SIGACTION(2)SIGACTION(2)NAMEsigaction - detailed signal management
SYNOPSIS
#include <signal.h>
int sigaction(int sig, const struct sigaction *restrict act,
struct sigaction *restrict oact);
DESCRIPTION
The sigaction() function allows the calling process to examine or spec‐
ify the action to be taken on delivery of a specific signal. See sig‐
nal.h(3HEAD) for an explanation of general signal concepts.
The sig argument specifies the signal and can be assigned any of the
signals specified in signal.h(3HEAD) except SIGKILL and SIGSTOP.
If the argument act is not NULL, it points to a structure specifying
the new action to be taken when delivering sig. If the argument oact is
not NULL, it points to a structure where the action previously associ‐
ated with sig is to be stored on return from sigaction().
The sigaction structure includes the following members:
void (*sa_handler)();
void (*sa_sigaction)(int, siginfo_t *, void *);
sigset_t sa_mask;
int sa_flags;
The storage occupied by sa_handler and sa_sigaction may overlap, and a
standard-conforming application (see standards(5)) must not use both
simultaneously.
The sa_handler member identifies the action to be associated with the
specified signal, if the SA_SIGINFO flag (see below) is cleared in the
sa_flags field of the sigaction structure. It may take any of the val‐
ues specified in signal.h(3HEAD) or that of a user specified signal
handler. If the SA_SIGINFO flag is set in the sa_flags field, the
sa_sigaction field specifies a signal-catching function.
The sa_mask member specifies a set of signals to be blocked while the
signal handler is active. On entry to the signal handler, that set of
signals is added to the set of signals already being blocked when the
signal is delivered. In addition, the signal that caused the handler to
be executed will also be blocked, unless the SA_NODEFER flag has been
specified. SIGSTOP and SIGKILL cannot be blocked (the system silently
enforces this restriction).
The sa_flags member specifies a set of flags used to modify the deliv‐
ery of the signal. It is formed by a logical OR of any of the following
values:
SA_ONSTACK
If set and the signal is caught, and if the thread that
is chosen to processes a delivered signal has an alter‐
nate signal stack declared with sigaltstack(2), then it
will process the signal on that stack. Otherwise, the
signal is delivered on the thread's normal stack.
SA_RESETHAND
If set and the signal is caught, the disposition of the
signal is reset to SIG_DFL and the signal will not be
blocked on entry to the signal handler (SIGILL, SIG‐
TRAP, and SIGPWR cannot be automatically reset when
delivered; the system silently enforces this restric‐
tion).
SA_NODEFER
If set and the signal is caught, the signal will not be
automatically blocked by the kernel while it is being
caught.
SA_RESTART
If set and the signal is caught, functions that are
interrupted by the execution of this signal's handler
are transparently restarted by the system, namely
fcntl(2), ioctl(2), wait(3C), waitid(2), and the fol‐
lowing functions on slow devices like terminals:
getmsg() and getpmsg() (see getmsg(2)); putmsg() and
putpmsg() (see putmsg(2)); pread(), read(), and readv()
(see read(2)); pwrite(), write(), and writev() (see
write(2)); recv(), recvfrom(), and recvmsg() (see
recv(3SOCKET)); and send(), sendto(), and sendmsg()
(see send(3SOCKET)). Otherwise, the function returns an
EINTR error.
SA_SIGINFO
If cleared and the signal is caught, sig is passed as
the only argument to the signal-catching function. If
set and the signal is caught, two additional arguments
are passed to the signal-catching function. If the
second argument is not equal to NULL, it points to a
siginfo_t structure containing the reason why the sig‐
nal was generated (see siginfo.h(3HEAD)); the third
argument points to a ucontext_t structure containing
the receiving process's context when the signal was
delivered (see ucontext.h(3HEAD)).
SA_NOCLDWAIT
If set and sig equals SIGCHLD, the system will not
create zombie processes when children of the calling
process exit. If the calling process subsequently
issues a wait(3C), it blocks until all of the calling
process's child processes terminate, and then returns
−1 with errno set to ECHILD.
SA_NOCLDSTOP
If set and sig equals SIGCHLD, SIGCHLD will not be sent
to the calling process when its child processes stop or
continue.
RETURN VALUES
Upon successful completion, 0 is returned. Otherwise, −1 is returned,
errno is set to indicate the error, and no new signal handler is
installed.
ERRORS
The sigaction() function will fail if:
EINVAL
The value of the sig argument is not a valid signal number or
is equal to SIGKILL or SIGSTOP. In addition, if in a multi‐
threaded process, it is equal to SIGWAITING, SIGCANCEL, or
SIGLWP.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬───────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼───────────────────┤
│Interface Stability │ Committed │
├────────────────────┼───────────────────┤
│MT-Level │ Async-Signal-Safe │
├────────────────────┼───────────────────┤
│Standard │ See standards(5). │
└────────────────────┴───────────────────┘
SEE ALSOkill(1), Intro(2), exit(2), fcntl(2), getmsg(2), ioctl(2), kill(2),
pause(2), putmsg(2), read(2), sigaltstack(2), sigprocmask(2),
sigsend(2), sigsuspend(2), waitid(2), write(2), recv(3SOCKET),
send(3SOCKET), siginfo.h(3HEAD), signal(3C), signal.h(3HEAD), sigse‐
tops(3C), ucontext.h(3HEAD), wait(3C), attributes(5), standards(5)NOTES
The handler routine can be declared:
void handler (int sig, siginfo_t *sip, ucontext_t *ucp);
The sig argument is the signal number. The sip argument is a pointer
(to space on the stack) to a siginfo_t structure, which provides addi‐
tional detail about the delivery of the signal. The ucp argument is a
pointer (again to space on the stack) to a ucontext_t structure
(defined in <sys/ucontext.h>) which contains the context from before
the signal. It is not recommended that ucp be used by the handler to
restore the context from before the signal delivery.
Mar 23, 2005 SIGACTION(2)
