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Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" From: @(#)sigaction.2 8.2 (Berkeley) 4/3/94 .\" $FreeBSD$ .\" .Dd April 3, 1994 .Dt SIGACTION 2 .Os .Sh NAME .Nm sigaction .Nd software signal facilities .Sh LIBRARY .Lb libc .Sh SYNOPSIS .Fd #include .Bd -literal struct sigaction { union { void (*__sa_handler) __P((int)); void (*__sa_sigaction) __P((int, struct __siginfo *, void *)); } __sigaction_u; /* signal handler */ int sa_flags; /* see signal options below */ sigset_t sa_mask; /* signal mask to apply */ }; #define sa_handler __sigaction_u.__sa_handler #define sa_sigaction __sigaction_u.__sa_sigaction .Ed .Ft int .Fn sigaction "int sig" "const struct sigaction *act" "struct sigaction *oact" .Sh DESCRIPTION The system defines a set of signals that may be delivered to a process. Signal delivery resembles the occurrence of a hardware interrupt: the signal is normally blocked from further occurrence, the current process context is saved, and a new one is built. A process may specify a .Em handler to which a signal is delivered, or specify that a signal is to be .Em ignored . A process may also specify that a default action is to be taken by the system when a signal occurs. A signal may also be .Em blocked , in which case its delivery is postponed until it is .Em unblocked . The action to be taken on delivery is determined at the time of delivery. Normally, signal handlers execute on the current stack of the process. This may be changed, on a per-handler basis, so that signals are taken on a special .Em "signal stack" . .Pp Signal routines normally execute with the signal that caused their invocation .Em blocked , but other signals may yet occur. A global .Em "signal mask" defines the set of signals currently blocked from delivery to a process. The signal mask for a process is initialized from that of its parent (normally empty). It may be changed with a .Xr sigprocmask 2 call, or when a signal is delivered to the process. .Pp When a signal condition arises for a process, the signal is added to a set of signals pending for the process. If the signal is not currently .Em blocked by the process then it is delivered to the process. Signals may be delivered any time a process enters the operating system (e.g., during a system call, page fault or trap, or clock interrupt). If multiple signals are ready to be delivered at the same time, any signals that could be caused by traps are delivered first. Additional signals may be processed at the same time, with each appearing to interrupt the handlers for the previous signals before their first instructions. The set of pending signals is returned by the .Xr sigpending 2 function. When a caught signal is delivered, the current state of the process is saved, a new signal mask is calculated (as described below), and the signal handler is invoked. The call to the handler is arranged so that if the signal handling routine returns normally the process will resume execution in the context from before the signal's delivery. If the process wishes to resume in a different context, then it must arrange to restore the previous context itself. .Pp When a signal is delivered to a process a new signal mask is installed for the duration of the process' signal handler (or until a .Xr sigprocmask call is made). This mask is formed by taking the union of the current signal mask set, the signal to be delivered, and the signal mask associated with the handler to be invoked. .Pp .Fn Sigaction assigns an action for a signal specified by .Fa sig . If .Fa act is non-zero, it specifies an action .Pf ( Dv SIG_DFL , .Dv SIG_IGN , or a handler routine) and mask to be used when delivering the specified signal. If .Fa oact is