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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" From: @(#)sigaction.2 8.2 (Berkeley) 4/3/94 29.\" $FreeBSD: src/lib/libc/sys/sigaction.2,v 1.22.2.10 2002/12/29 16:35:34 schweikh Exp $ 30.\" 31.Dd April 3, 1994 32.Dt SIGACTION 2 33.Os 34.Sh NAME 35.Nm sigaction 36.Nd software signal facilities 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In signal.h 41.Bd -literal 42struct sigaction { 43 /* 44 * Signal handler function if flag SA_SIGINFO is not used and for 45 * SIG_DFL and SIG_IGN. 46 */ 47 void (*sa_handler)(int); 48 49 /* Signal handler function if flag SA_SIGINFO is used */ 50 void (*sa_sigaction)(int, siginfo_t *, void *); 51 52 sigset_t sa_mask; /* signal mask to apply */ 53 int sa_flags; /* see signal options below */ 54}; 55.Ed 56.Ft int 57.Fn sigaction "int sig" "const struct sigaction *act" "struct sigaction *oact" 58.Sh DESCRIPTION 59The system defines a set of signals that may be delivered to a process. 60Signal delivery resembles the occurrence of a hardware interrupt: 61the signal is normally blocked from further occurrence, the current process 62context is saved, and a new one is built. 63A process may specify a 64.Em handler 65to which a signal is delivered, or specify that a signal is to be 66.Em ignored . 67A process may also specify that a default action is to be taken 68by the system when a signal occurs. 69A signal may also be 70.Em blocked , 71in which case its delivery is postponed until it is 72.Em unblocked . 73The action to be taken on delivery is determined at the time 74of delivery. 75Normally, signal handlers execute on the current stack 76of the process. 77This may be changed, on a per-handler basis, 78so that signals are taken on a special 79.Em "signal stack" . 80.Pp 81Signal routines normally execute with the signal that caused their 82invocation 83.Em blocked , 84but other signals may yet occur. 85A global 86.Em "signal mask" 87defines the set of signals currently blocked from delivery 88to a process. 89The signal mask for a process is initialized 90from that of its parent (normally empty). 91It may be changed with a 92.Xr sigprocmask 2 93call, or when a signal is delivered to the process. 94.Pp 95When a signal 96condition arises for a process, the signal is added to a set of 97signals pending for the process. 98If the signal is not currently 99.Em blocked 100by the process then it is delivered to the process. 101Signals may be delivered any time a process enters the operating system 102(e.g., during a system call, page fault or trap, or clock interrupt). 103If multiple signals are ready to be delivered at the same time, 104any signals that could be caused by traps are delivered first. 105Additional signals may be processed at the same time, with each 106appearing to interrupt the handlers for the previous signals 107before their first instructions. 108The set of pending signals is returned by the 109.Xr sigpending 2 110function. 111When a caught signal 112is delivered, the current state of the process is saved, 113a new signal mask is calculated (as described below), 114and the signal handler is invoked. 115The call to the handler 116is arranged so that if the signal handling routine returns 117normally the process will resume execution in the context 118from before the signal's delivery. 119If the process wishes to resume in a different context, then it 120must arrange to restore the previous context itself. 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 set, 128the signal to be delivered, and 129the signal mask associated with the handler to be invoked. 130.Pp 131.Fn Sigaction 132assigns an action for a signal specified by 133.Fa sig . 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 normally 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 . 158The defaults are process termination, possibly with core dump; 159no action; stopping the process; or continuing the process. 