7.3 Signal Chaining

If an application with native code requires its own signal handlers, then it might need to be used with the signal chaining facility. The signal chaining facility offers the following features:

• Support for pre-installed signal handlers when the HotSpot VM is created.

  When the VM is first created, existing signal handlers (that is, handlers for signals that are used by the VM) are saved. During execution, when any of these signals are raised and found not to be targeted at the Java HotSpot VM, the pre-installed handlers are invoked. In other words, pre-installed handlers are chained behind the VM handlers for these signals.

• Support for signal handler installation after the HotSpot VM is created, either inside JNI code or from another native thread.

  An application can link and load the libjsig.so shared library before libc/libthread/libpthread. This library ensures that calls such as signal(), sigset(), and sigaction() are intercepted so that they do not actually replace the Java HotSpot VM's signal handlers if the handlers conflict with those already installed by the VM. Instead, these calls save the new signal handlers (or chain them behind the VM-installed handlers). During execution, when any of these signals are raised and found not to be targeted at the Java HotSpot VM, the pre-installed handlers are invoked.

  If support for signal handler installation after the creation of the VM is not required, then the libjsig.so shared library is not needed.

  Perform one of these two procedures to use the libjsig.so shared library:

  • Link it with the application that creates/embeds the HotSpot VM:

    cc -L libjvm.so-directory -ljsig -ljvm java_application.c
    

  • Use the LD_PRELOAD environment variable:

    • KornShell (ksh):

      export LD_PRELOAD=libjvm.so-directory/libjsig.so; java_application(ksh)
      

    • C shell (csh):

      setenv LD_PRELOAD libjvm.so-directory/libjsig.so; java_application(csh)
      

  The interposed signal(), sigset(), and sigaction() return the saved signal handlers, not the signal handlers installed by the Java HotSpot VM and which are seen by the operating system.

Also, the SIGQUIT, SIGTERM, SIGINT, and SIGHUP signals cannot be chained. If the application needs to handle these signals, consider using the -Xrs option.

On Oracle Solaris operating system, the SIGUSR2 signal can be chained only for non-Java and non-VM threads (that is, it can only be used for native threads created by applications that do not attach to the VM).