Ubuntu Manpage: sigvec — software signal facilities

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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 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 sigvec() function 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>:

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 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 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 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.

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 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.

RETURN VALUES

     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.

EXAMPLES

     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.  The 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 scp argument is a pointer
     to the sigcontext structure (defined in <signal.h>), used to restore the context from before
     the signal.

ERRORS

     The sigvec() function 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]           The sig argument is not a valid signal number.

     [EINVAL]           An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.

BUGS

     This manual page is still confusing.