non-zero, the previous handling information for the signal is returned to the user. .Pp Once a signal handler is installed, it normally remains installed until another .Fn sigaction call is made, or an .Xr execve 2 is performed. A signal-specific default action may be reset by setting .Fa sa_handler to .Dv SIG_DFL . The defaults are process termination, possibly with core dump; no action; stopping the process; or continuing the process. See the signal list below for each signal's default action. If .Fa sa_handler is .Dv SIG_DFL , the default action for the signal is to discard the signal, and if a signal is pending, the pending signal is discarded even if the signal is masked. If .Fa sa_handler is set to .Dv SIG_IGN current and pending instances of the signal are ignored and discarded. .Pp Options may be specified by setting .Em sa_flags . The meaning of the various bits is as follows: .Bl -tag -offset indent -width SA_RESETHANDXX .It Dv SA_NOCLDSTOP If this bit is set when installing a catching function for the .Dv SIGCHLD signal, the .Dv SIGCHLD signal will be generated only when a child process exits, not when a child process stops. .It Dv SA_NOCLDWAIT If this bit is set when calling .Fn sigaction for the .Dv SIGCHLD signal, the system will not create zombie processes when children of the calling process exit. If the calling process subsequently issues a .Xr wait 2 (or equivalent), it blocks until all of the calling process's child processes terminate, and then returns a value of -1 with errno set to .Er ECHILD . .It Dv SA_ONSTACK If this bit is set, the system will deliver the signal to the process on a .Em "signal stack" , specified with .Xr sigaltstack 2 . .It Dv SA_NODEFER If this bit is set, further occurrences of the delivered signal are not masked during the execution of the handler. .It Dv SA_RESETHAND If this bit is set, the handler is reset back to .Dv SIG_DFL at the moment the signal is delivered. .It Dv SA_RESTART See paragraph below. .It Dv SA_SIGINFO If this bit is set, the handler function is assumed to be pointed to by the .Dv sa_sigaction member of struct sigaction and should match the prototype shown above or as below in .Sx EXAMPLES . This bit should not be set when assigning .Dv SIG_DFL or .Dv SIG_IGN . .El .Pp If a signal is caught during the system calls listed below, the call may be forced to terminate with the error .Er EINTR , the call may return with a data transfer shorter than requested, or the call may be restarted. Restart of pending calls is requested by setting the .Dv SA_RESTART bit in .Ar sa_flags . The affected system calls include .Xr open 2 , .Xr read 2 , .Xr write 2 , .Xr sendto 2 , .Xr recvfrom 2 , .Xr sendmsg 2 and .Xr recvmsg 2 on a communications channel or a slow device (such as a terminal, but not a regular file) and during a .Xr wait 2 or .Xr ioctl 2 . However, calls that have already committed are not restarted, but instead return a partial success (for example, a short read count). .Pp After a .Xr fork 2 or .Xr vfork 2 all signals, the signal mask, the signal stack, and the restart/interrupt flags are inherited by the child. .Pp .Xr Execve 2 reinstates the default action for all signals which were caught and resets all signals to be caught on the user stack. Ignored signals remain ignored; the signal mask remains the same; signals that restart pending system calls continue to do so. .Pp The following is a list of all signals with names as in the include file .Aq Pa signal.h : .Bl -column SIGVTALARMXX "create core imagexxx" .It Sy "NAME Default Action Description" .It Dv SIGHUP No " terminate process" " terminal line hangup" .It Dv SIGINT No " terminate process" " interrupt program" .It Dv SIGQUIT No " create core image" " quit program" .It Dv