SIGACTION(2) System Calls SIGACTION(2)
NAME
sigaction - 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 specify
the action to be taken on delivery of a specific signal. See
signal.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 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
standards(7)) 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.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 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. 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,
SIGTRAP,
and
SIGPWR cannot be automatically reset when delivered;
the system silently enforces this restriction).
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 following
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 signal 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
multithreaded process, it is equal to
SIGWAITING,
SIGCANCEL, or
SIGLWP.
ATTRIBUTES
See
attributes(7) for descriptions of the following attributes:
+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|MT-Level | Async-Signal-Safe |
+--------------------+-------------------+
|Standard | See
standards(7). |
+--------------------+-------------------+
SEE ALSO
kill(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),
signal(3C),
sigsetops(3C),
wait(3C),
siginfo.h(3HEAD),
signal.h(3HEAD),
ucontext.h(3HEAD),
recv(3SOCKET),
send(3SOCKET),
attributes(7),
standards(7)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.
March 23, 2005
SIGACTION(2)