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SIGACTION(2)                  System Calls Manual                 SIGACTION(2)

     sigaction - software signal facilities

     Standard C Library (libc, -lc)

     #include <signal.h>

     sigaction(int sig, const struct sigaction * restrict act,
         struct sigaction * restrict oact);

     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 blocked from further occurrence, the current process context is
     saved, and a new one is built.  A process may specify a handler to which
     a signal is delivered, or specify that a signal is to be 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 blocked, in which case
     its delivery is postponed until it is 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
     signal stack.

     Signal routines execute with the signal that caused their invocation
     blocked, but other signals may yet occur.  A global 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 sigprocmask(2) call, or when
     a signal is delivered to the process.  Signal masks are represented using
     the sigset_t type; the sigsetops(3) interface is used to modify such

     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
     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 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.

     struct sigaction includes the following members:

           void      (*sa_sigaction)(int sig, siginfo_t *info, void *ctx);
           void      (*sa_handler)(int sig);
           sigset_t  sa_mask;
           int       sa_flags;

     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
     sigprocmask(2) call is made).  This mask is formed by taking the union of
     the current signal mask, the signal to be delivered, and the signal mask
     associated with the handler to be invoked, sa_mask.

     sigaction() assigns an action for a specific signal.  If act is non-zero,
     it specifies an action (SIG_DFL, SIG_IGN, or a handler routine) and mask
     to be used when delivering the specified signal.  If oact is non-zero,
     the previous handling information for the signal is returned to the user.

     Once a signal handler is installed, it remains installed until another
     sigaction() call is made, or an execve(2) is performed.  A signal-
     specific default action may be reset by setting sa_handler to 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 sa_handler is set to 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 sa_handler is set to SIG_IGN, current and pending instances
     of the signal are ignored and discarded.

     Options may be specified by setting sa_flags.

     SA_NODEFER     If set, then the signal that caused the handler to be
                    executed is not added to the list of block signals.
                    Please note that sa_mask takes precedence over SA_NODEFER,
                    so that if the specified signal is blocked in sa_mask,
                    then SA_NODEFER will have no effect.

     SA_NOCLDSTOP   If set when installing a catching function for the SIGCHLD
                    signal, the SIGCHLD signal will be generated only when a
                    child process exits, not when a child process stops or

     SA_NOCLDWAIT   If set, the system will not create a zombie when the child
                    exits, but the child process will be automatically waited
                    for.  The same effect can be achieved by setting the
                    signal handler for SIGCHLD to SIG_IGN.

     SA_ONSTACK     If set, the system will deliver the signal to the process
                    on a signal stack, specified with sigaltstack(2).

     SA_RESETHAND   If set, the default action will be reinstated when the
                    signal is first posted.

     SA_RESTART     Normally, if a signal is caught during the system calls
                    listed below, the call may be forced to terminate with the
                    error EINTR, the call may return with a data transfer
                    shorter than requested, or the call may be restarted.
                    Restarting of pending calls is requested by setting the
                    SA_RESTART bit in sa_flags.  The affected system calls
                    include open(2), read(2), write(2), sendto(2),
                    recvfrom(2), sendmsg(2) and recvmsg(2) on a communications
                    channel or a slow device (such as a terminal, but not a
                    regular file) and during a wait(2) or ioctl(2).  However,
                    calls that have already committed are not restarted, but
                    instead return a partial success (for example, a short
                    read count).

                    After a fork(2) or vfork(2) all signals, the signal mask,
                    the signal stack, and the restart/interrupt flags are
                    inherited by the child.

                    The execve(2) system call 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.

                    See signal(7) for comprehensive list of supported signals.

     SA_SIGINFO     If set, the signal handler function will receive
                    additional information about the caught signal.  An
                    alternative handler that gets passed additional arguments
                    will be called which is named sa_sigaction.  The sig
                    argument of this handler contains the signal number that
                    was caught.  The info argument contains additional signal
                    specific information which is listed in siginfo(2).  The
                    ctx argument is a pointer to the ucontext(2) context where
                    the signal handler will return to.

     SA_NOKERNINFO  This flag is relevant only to SIGINFO, and turns off
                    printing kernel messages on the tty.  It is similar to the
                    NOKERNINFO flag in termios(4).

     If the signal handler is called due to signal delivery resulting from
     reasons other than direct calls to kill(2) or _lwp_kill(2) or indirect
     calls to _lwp_kill(2) via abort(3) or raise(3) any activity (such as
     calling functions or assigning variables in the global or static scopes)
     other than setting a variable of the type volatile sig_atomic_t is

   Signal-safe functions
     Only functions that are guaranteed to be async-signal-safe can safely be
     used in signal handlers.  These are functions that are either reentrant
     or non-interruptible.  (These functions are also the only functions that
     may be used in a child process after doing fork(2) in a threaded

     The following functions are async-signal-safe.  Any function not listed
     below is unsafe to use in signal handlers.

     _Exit(2), _exit(2), abort(3), accept(2), access(2), alarm(3), bind(2),
     cfgetispeed(3), cfgetospeed(3), cfsetispeed(3), cfsetospeed(3), chdir(2),
     chmod(2), chown(2), clock_gettime(2), close(2), connect(2), creat(3),
     dup(2), dup2(2), execle(3), execve(2), fchmod(2), fchown(2), fcntl(2),
     fdatasync(2), fork(2), fpathconf(2), fstat(2), fsync(2), ftruncate(2),
     getegid(2), geteuid(2), getgid(2), getgroups(2), getpeername(2),
     getpgrp(2), getpid(2), getppid(2), getsockname(2), getsockopt(2),
     getuid(2), kill(2), link(2), listen(2), lseek(2), lstat(2), mkdir(2),
     mkfifo(2), open(2), pathconf(2), pause(3), pipe(2), poll(2),
     pthread_mutex_unlock(3), raise(3), read(2), readlink(2), recv(2),
     recvfrom(2), recvmsg(2), rename(2), rmdir(2), select(2), sem_post(3),
     send(2), sendmsg(2), sendto(2), setgid(2), setpgid(2), setsid(2),
     setsockopt(2), setuid(2), shutdown(2), sigaddset(3), sigdelset(3),
     sigemptyset(3), sigfillset(3), sigismember(3), sleep(3), signal(3),
     sigpause(3), sigpending(2), sigprocmask(2), sigset(3), sigsuspend(2),
     sockatmark(3), socket(2), socketpair(2), stat(2), symlink(2), sysconf(3),
     tcdrain(3), tcflow(3), tcflush(3), tcgetattr(3), tcgetpgrp(3),
     tcsendbreak(3), tcsetattr(3), tcsetpgrp(3), time(3), timer_getoverrun(2),
     timer_gettime(2), timer_settime(2), times(3), umask(2), uname(3),
     unlink(2), utime(3), wait(2), waitpid(2), write(2).

     The mask specified in act is not allowed to block SIGKILL or SIGSTOP.
     This is enforced silently by the system.

     A 0 value indicates that the call succeeded.  A -1 return value indicates
     an error occurred and errno is set to indicate the reason.

     sigaction() will fail and no new signal handler will be installed if one
     of the following occurs:

     [EFAULT]           Either act or oact points to memory that is not a
                        valid part of the process address space.

     [EINVAL]           sig is not a valid signal number; or an attempt is
                        made to ignore or supply a handler for SIGKILL or
                        SIGSTOP; or the sa_flags word contains bits other than
                        SA_RESETHAND, SA_RESTART, and SA_SIGINFO.

     kill(1), kill(2), ptrace(2), sigaltstack(2), sigprocmask(2), sigstack(2),
     sigsuspend(2), fpgetmask(3), fpsetmask(3), setjmp(3), sigblock(3),
     siginterrupt(3), signal(3), sigpause(3), sigsetmask(3), sigsetops(3),

     The sigaction() function conforms to IEEE Std 1003.1-1990 ("POSIX.1").
     The SA_ONSTACK and SA_RESTART flags are Berkeley extensions, available on
     most BSD-derived systems.

NetBSD 9.99                      May 22, 2018                      NetBSD 9.99