SIGILL No " create core image" " illegal instruction" .It Dv SIGTRAP No " create core image" " trace trap" .It Dv SIGABRT No " create core image" Ta Xr abort 3 call (formerly .Dv SIGIOT ) .It Dv SIGEMT No " create core image" " emulate instruction executed" .It Dv SIGFPE No " create core image" " floating-point exception" .It Dv SIGKILL No " terminate process" " kill program" .It Dv SIGBUS No " create core image" " bus error" .It Dv SIGSEGV No " create core image" " segmentation violation" .It Dv SIGSYS No " create core image" " non-existent system call invoked" .It Dv SIGPIPE No " terminate process" " write on a pipe with no reader" .It Dv SIGALRM No " terminate process" " real-time timer expired" .It Dv SIGTERM No " terminate process" " software termination signal" .It Dv SIGURG No " discard signal" " urgent condition present on socket" .It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)" .It Dv SIGTSTP No " stop process" " stop signal generated from keyboard" .It Dv SIGCONT No " discard signal" " continue after stop" .It Dv SIGCHLD No " discard signal" " child status has changed" .It Dv SIGTTIN No " stop process" " background read attempted from control terminal" .It Dv SIGTTOU No " stop process" " background write attempted to control terminal" .It Dv SIGIO No " discard signal" Tn " I/O" is possible on a descriptor (see .Xr fcntl 2 ) .It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see" .Xr setrlimit 2 ) .It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see" .Xr setrlimit 2 ) .It Dv SIGVTALRM No " terminate process" " virtual time alarm (see" .Xr setitimer 2 ) .It Dv SIGPROF No " terminate process" " profiling timer alarm (see" .Xr setitimer 2 ) .It Dv SIGWINCH No " discard signal" " Window size change" .It Dv SIGINFO No " discard signal" " status request from keyboard" .It Dv SIGUSR1 No " terminate process" " User defined signal 1" .It Dv SIGUSR2 No " terminate process" " User defined signal 2" .El .Sh NOTE The .Fa sa_mask field specified in .Fa act is not allowed to block .Dv SIGKILL or .Dv SIGSTOP . Any attempt to do so will be silently ignored. .Pp The following functions are either reentrant or not interruptible by signals and are async-signal safe. Therefore applications may invoke them, without restriction, from signal-catching functions: .Pp Base Interfaces: .Pp .Fn _exit , .Fn access , .Fn alarm , .Fn cfgetispeed , .Fn cfgetospeed , .Fn cfsetispeed , .Fn cfsetospeed , .Fn chdir , .Fn chmod , .Fn chown , .Fn close , .Fn creat , .Fn dup , .Fn dup2 , .Fn execle , .Fn execve , .Fn fcntl , .Fn fork , .Fn fpathconf , .Fn fstat , .Fn fsync , .Fn getegid , .Fn geteuid , .Fn getgid , .Fn getgroups , .Fn getpgrp , .Fn getpid , .Fn getppid , .Fn getuid , .Fn kill , .Fn link , .Fn lseek , .Fn mkdir , .Fn mkfifo , .Fn open , .Fn pathconf , .Fn pause , .Fn pipe , .Fn raise , .Fn read , .Fn rename , .Fn rmdir , .Fn setgid , .Fn setpgid , .Fn setsid , .Fn setuid , .Fn sigaction , .Fn sigaddset , .Fn sigdelset , .Fn sigemptyset , .Fn sigfillset , .Fn sigismember , .Fn signal , .Fn sigpending , .Fn sigprocmask , .Fn sigsuspend , .Fn sleep , .Fn stat , .Fn sysconf , .Fn tcdrain , .Fn tcflow , .Fn tcflush , .Fn tcgetattr , .Fn tcgetpgrp , .Fn tcsendbreak , .Fn tcsetattr , .Fn tcsetpgrp , .Fn time , .Fn times , .Fn umask , .Fn uname , .Fn unlink , .Fn utime , .Fn wait , .Fn waitpid , .Fn write . .Pp Realtime Interfaces: .Pp .Fn aio_error , .Fn clock_gettime , .Fn sigpause , .Fn timer_getoverrun , .Fn aio_return , .Fn fdatasync , .Fn sigqueue , .Fn timer_gettime , .Fn aio_suspend , .Fn sem_post , .Fn sigset , .Fn timer_settime . .Pp ANSI C Interfaces: .Pp .Fn strcpy , .Fn strcat , .Fn strncpy , .Fn strncat , and perhaps some others. .Pp Extension Interfaces: .Pp .Fn strlcpy , .Fn