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. Signal masks are represented using the
sigset_t type; the
sigsetops(3) interface is used to modify such data.
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);
sigset_t sa_mask;
int sa_flags;
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 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.
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.