrupt the handlers for the previous signals 102before their first instructions. 103The set of pending signals is returned by the 104.Xr sigpending 2 105function. 106When a caught signal 107is delivered, the current state of the process is saved, 108a new signal mask is calculated (as described below), 109and the signal handler is invoked. 110The call to the handler is arranged so that if the signal handling 111routine returns normally the process will resume execution in the 112context from before the signal's delivery. 113If the process wishes to resume in a different context, then it 114must arrange to restore the previous context itself. 115.Pp 116When a signal is delivered to a process a new signal mask is 117installed for the duration of the process' signal handler 118(or until a 119.Xr sigprocmask 2 120call is made). 121This mask is formed by taking the union of the current signal mask, 122the signal to be delivered, and 123the signal mask associated with the handler to be invoked, 124.Em sa_mask . 125.Pp 126.Fn sigaction 127assigns an action for a specific signal. 128If 129.Fa act 130is non-zero, it 131specifies an action 132.Pf ( Dv SIG_DFL , 133.Dv SIG_IGN , 134or a handler routine) and mask 135to be used when delivering the specified signal. 136If 137.Fa oact 138is non-zero, the previous handling information for the signal 139is returned to the user. 140.Pp 141Once a signal handler is installed, it remains installed 142until another 143.Fn sigaction 144call is made, or an 145.Xr execve 2 146is performed. 147A signal-specific default action may be reset by 148setting 149.Fa sa_handler 150to 151.Dv SIG_DFL . 152Alternately, if the 153.Dv SA_RESETHAND 154bit is set the default action will be reinstated when the signal 155is first posted. 156The defaults are process termination, possibly with core dump; 157no action; stopping the process; or continuing the process. 158See the signal list below for each signal's default action. 159If 160.Fa sa_handler 161is set to 162.Dv SIG_DFL , 163the default action for the signal is to discard the signal, 164and if a signal is pending, 165the pending signal is discarded even if the signal is masked. 166If 167.Fa sa_handler 168is set to 169.Dv SIG_IGN , 170current and pending instances 171of the signal are ignored and discarded. 172.Pp 173Options may be specified by setting 174.Em sa_flags . 175If the 176.Dv SA_NOCLDSTOP 177bit is set when installing a catching function 178for the 179.Dv SIGCHLD 180signal, 181the 182.Dv SIGCHLD 183signal will be generated only when a child process exits, 184not when a child process stops. 185Further, if the 186.Dv SA_ONSTACK 187bit is set in 188.Em sa_flags , 189the system will deliver the signal to the process on a 190.Em "signal stack" , 191specified with 192.Xr sigaltstack 2 . 193Finally, if the 194.Dv SA_NOCLDWAIT 195bit is set in 196.Em sa_flags , 197the system will not create a zombie when the child exits, but the child 198process will be automatically waited for. 199.Pp 200If a signal is caught during the system calls listed below, 201the call may be forced to terminate 202with the error 203.Er EINTR , 204the call may return with a data transfer shorter than requested, 205or the call may be restarted. 206Restarting of pending calls is requested 207by setting the 208.Dv SA_RESTART 209bit in 210.Ar sa_flags . 211The affected system calls include 212.Xr open 2 , 213.Xr read 2 , 214.Xr write 2 , 215.Xr sendto 2 , 216.Xr recvfrom 2 , 217.Xr sendmsg 2 218and 219.Xr recvmsg 2 220on a communications channel or a slow device (such as a terminal, 221but not a regular file) 222and during a 223.Xr wait 2 224or 225.Xr ioctl 2 . 226However, calls that have already committed are not restarted, 227but instead return a partial success (for example, a short read count). 228.Pp 229After a 230.Xr fork 2 231or 232.Xr vfork 2 233all signals, the signal mask, the signal stack, 234and the restart/interrupt flags are inherited by the child. 235.Pp 236The 237.Xr execve 2 238system call reinstates the default 239action for all signals which were caught and 240resets all signals to be caught on the user stack. 241Ignored signals remain ignored; 242the signal mask remains the same; 243signals that restart pending system calls continue to do so. 244.Pp 245See 246.Xr signal 7 247for comprehensive list of supported signals. 248.Sh NOTES 249The mask specified in 250.Fa act 251is not allowed to block 252.Dv SIGKILL 253or 254.Dv SIGSTOP . 255This is enforced silently by the system. 256.Sh RETURN VALUES 257A 0 value indicates that the call succeeded. 258A \-1 return value indicates an error occurred and 259.Va errno 260is set to indicate the reason. 261.Sh EXAMPLES 262The handler routine can be declared: 263.Bd -literal -offset indent 264void 265handler(sig, code, scp) 266 int sig, code; 267 struct sigcontext *scp; 268.Ed 269.Pp 270Here 271.Fa sig 272is the signal number, into which the hardware faults and traps are 273mapped. 274.Fa code 275is a parameter that is either a constant 276or the code provided by the hardware. 277.Fa scp 278is a pointer to the 279.Fa sigcontext 280structure (defined in 281.Aq Pa signal.h ) , 282used to restore the context from before the signal. 283.Pp 284For POSIX compliance, the 285.Fa sa_handler 286is declared to be (void (*)(int)) and the above handler will need to be 287casted to that type. 288Future versions of 289.Nx 290will replace the 291.Fa sigcontext 292interface with the 293.Fa siginfo 294interface. 295.Sh ERRORS 296.Fn sigaction 297will fail and no new signal handler will be installed if one 298of the following occurs: 299.Bl -tag -width Er 300.It Bq Er EFAULT 301Either 302.Fa act 303or 304.Fa oact 305points to memory that is not a valid part of the process 306address space. 307.It Bq Er EINVAL 308.Fa sig 309is not a valid signal number. 310.It Bq Er EINVAL 311An attempt is made to ignore or supply a handler for 312.Dv SIGKILL 313or 314.Dv SIGSTOP . 315.El 316.Sh SEE ALSO 317.Xr kill 1 , 318.Xr kill 2 , 319.Xr ptrace 2 , 320.Xr sigaltstack 2 , 321.Xr sigprocmask 2 , 322.Xr sigsuspend 2 , 323.Xr setjmp 3 , 324.Xr sigsetops 3 , 325.Xr tty 4 , 326.Xr signal 7 327.Sh STANDARDS 328The 329.Fn sigaction 330function conforms to 331.St -p1003.1-90 . 332The 333.Dv SA_ONSTACK 334and 335.Dv SA_RESTART 336flags are Berkeley extensions, available on most 337.Bx Ns \-derived 338systems. 339