sigaction - software signal facilities
Standard C Library (libc, -lc)
#include <signal.h>
struct sigaction {
void (*sa_handler)(int);
sigset_t sa_mask;
int sa_flags;
};
int
sigaction(int sig, const struct sigaction *act, struct sigaction *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.
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.
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.
Alternately, if the SA_RESETHAND bit is set the default action will be
reinstated when the signal is first posted. 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. If the SA_NOCLDSTOP bit is
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. Further, if the SA_ONSTACK bit is set in
sa_flags, the system will deliver the signal to the process on a signal
stack, specified with sigaltstack(2). Finally, if the SA_NOCLDWAIT bit
is set in sa_flags, the system will not create a zombie when the child
exits, but the child process will be automatically waited for.
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.
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.
The handler routine can be declared:
void
handler(sig, code, scp)
int sig, code;
struct sigcontext *scp;
Here sig is the signal number, into which the hardware faults and traps
are mapped. code is a parameter that is either a constant or the code
provided by the hardware. scp is a pointer to the sigcontext structure
(defined in <signal.h>), used to restore the context from before the signal.
For POSIX compliance, the sa_handler is declared to be (void (*)(int))
and the above handler will need to be casted to that type. Future versions
of NetBSD will replace the sigcontext interface with the siginfo
interface.
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.
kill(1), kill(2), ptrace(2), sigaltstack(2), sigprocmask(2),
sigsuspend(2), setjmp(3), sigsetops(3), tty(4), signal(7)
The sigaction() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'').
The SA_ONSTACK and SA_RESTART flags are Berkeley extensions, available on
most BSD-derived systems.
BSD November 1, 1997 BSD
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