SIGACTION(2) System Calls Manual SIGACTION(2)
sigaction -- software signal facilities
Standard C Library (libc, -lc)
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 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);
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.
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
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).
Only functions that are async-signal-safe can safely be used in signal
handlers, see signal(7) for a complete list.
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.
[EINVAL] An attempt is made to ignore or supply a handler for
SIGKILL or SIGSTOP.
[EINVAL] The sa_flags word contains bits other than
SA_NOCLDSTOP, SA_NOCLDWAIT, SA_NODEFER, SA_ONSTACK,
SA_RESETHAND, SA_RESTART, and SA_SIGINFO.
kill(1), kill(2), ptrace(2), sigaltstack(2), siginfo(2), sigprocmask(2),
sigsuspend(2), setjmp(3), sigsetops(3), tty(4), signal(7)
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 6.1.5 June 3, 2006 NetBSD 6.1.5