NAME

sigaction — software signal facilities

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <signal.h>

int
sigaction(int sig, const struct sigaction * restrict act, struct sigaction * restrict oact);

DESCRIPTION

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.

NOTES

The mask specified in act is not allowed to block SIGKILL or SIGSTOP. This is enforced silently by the system.

RETURN VALUES

A 0 value indicates that the call succeeded. A -1 return value indicates an error occurred and errno is set to indicate the reason.

ERRORS

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; or an attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP; or the sa_flags word contains bits other than SA_NOCLDSTOP, SA_NOCLDWAIT, SA_NODEFER, SA_ONSTACK, SA_RESETHAND, SA_RESTART, and SA_SIGINFO.

SEE ALSO

kill(1), kill(2), ptrace(2), sigaltstack(2), siginfo(2), sigprocmask(2), sigsuspend(2), setjmp(3), sigsetops(3), tty(4), signal(7)

STANDARDS

The sigaction() function conforms to IEEE Std 1003.1-1990 (“POSIX.1”). The SA_ONSTACK and SA_RESTART flags are Berkeley extensions, available on most BSD-derived systems.