SIGVEC(2)

NAME
sigvec - software signal facilities

SYNOPSIS
#include <signal.h>

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

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

DESCRIPTION
The system defines a set of signals that may be delivered to a process. Signal delivery resembles the occurence 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 blocked or ignored. 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 priority. 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 - but see the comments below about the restrictions on signal handlers.

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 or sigsetmask call is made). This mask is formed by taking the current signal mask, adding the signal to be delivered, and or'ing in 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 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 sigstack(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>:

SIGHUP	1	hangup
SIGINT	2	interrupt
SIGQUIT	3*	quit
SIGILL	4*	illegal instruction
SIGTRAP	5*	trace trap
SIGIOT	6*	IOT instruction
SIGEMT	7*	EMT instruction
SIGFPE	8*	floating point exception
SIGKILL	9	kill (cannot be caught, blocked, or ignored)
SIGBUS	10*	bus error
SIGSEGV	11*	segmentation violation
SIGSYS	12*	bad argument to system call
SIGPIPE	13	write on a pipe with no one to read it
SIGALRM	14	alarm clock
SIGTERM	15	software termination signal
SIGURG	16@	urgent condition present on socket
SIGSTOP	17'|+'	stop (cannot be caught, blocked, or ignored)
SIGTSTP	18'|+'	stop signal generated from keyboard
SIGCONT	19@	continue after stop (cannot be blocked)
SIGCHLD	20@	child status has changed
SIGTTIN	21'|+'	background read attempted from control terminal
SIGTTOU	22'|+'	background write attempted to control terminal
SIGIO	23@	i/o is possible on a descriptor (see fcntl(2))
SIGXCPU	24	cpu time limit exceeded (see setrlimit(2))
SIGXFSZ	25	file size limit exceeded (see setrlimit(2))
SIGVTALRM	26	virtual time alarm (see setitimer(2))
SIGPROF	27	profiling timer alarm (see setitimer(2))
SIGWINCH	28@	window size change
SIGUSR1	30	user defined signal 1
SIGUSR2	31	user defined signal 2

The starred signals in the list above cause a core image if not caught or ignored.

Once a signal handler is installed, it remains installed until another sigvec call is made, or an execve(2) is performed. The default action for a signal may be reinstated by setting sv_handler to SIG_DFL; this default is termination (with a core image for starred signals) except for signals marked with @ or '|+'. Signals marked with @ are discarded if the action is SIG_DFL; signals marked with '|+' cause the process to stop. If sv_handler is SIG_IGN the signal is subsequently ignored, and pending instances of the signal are discarded.

If a caught signal occurs during certain system calls, 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 are read(2) or write(2) on a slow device (such as a terminal; but not a file) and during a wait(2).

After a fork(2) or vfork(2) the child inherits all signals, the signal mask, the signal stack, and the restart/interrupt flags.

Execve(2) resets all caught signals to default action 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.

NOTES
The mask specified in vec is not allowed to block SIGKILL, SIGSTOP, or SIGCONT. 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.

A signal handler executes in the environment of the process which receives the the signal, therefore it must be very careful about what it does to that environment. In particular signal handlers may interrupt the execution of sensitive library code, but no UNIX library code is protected against reentry from a signal handler therefore such code cannot safely be called in a signal handler. The best example of such code is malloc(3). A handler which exits by any method other than a normal return - in particular by using sigreturn(2) or longjmp(3) - must take care to leave the interrupted code in a safe state. Since this is not normally possible unless the handler knows exactly what code was executing when it was called it is not normally a good idea to exit from a signal handler in any way other than by a normal return.

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

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

[EFAULT]
    Either vec or ovec 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.

[EINVAL]
    An attempt is made to ignore SIGCONT (by default SIGCONT is ignored).

SEE ALSO
kill(1), ptrace(2), kill(2), sigblock(2), sigsetmask(2), sigpause(2), sigstack(2), sigvec(2), setjmp(3), siginterrupt(3), tty(4)