Java example source code file (jsig.c)

This example Java source code file (jsig.c) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

bsd, jvm_begin_signal_setting, jvm_end_signal_setting, jvm_get_signal_action, mask, maxsignum, null, pthread_cond_initializer, pthread_mutex_initializer

The jsig.c Java example source code

/*
 * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

/* CopyrightVersion 1.2 */

/* This is a special library that should be loaded before libc &
 * libthread to interpose the signal handler installation functions:
 * sigaction(), signal(), sigset().
 * Used for signal-chaining. See RFE 4381843.
 */

#include <signal.h>
#include <dlfcn.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

#define MAXSIGNUM 32
#define MASK(sig) ((unsigned int)1 << sig)

static struct sigaction sact[MAXSIGNUM]; /* saved signal handlers */
static unsigned int jvmsigs = 0; /* signals used by jvm */

/* used to synchronize the installation of signal handlers */
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
static pthread_t tid = 0;

typedef void (*sa_handler_t)(int);
typedef void (*sa_sigaction_t)(int, siginfo_t *, void *);
typedef sa_handler_t (*signal_t)(int, sa_handler_t);
typedef int (*sigaction_t)(int, const struct sigaction *, struct sigaction *);

static signal_t os_signal = 0; /* os's version of signal()/sigset() */
static sigaction_t os_sigaction = 0; /* os's version of sigaction() */

static bool jvm_signal_installing = false;
static bool jvm_signal_installed = false;

static void signal_lock() {
  pthread_mutex_lock(&mutex);
  /* When the jvm is installing its set of signal handlers, threads
   * other than the jvm thread should wait */
  if (jvm_signal_installing) {
    if (tid != pthread_self()) {
      pthread_cond_wait(&cond, &mutex);
    }
  }
}

static void signal_unlock() {
  pthread_mutex_unlock(&mutex);
}

static sa_handler_t call_os_signal(int sig, sa_handler_t disp,
                                   bool is_sigset) {
  if (os_signal == NULL) {
    if (!is_sigset) {
      os_signal = (signal_t)dlsym(RTLD_NEXT, "signal");
    } else {
      os_signal = (signal_t)dlsym(RTLD_NEXT, "sigset");
    }
    if (os_signal == NULL) {
      printf("%s\n", dlerror());
      exit(0);
    }
  }
  return (*os_signal)(sig, disp);
}

static void save_signal_handler(int sig, sa_handler_t disp) {
  sigset_t set;
  sact[sig].sa_handler = disp;
  sigemptyset(&set);
  sact[sig].sa_mask = set;
  sact[sig].sa_flags = 0;
}

static sa_handler_t set_signal(int sig, sa_handler_t disp, bool is_sigset) {
  sa_handler_t oldhandler;
  bool sigused;

  signal_lock();

  sigused = (MASK(sig) & jvmsigs) != 0;
  if (jvm_signal_installed && sigused) {
    /* jvm has installed its signal handler for this signal. */
    /* Save the handler. Don't really install it. */
    oldhandler = sact[sig].sa_handler;
    save_signal_handler(sig, disp);

    signal_unlock();
    return oldhandler;
  } else if (jvm_signal_installing) {
    /* jvm is installing its signal handlers. Install the new
     * handlers and save the old ones. jvm uses sigaction().
     * Leave the piece here just in case. */
    oldhandler = call_os_signal(sig, disp, is_sigset);
    save_signal_handler(sig, oldhandler);

    /* Record the signals used by jvm */
    jvmsigs |= MASK(sig);

    signal_unlock();
    return oldhandler;
  } else {
    /* jvm has no relation with this signal (yet). Install the
     * the handler. */
    oldhandler = call_os_signal(sig, disp, is_sigset);

    signal_unlock();
    return oldhandler;
  }
}

sa_handler_t signal(int sig, sa_handler_t disp) {
  return set_signal(sig, disp, false);
}

sa_handler_t sigset(int sig, sa_handler_t disp) {
  printf("sigset() is not supported by BSD");
  exit(0);
 }

static int call_os_sigaction(int sig, const struct sigaction  *act,
                             struct sigaction *oact) {
  if (os_sigaction == NULL) {
    os_sigaction = (sigaction_t)dlsym(RTLD_NEXT, "sigaction");
    if (os_sigaction == NULL) {
      printf("%s\n", dlerror());
      exit(0);
    }
  }
  return (*os_sigaction)(sig, act, oact);
}

int sigaction(int sig, const struct sigaction *act, struct sigaction *oact) {
  int res;
  bool sigused;
  struct sigaction oldAct;

  signal_lock();

  sigused = (MASK(sig) & jvmsigs) != 0;
  if (jvm_signal_installed && sigused) {
    /* jvm has installed its signal handler for this signal. */
    /* Save the handler. Don't really install it. */
    if (oact != NULL) {
      *oact = sact[sig];
    }
    if (act != NULL) {
      sact[sig] = *act;
    }

    signal_unlock();
    return 0;
  } else if (jvm_signal_installing) {
    /* jvm is installing its signal handlers. Install the new
     * handlers and save the old ones. */
    res = call_os_sigaction(sig, act, &oldAct);
    sact[sig] = oldAct;
    if (oact != NULL) {
      *oact = oldAct;
    }

    /* Record the signals used by jvm */
    jvmsigs |= MASK(sig);

    signal_unlock();
    return res;
  } else {
    /* jvm has no relation with this signal (yet). Install the
     * the handler. */
    res = call_os_sigaction(sig, act, oact);

    signal_unlock();
    return res;
  }
}

/* The three functions for the jvm to call into */
void JVM_begin_signal_setting() {
  signal_lock();
  jvm_signal_installing = true;
  tid = pthread_self();
  signal_unlock();
}

void JVM_end_signal_setting() {
  signal_lock();
  jvm_signal_installed = true;
  jvm_signal_installing = false;
  pthread_cond_broadcast(&cond);
  signal_unlock();
}

struct sigaction *JVM_get_signal_action(int sig) {
  /* Does race condition make sense here? */
  if ((MASK(sig) & jvmsigs) != 0) {
    return &sact[sig];
  }
  return NULL;
}