160See the signal list below for each signal's default action. 161If 162.Fa sa_handler 163is 164.Dv SIG_DFL , 165the default action for the signal is to discard the signal, 166and if a signal is pending, 167the pending signal is discarded even if the signal is masked. 168If 169.Fa sa_handler 170is set to 171.Dv SIG_IGN 172current and pending instances 173of the signal are ignored and discarded. 174.Pp 175Options may be specified by setting 176.Em sa_flags . 177The meaning of the various bits is as follows: 178.Bl -tag -offset indent -width SA_RESETHANDXX 179.It Dv SA_NOCLDSTOP 180If this bit is set when installing a catching function 181for the 182.Dv SIGCHLD 183signal, 184the 185.Dv SIGCHLD 186signal will be generated only when a child process exits, 187not when a child process stops. 188.It Dv SA_NOCLDWAIT 189If this bit is set when calling 190.Fn sigaction 191for the 192.Dv SIGCHLD 193signal, the system will not create zombie processes when children of 194the calling process exit. 195If the calling process subsequently issues a 196.Xr wait 2 197(or equivalent), it blocks until all of the calling process's child 198processes terminate, and then returns a value of -1 with 199.Va errno 200set to 201.Er ECHILD . 202.It Dv SA_ONSTACK 203If this bit is set, the system will deliver the signal to the process 204on a 205.Em "signal stack" , 206specified with 207.Xr sigaltstack 2 . 208.It Dv SA_NODEFER 209If this bit is set, further occurrences of the delivered signal are 210not masked during the execution of the handler. 211.It Dv SA_RESETHAND 212If this bit is set, the handler is reset back to 213.Dv SIG_DFL 214at the moment the signal is delivered. 215.It Dv SA_SIGINFO 216If this bit is set, the handler function is assumed to be pointed to by the 217.Dv sa_sigaction 218member of struct sigaction and should match the prototype shown above or as 219below in 220.Sx EXAMPLES . 221This bit should not be set when assigning 222.Dv SIG_DFL 223or 224.Dv SIG_IGN . 225.El 226.Pp 227If a signal is caught during the system calls listed below, 228the call may be forced to terminate 229with the error 230.Er EINTR , 231the call may return with a data transfer shorter than requested, 232or the call may be restarted. 233Restart of pending calls is requested 234by setting the 235.Dv SA_RESTART 236bit in 237.Ar sa_flags . 238The affected system calls include 239.Xr open 2 , 240.Xr read 2 , 241.Xr write 2 , 242.Xr sendto 2 , 243.Xr recvfrom 2 , 244.Xr sendmsg 2 245and 246.Xr recvmsg 2 247on a communications channel or a slow device (such as a terminal, 248but not a regular file) 249and during a 250.Xr wait 2 251or 252.Xr ioctl 2 . 253However, calls that have already committed are not restarted, 254but instead return a partial success (for example, a short read count). 255.Pp 256After a 257.Xr fork 2 258or 259.Xr vfork 2 260all signals, the signal mask, the signal stack, 261and the restart/interrupt flags are inherited by the child. 262.Pp 263.Xr Execve 2 264reinstates the default 265action for all signals which were caught and 266resets all signals to be caught on the user stack. 267Ignored signals remain ignored; 268the signal mask remains the same; 269signals that restart pending system calls continue to do so. 270.Pp 271The following is a list of all signals 272with names as in the include file 273.In signal.h : 274.Bl -column SIGVTALARMXX "create core imagexxx" 275.It Sy "NAME Default Action Description" 276.It Dv SIGHUP No " terminate process" " terminal line hangup" 277.It Dv SIGINT No " terminate process" " interrupt program" 278.It Dv SIGQUIT No " create core image" " quit program" 279.It Dv SIGILL No " create core image" " illegal instruction" 280.It Dv SIGTRAP No " create core image" " trace trap" 281.It Dv SIGABRT No " create core image" Ta Xr abort 3 282call (formerly 283.Dv SIGIOT ) 284.It Dv SIGEMT No " create core image" " emulate instruction executed" 285.It Dv SIGFPE No " create core image" " floating-point exception" 286.It Dv SIGKILL No " terminate process" " kill program" 287.It Dv SIGBUS No " create core image" " bus error" 288.It Dv SIGSEGV No " create core image" " segmentation violation" 289.It Dv SIGSYS No " create core image" " non-existent system call invoked" 290.It Dv SIGPIPE No " terminate process" " write on a pipe with no reader" 291.It Dv SIGALRM No " terminate process" " real-time timer expired" 292.It Dv SIGTERM No " terminate process" " software termination signal" 293.It Dv SIGURG No " discard signal" " urgent condition present on socket" 294.It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)" 295.It Dv SIGTSTP No " stop process" " stop signal generated from keyboard" 296.It Dv SIGCONT No " discard signal" " continue after stop" 297.It Dv SIGCHLD No " discard signal" " child status has changed" 298.It Dv SIGTTIN No " stop process" " background read attempted from control terminal" 299.It Dv SIGTTOU No " stop process" " background write attempted to control terminal" 300.It Dv SIGIO No " discard signal" Tn " I/O" 301is possible on a descriptor (see 302.Xr fcntl 2 ) 303.It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see" 304.Xr setrlimit 2 ) 305.It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see" 306.Xr setrlimit 2 ) 307.It Dv SIGVTALRM No " terminate process" " virtual time alarm (see" 308.Xr setitimer 2 ) 309.It Dv SIGPROF No " terminate process" " profiling timer alarm (see" 310.Xr setitimer 2 ) 311.It Dv SIGWINCH No " discard signal" " Window size change" 312.It Dv SIGINFO No " discard signal" " status request from keyboard" 313.It Dv SIGUSR1 No " terminate process" " User defined signal 1" 314.It Dv SIGUSR2 No " terminate process" " User defined signal 2" 315.El 316.Sh NOTE 317The 318.Fa sa_mask 319field specified in 320.Fa act 321is not allowed to block 322.Dv SIGKILL 323or 324.Dv SIGSTOP . 325Any attempt to do so will be silently ignored. 326.Pp 327The following functions are either reentrant or not interruptible 328by signals and are async-signal safe. 329Therefore applications may 330invoke them, without restriction, from signal-catching functions: 331.Pp 332Base Interfaces: 333.Pp 334.Fn _exit , 335.Fn access , 336.Fn alarm , 337.Fn cfgetispeed , 338.Fn cfgetospeed , 339.Fn cfsetispeed , 340.Fn cfsetospeed , 341.Fn chdir , 342.Fn chmod , 343.Fn chown , 344.Fn close , 345.Fn creat , 346.Fn dup , 347.Fn dup2 , 348.Fn execle , 349.Fn execve , 350.Fn fcntl , 351.Fn fork , 352.Fn fpathconf , 353.Fn fstat , 354.Fn fsync , 355.Fn getegid , 356.Fn geteuid , 357.Fn getgid , 358.Fn getgroups , 359.Fn getpgrp , 360.Fn getpid , 361.Fn getppid , 362.Fn getuid , 363.Fn kill , 364.Fn link , 365.Fn lseek , 366.Fn mkdir , 367.Fn mkfifo , 368.Fn open , 369.Fn pathconf , 370.Fn pause , 371.Fn pipe , 372.Fn raise , 373.Fn read , 374.Fn rename , 375.Fn rmdir , 376.Fn setgid , 377.Fn setpgid , 378.Fn setsid , 379.Fn setuid , 380.Fn sigaction , 381.Fn sigaddset , 382.Fn sigdelset , 383.Fn sigemptyset , 384.Fn sigfillset , 385.Fn sigismember , 386.Fn signal , 387.Fn sigpending , 388.Fn sigprocmask , 389.Fn sigsuspend , 390.Fn sleep , 391.Fn stat , 392.Fn sysconf , 393.Fn tcdrain , 394.Fn tcflow , 395.Fn tcflush , 396.Fn tcgetattr , 397.Fn tcgetpgrp , 398.Fn tcsendbreak , 399.Fn tcsetattr , 400.Fn tcsetpgrp , 401.Fn time , 402.Fn times , 403.Fn umask , 404.Fn uname , 405.Fn unlink , 406.Fn utime , 407.Fn wait , 408.Fn waitpid , 409.Fn write . 410.Pp 411Realtime Interfaces: 412.Pp 413.Fn aio_error , 414.Fn clock_gettime , 415.Fn sigpause , 416.\".Fn timer_getoverrun , 417.Fn aio_return , 418.\".Fn fdatasync , 419.Fn sigqueue , 420.\".Fn timer_gettime , 421.Fn aio_suspend , 422.Fn sem_post , 423.Fn sigset . 424.\".Fn timer_settime . 