NAME

sigvec — software signal facilities

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <signal.h>

struct sigvec { 
	void	(*sv_handler)(); 
	int	sv_mask; 
	int	sv_flags; 
};

int
sigvec(int sig, struct sigvec *vec, struct sigvec *ovec);

DESCRIPTION

This interface is made obsolete by sigaction(2). The structure, flags, and function declaration have been removed from the header files but the function is kept in the c library for binary compatibility.

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 0). It may be changed with a sigblock(3) or sigsetmask(3) 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. 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 sigblock(3) or sigsetmask(3) 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.

sigvec() assigns a handler for a specific signal. If vec 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. Further, if the SV_ONSTACK bit is set in sv_flags, the system will deliver the signal to the process on a signal stack, specified with sigaltstack(2). If ovec 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 sigvec() call is made, or an execve(2) is performed. A signal-specific default action may be reset by setting sv_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 sv_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 sv_handler is set to SIG_IGN, current and pending instances of the signal are ignored and discarded.

Options may be specified by setting sv_flags. If the SV_ONSTACK bit is set in sv_flags, the system will deliver the signal to the process on a signal stack, specified with sigstack(2).

If a signal is caught during the system calls listed below, the call may be restarted, the call may return with a data transfer shorter than requested, or the call may be forced to terminate with the error EINTR. Interrupting of pending calls is requested by setting the SV_INTERRUPT bit in sv_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 interrupt/restart 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 interrupt pending system calls continue to do so.

See signal(7) for comprehensive list of supported signals.

NOTES

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

The SV_INTERRUPT flag is not available in 4.2BSD, hence it should not be used if backward compatibility is needed.

RETURN VALUES

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

EXAMPLES

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 as defined below. 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.

ERRORS

sigvec() will fail and no new signal handler will be installed if one of the following occurs: