SIGVEC(2) — SYSTEM CALLS

NAME

sigvec − software signal facilities

SYNOPSIS

#include <signal.h>

struct sigvec {
int(∗sv_handler)();
intsv_mask;
intsv_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 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 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 mask specified in vec is not allowed to block SIGKILL, SIGSTOP, or SIGCONT. The system enforces this restriction silently.

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

SIGHUP1hangup
SIGINT2interrupt
SIGQUIT3∗quit
SIGILL4∗illegal instruction (other than A-line or F-line op code)
SIGTRAP5∗trace trap
SIGIOT6∗IOT trap (not generated on Suns)
SIGEMT7∗EMT trap (A-line or F-line op code)
SIGFPE8∗arithmetic exception
SIGKILL9kill (cannot be caught, blocked, or ignored)
SIGBUS10∗bus error
SIGSEGV11∗segmentation violation
SIGSYS12∗bad argument to system call
SIGPIPE13write on a pipe or other socket with no one to read it
SIGALRM14alarm clock
SIGTERM15software termination signal
SIGURG16•urgent condition present on socket
SIGSTOP17†stop (cannot be caught, blocked, or ignored)
SIGTSTP18†stop signal generated from keyboard
SIGCONT19•continue after stop (cannot be blocked)
SIGCHLD20•child status has changed
SIGTTIN21†background read attempted from control terminal
SIGTTOU22†background write attempted to control terminal
SIGIO23•I/O is possible on a descriptor (see
fcntl(2))
SIGXCPU24cpu time limit exceeded (see
setrlimit(2))
SIGXFSZ25file size limit exceeded (see
setrlimit(2))
SIGVTALRM26virtual time alarm (see
setitimer(2))
SIGPROF27profiling timer alarm (see
setitimer(2))
SIGWINCH28•window changed (see
win(4S))
SIGLOST29∗resource lost (see
lockd(8C))
SIGUSR130user-defined signal 1
SIGUSR231user-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, except that if the SV_RESETHAND bit is set in sv_flags, the value of sv_handler for the caught signal will be set to SIG_DFL before entering the signal-catching function, unless the signal is SIGILL or SIGTRAP. If this bit is set, the bit for that signal in the signal mask will not be set; unless the signal mask associated with that signal blocks that signal, further occurrences of that signal will not be blocked. The SV_RESETHAND flag is not available in 4.2BSD, hence it should not be used if backward compatibility is needed.

The default action for a signal may be reinstated by setting sv_handler to SIG_DFL; this default is termination 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 the process is terminated, a “core image” will be made in the current working directory of the receiving process if the signal is one for which an asterisk appears in the above list and the following conditions are met:

The effective user ID and the real user ID of the receiving process are equal.

The effective group ID and the real group ID of the receiving process are equal.

An ordinary file named core exists and is writable or can be created. If the file must be created, it will have the following properties:

a mode of 0666 modified by the file creation mask (see umask(2))

a file owner ID that is the same as the effective user ID of the receiving process.

a file group ID that is the same as the file group ID of the current directory

If sv_handler is SIG_IGN the signal is subsequently ignored, and pending instances of the signal are discarded.

Note: the signals SIGKILL, SIGSTOP, and SIGCONT cannot be ignored.

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 SV_INTERRUPT flag is not available in 4.2BSD, hence it should not be used if backward compatibility is needed. The affected system calls are read(2V) or write(2V) on a slow device (such as a terminal or pipe or other socket, 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 and reset-signal-handler flags.

The execve(2) call 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 handler routine can be declared:

    handler(sig, code, scp)
    int sig, code;
    struct sigcontext ∗scp;

Here sig is the signal number. Code is a parameter of certain signals that provides additional detail. Scp is a pointer to the sigcontext structure (defined in <signal.h>), used to restore the context from before the signal.

Programs that must be portable to UNIX systems other than 4.2 BSD should use the signal(3) interface instead.

CODES

The following defines the codes for signals which produce them. All of these symbols are defined in <signal.h>:

Hardware conditionSignalCode
Illegal instructionSIGILLILL_INSTR_FAULT
Privilege violationSIGILLILL_PRIVVIO_FAULT
Coprocessor protocol errorSIGILLILL_INSTR_FAULT
Trap #n (1 <= n <= 14)SIGILLILL_TRAPn_FAULT
A-line op codeSIGEMTEMT_EMU1010
F-line op codeSIGEMTEMT_EMU1111
Integer division by zeroSIGFPEFPE_INTDIV_TRAP
CHK or CHK2 instructionSIGFPEFPE_CHKINST_TRAP
TRAPV or TRAPcc or cpTRAPccSIGFPEFPE_TRAPV_TRAP
IEEE floating point compare unorderedSIGFPEFPE_FLTBSUN_TRAP
IEEE floating point inexactSIGFPEFPE_FLTINEX_TRAP
IEEE floating point division by zeroSIGFPEFPE_FLTDIV_TRAP
IEEE floating point underflowSIGFPEFPE_FLTUND_TRAP
IEEE floating point operand errorSIGFPEFPE_FLTOPERR_TRAP
IEEE floating point overflowSIGFPEFPE_FLTOVF_FAULT
IEEE floating point signaling NaNSIGFPEFPE_FLTNAN_TRAP

RETURN VALUE

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

Museum

Related Articles