LIBRARY

SYNOPSIS

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

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 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.

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 or 'ing in the signal mask associated with the handler to be invoked.

The Fn sigvec function assigns a handler for a specific signal. If Fa 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 Fa sv_flags , the system will deliver the signal to the process on a signal stack specified with sigaltstack(2). If Fa 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 In signal.h :

NAME  Default Action  Description

SIGHUP         terminate process     terminal line hangup

SIGINT terminate process interrupt program

SIGQUIT create core image quit program

SIGILL create core image illegal instruction

SIGTRAP create core image trace trap

SIGABRT create core image Ta abort(3)

call (formerly
SIGIOT

SIGEMT         create core image     emulate instruction executed

SIGFPE      create core image floating-point exception

SIGKILL terminate process kill program

SIGBUS create core image bus error

SIGSEGV create core image segmentation violation

SIGSYS create core image non-existent system call invoked

SIGPIPE terminate process write on a pipe with no reader

SIGALRM terminate process real-time timer expired

SIGTERM terminate process software termination signal

SIGURG discard signal urgent condition present on socket

SIGSTOP stop process stop (cannot be caught or ignored)

SIGTSTP stop process stop signal generated from keyboard

SIGCONT discard signal continue after stop

SIGCHLD discard signal child status has changed

SIGTTIN stop process background read attempted from control terminal

SIGTTOU stop process background write attempted to control terminal

SIGIO discard signal I/O

is possible on a descriptor (see
fcntl(2))

SIGXCPU        terminate process     cpu time limit exceeded (see

setrlimit(2))

SIGXFSZ        terminate process     file size limit exceeded (see

setrlimit(2))

SIGVTALRM      terminate process     virtual time alarm (see

setitimer(2))

SIGPROF        terminate process     profiling timer alarm (see

setitimer(2))

SIGWINCH       discard signal        Window size change

SIGINFO       discard signal status request from keyboard

SIGUSR1 terminate process User defined signal 1

SIGUSR2 terminate process User defined signal 2

Once a signal handler is installed, it remains installed
until another Fn sigvec call is made, or an execve(2) is performed. A signal-specific default action may be reset by setting Fa 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 Fa 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 Er EINTR error return by setting the SV_INTERRUPT bit in Fa 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.

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 system calls continue to do so.

NOTES

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

RETURN VALUES

EXAMPLES

void handler(sig, code, scp)
int sig, code;
struct sigcontext *scp;

Here Fa sig is the signal number, into which the hardware faults and traps are mapped as defined below. The Fa code argument 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). The Fa scp argument is a pointer to the Fa sigcontext structure (defined in In signal.h ) , used to restore the context from before the signal.