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SIGVEC(2) System Calls Manual SIGVEC(2)

sigvec — software signal facilities

Standard C Library (libc, -lc)

#include <signal.h>

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

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

This interface is made obsolete by sigaction(2).

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 to which a signal is delivered, or specify that a signal is to be blocked or . A process may also specify that a default action is to be taken by the system when a signal occurs. 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.

All signals have the same . 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(2) or sigsetmask(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. When a 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(2) or sigsetmask(2) call is made). This mask is formed by taking the current signal mask, adding the signal to be delivered, and 'ing in the signal mask associated with the handler to be invoked.

() assigns a handler for a specific signal. If vec is non-zero, it specifies 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.

The following is a list of all signals with names as in the include file ⟨signal.h⟩:

Default Action Description
terminate process terminal line hangup
terminate process interrupt program
create core image quit program
create core image illegal instruction
create core image trace trap
create core image abort(3) call (formerly SIGIOT)
create core image emulate instruction executed
create core image floating-point exception
terminate process kill program
create core image bus error
create core image segmentation violation
create core image non-existent system call invoked
terminate process write on a pipe with no reader
terminate process real-time timer expired
terminate process software termination signal
discard signal urgent condition present on socket
stop process stop (cannot be caught or ignored)
stop process stop signal generated from keyboard
discard signal continue after stop
discard signal child status has changed
stop process background read attempted from control terminal
stop process background write attempted to control terminal
discard signal I/O is possible on a descriptor (see fcntl(2))
terminate process cpu time limit exceeded (see setrlimit(2))
terminate process file size limit exceeded (see setrlimit(2))
terminate process virtual time alarm (see setitimer(2))
terminate process profiling timer alarm (see setitimer(2))
discard signal Window size change
discard signal status request from keyboard
terminate process User defined signal 1
terminate process User defined signal 2

Once a signal handler is installed, it remains installed until another () 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 above signal list for each signal's default action. If sv_handler is SIG_IGN current and pending instances of the signal are ignored and discarded.

If a signal is caught during the system calls listed below, the call is normally restarted. The call can be forced to terminate prematurely with an EINTR error return by setting the SV_INTERRUPT bit in sv_flags. The affected system calls include 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.

Execve(2) 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 system calls continue to do so.

The mask specified in vec is not allowed to block SIGKILL or SIGSTOP. This is done 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.

The sigvec() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

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

[]
Either vec or ovec points to memory that is not a valid part of the process address space.
[]
Sig is not a valid signal number.
[]
An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.

kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2), sigblock(2), sigpause(2), sigprocmask(2), sigsetmask(2), sigsuspend(2), setjmp(3), siginterrupt(3), signal(3), sigsetops(3), tty(4)

On the VAX-11 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 as given below or, for compatibility mode faults, the code provided by the hardware (Compatibility mode faults are distinguished from the other SIGILL traps by having PSL_CM set in the psl). Scp is a pointer to the sigcontext structure (defined in ⟨signal.h⟩), used to restore the context from before the signal.

This manual page is still confusing.

April 19, 1994 macOS