425.Pp 426All functions not in the above lists are considered to be unsafe 427with respect to signals. 428That is to say, the behaviour of such 429functions when called from a signal handler is undefined. 430.Sh RETURN VALUES 431.Rv -std sigaction 432.Sh EXAMPLES 433There are three possible prototypes the handler may match: 434.Bl -tag -offset indent -width short 435.It ANSI C: 436.Ft void 437.Fn handler int ; 438.It Traditional BSD style: 439.Ft void 440.Fn handler int "int code" "struct sigcontext *scp" ; 441.It POSIX SA_SIGINFO: 442.Ft void 443.Fn handler int "siginfo_t *info" "ucontext_t *uap" ; 444.El 445.Pp 446The handler function should match the 447.Dv SA_SIGINFO 448prototype if the 449.Dv SA_SIGINFO 450bit is set in flags. 451It then should be pointed to by the 452.Dv sa_sigaction 453member of 454.Dv struct sigaction . 455Note that you should not assign 456.Dv SIG_DFL 457or 458.Dv SIG_IGN 459this way. 460.Pp 461If the 462.Dv SA_SIGINFO 463flag is not set, the handler function should match 464either the ANSI C or traditional 465.Bx 466prototype and be pointed to by 467the 468.Dv sa_handler 469member of 470.Dv struct sigaction . 471In practice, 472.Dx 473always sends the three arguments of the latter and since the ANSI C 474prototype is a subset, both will work. 475The 476.Dv sa_handler 477member declaration in 478.Dx 479include files is that of ANSI C (as required by POSIX), 480so a function pointer of a 481.Bx Ns -style 482function needs to be casted to 483compile without warning. 484The traditional 485.Bx 486style is not portable and since its capabilities are a full subset of a 487.Dv SA_SIGINFO 488handler, its use is deprecated. 489.Pp 490The 491.Fa sig 492argument is the signal number, one of the 493.Dv SIG... 494.In signal.h . 495.Pp 496The 497.Fa code 498argument of the 499.Bx Ns -style 500handler and the 501.Dv si_code 502member of the 503.Dv info 504argument to a 505.Dv SA_SIGINFO 506handler contain a numeric code explaining the 507cause of the signal, usually one of the 508.Dv SI_... 509values from 510.In sys/signal.h 511or codes specific to a signal, i.e.\& one of the 512.Dv FPE_... 513values for 514.Dv SIGFPE . 515.Pp 516The 517.Fa scp 518argument to a 519.Bx Ns -style 520handler points to an instance of struct 521sigcontext. 522.Pp 523The 524.Fa uap 525argument to a POSIX 526.Dv SA_SIGINFO 527handler points to an instance of ucontext_t. 528.Sh ERRORS 529.Fn Sigaction 530will fail and no new signal handler will be installed if one 531of the following occurs: 532.Bl -tag -width Er 533.It Bq Er EFAULT 534Either 535.Fa act 536or 537.Fa oact 538points to memory that is not a valid part of the process 539address space. 540.It Bq Er EINVAL 541.Fa Sig 542is not a valid signal number. 543.It Bq Er EINVAL 544An attempt is made to ignore or supply a handler for 545.Dv SIGKILL 546or 547.Dv SIGSTOP . 548.El 549.Sh SEE ALSO 550.Xr kill 1 , 551.Xr kill 2 , 552.Xr ptrace 2 , 553.Xr sigaltstack 2 , 554.Xr sigpending 2 , 555.Xr sigprocmask 2 , 556.Xr sigsuspend 2 , 557.Xr wait 2 , 558.Xr fpsetmask 3 , 559.Xr setjmp 3 , 560.Xr siginterrupt 3 , 561.Xr sigsetops 3 , 562.Xr ucontext 3 , 563.Xr tty 4 564.Sh STANDARDS 565The 566.Fn sigaction 567function call is expected to conform to 568.St -p1003.1-90 . 569The 570.Dv SA_ONSTACK 571and 572.Dv SA_RESTART 573flags are Berkeley extensions, 574as are the signals, 575.Dv SIGTRAP , 576.Dv SIGEMT , 577.Dv SIGBUS , 578.Dv SIGSYS , 579.Dv SIGURG , 580.Dv SIGIO , 581.Dv SIGXCPU , 582.Dv SIGXFSZ , 583.Dv SIGVTALRM , 584.Dv SIGPROF , 585.Dv SIGWINCH , 586and 587.Dv SIGINFO . 588Those signals are available on most 589.Bx Ns \-derived 590systems. 591The 592.Dv SA_NODEFER 593and 594.Dv SA_RESETHAND 595flags are intended for backwards compatibility with other operating 596systems. 597The 598.Dv SA_NOCLDSTOP , 599and 600.Dv SA_NOCLDWAIT 601.\" and 602.\" SA_SIGINFO 603flags are featuring options commonly found in other operating systems. 604