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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This file implements runtime support for signal handling.
//
// Most synchronization primitives are not available from
// the signal handler (it cannot block and cannot use locks)
// so the handler communicates with a processing goroutine
// via struct sig, below.
//
// Ownership for sig.Note passes back and forth between
// the signal handler and the signal goroutine in rounds.
// The initial state is that sig.note is cleared (setup by siginit).
// At the beginning of each round, mask == 0.
// The round goes through three stages:
//
// (In parallel)
// 1a) One or more signals arrive and are handled
// by sigsend using cas to set bits in sig.mask.
// The handler that changes sig.mask from zero to non-zero
// calls notewakeup(&sig).
// 1b) Sigrecv calls notesleep(&sig) to wait for the wakeup.
//
// 2) Having received the wakeup, sigrecv knows that sigsend
// will not send another wakeup, so it can noteclear(&sig)
// to prepare for the next round. (Sigsend may still be adding
// signals to sig.mask at this point, which is fine.)
//
// 3) Sigrecv uses cas to grab the current sig.mask and zero it,
// triggering the next round.
//
// The signal handler takes ownership of the note by atomically
// changing mask from a zero to non-zero value. It gives up
// ownership by calling notewakeup. The signal goroutine takes
// ownership by returning from notesleep (caused by the notewakeup)
// and gives up ownership by clearing mask.

package runtime
#include "config.h"
#include "runtime.h"
#include "arch.h"
#include "malloc.h"
#include "defs.h"

static struct {
        Note;
        uint32 mask;
        bool inuse;
} sig;

void
siginit(void)
{
        runtime_noteclear(&sig);
}

// Called from sighandler to send a signal back out of the signal handling thread.
bool
__go_sigsend(int32 s)
{
        uint32 bit, mask;

        if(!sig.inuse)
                return false;
        bit = 1 << s;
        for(;;) {
                mask = sig.mask;
                if(mask & bit)
                        break;          // signal already in queue
                if(runtime_cas(&sig.mask, mask, mask|bit)) {
                        // Added to queue.
                        // Only send a wakeup for the first signal in each round.
                        if(mask == 0)
                                runtime_notewakeup(&sig);
                        break;
                }
        }
        return true;
}

// Called to receive a bitmask of queued signals.
func Sigrecv() (m uint32) {
        runtime_entersyscall();
        runtime_notesleep(&sig);
        runtime_exitsyscall();
        runtime_noteclear(&sig);
        for(;;) {
                m = sig.mask;
                if(runtime_cas(&sig.mask, m, 0))
                        break;
        }
}

func Signame(sig int32) (name String) {
        const char* s = NULL;
        char buf[100];
#if defined(HAVE_STRSIGNAL)
        s = strsignal(sig);
#endif
        if (s == NULL) {
                snprintf(buf, sizeof buf, "signal %d", sig);
                s = buf;
        }
        int32 len = __builtin_strlen(s);
        unsigned char *data = runtime_mallocgc(len, FlagNoPointers, 0, 0);
        __builtin_memcpy(data, s, len);
        name.__data = data;
        name.__length = len;
}

func Siginit() {
        runtime_initsig(SigQueue);
        sig.inuse = true;       // enable reception of signals; cannot disable
}