strlcat . .Pp All functions not in the above lists are considered to be unsafe with respect to signals. That is to say, the behaviour of such functions when called from a signal handler is undefined. In general though, signal handlers should do little more than set a flag; most other actions are not safe. .Pp Also, it is good practice to make a copy of the global variable .Va errno and restore it before returning from the signal handler. This protects against the side effect of .Va errno being set by functions called from inside the signal handler. .Sh RETURN VALUES .Rv -std sigaction .Sh EXAMPLES There are three possible prototypes the handler may match: .Bl -tag -offset indent -width short .It ANSI C: .Ft void .Fn handler int ; .It Traditional BSD style: .Ft void .Fn handler int "int code" "struct sigcontext *scp" ; .It POSIX SA_SIGINFO: .Ft void .Fn handler int "siginfo_t *info" "void *context" ; .El .Pp The handler function should match the SA_SIGINFO prototype if the SA_SIGINFO bit is set in flags. It then should be pointed to by the .Dv sa_sigaction member of .Dv struct sigaction . Note that you should not assign SIG_DFL or SIG_IGN this way. .Pp If the SA_SIGINFO flag is not set, the handler function should match either the ANSI C or traditional .Bx prototype and be pointed to by the .Dv sa_handler member of .Dv struct sigaction . In pratice, .Fx always sends the three arguments of the latter and since the ANSI C prototype is a subset, both will work. The .Dv sa_handler member declaration in .Fx include files is that of ANSI C (as required by POSIX), so a function pointer of a .Bx Ns -style function needs to be casted to compile without warning. The traditional .Bx style is not portable and since its capabilities are a full subset of a SA_SIGINFO handler, its use is deprecated. .Pp The .Fa sig argument is the signal number, one of the .Dv SIG... values from . .Pp The .Fa code argument of the .Bx Ns -style handler and the .Dv si_code member of the .Dv info argument to a SA_SIGINFO handler contain a numeric code explaning the cause of the signal, usually one of the .Dv SI_... values from or codes specific to a signal, i.e. one of the .Dv FPE_... values for SIGFPE. .Pp The .Fa scp argument to a .Bx Ns -style handler points to an instance of struct sigcontext. .Pp The .Fa context argument to a POSIX SA_SIGINFO handler points to an instance of ucontext_t. .Sh ERRORS .Fn Sigaction will fail and no new signal handler will be installed if one of the following occurs: .Bl -tag -width Er .It Bq Er EFAULT Either .Fa act or .Fa oact points to memory that is not a valid part of the process address space. .It Bq Er EINVAL .Fa Sig is not a valid signal number. .It Bq Er EINVAL An attempt is made to ignore or supply a handler for .Dv SIGKILL or .Dv SIGSTOP . .El .Sh STANDARDS The .Fn sigaction function call is expected to conform to .St -p1003.1-90 . The .Dv SA_ONSTACK and .Dv SA_RESTART flags are Berkeley extensions, as are the signals, .Dv SIGTRAP , .Dv SIGEMT , .Dv SIGBUS , .Dv SIGSYS , .Dv SIGURG , .Dv SIGIO , .Dv SIGXCPU , .Dv SIGXFSZ , .Dv SIGVTALRM , .Dv SIGPROF , .Dv SIGWINCH , and .Dv SIGINFO . Those signals are available on most .Bx Ns \-derived systems. The .Dv SA_NODEFER and .Dv SA_RESETHAND flags are intended for backwards compatibility with other operating systems. The .Dv SA_NOCLDSTOP , and .Dv SA_NOCLDWAIT .\" and .\" SA_SIGINFO flags are featuring options commonly found in other operating systems. .Sh SEE ALSO .Xr kill 1 , .Xr kill 2 , .Xr ptrace 2 , .Xr sigaltstack 2 , .Xr sigblock 2 , .Xr sigpause 2 , .Xr sigpending 2 , .Xr sigprocmask 2 , .Xr sigsetmask 2 , .Xr sigsuspend 2 , .Xr sigvec 2 , .Xr wait 2 , .Xr fpsetmask 3 , .Xr setjmp 3 , .Xr siginterrupt 3 , .Xr sigsetops 3 , .Xr tty 4