sigaction(2)
NAME
sigaction - detailed signal management
SYNOPSIS
#include <signal.h>
int sigaction(int sig, const struct sigaction *act, struct
sigaction *oact);
DESCRIPTION
The sigaction() function allows the calling process to exam-
ine or specify the action to be taken on delivery of a
specific signal. See signal(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(3HEAD) except SIG-
KILL and SIGSTOP. In a multithreaded process, sig cannot be
SIGWAITING, SIGCANCEL, or SIGLWP.
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 associated 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 stan-
dards(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 values specified in
signal(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 addi-
tion, the signal that caused the handler to be executed will
also be blocked, unless the SA_NODEFER flag has been speci-
fied. 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 delivery 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
alternate signal stack declared with sigaltstack(2),
then it will process the signal on that stack. Other-
wise, 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,
SIGTRAP, and SIGPWR cannot be automatically reset
when delivered; the system silently enforces this res-
triction).
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(2), 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 argu-
ments 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
signal was generated (see siginfo(3HEAD)); the third
argument points to a ucontext_t structure containing
the receiving process's context when the signal was
delivered (see ucontext(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(2), 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 addi-
tion, if in a multithreaded process, it is equal to
SIGWAITING, SIGCANCEL, or SIGLWP.
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | Standard |
|_____________________________|_____________________________|
| MT-Level | Async-Signal-Safe |
|_____________________________|_____________________________|
SEE ALSO
kill(1), intro(2), exit(2), fcntl(2), getmsg(2), ioctl(2),
kill(2), pause(2), putmsg(2), read(2), sigaltstack(2), sig-
procmask(2), sigsend(2), sigsuspend(2), wait(2), waitid(2),
write(2), recv(3SOCKET), send(3SOCKET), siginfo(3HEAD),
signal(3C), signal(3HEAD), sigsetops(3C), thr_create(3THR),
ucontext(3HEAD), 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 additional 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.
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