6 Integrating Signal and Exception Handling

Signal Handling on Solaris OS and Linux

The Java HotSpot VM installs signal handlers to implement various features and to handle fatal error conditions. For example, in an optimization to avoid explicit null checks in cases where java.lang.NullPointerException will be thrown rarely, the SIGSEGV signal is caught and handled, and the NullPointerException is thrown.

In general there are two categories of situations where signal/traps arise:

• When signals are expected and handled, like the implicit null-handling cited above. Another example is the safepoint polling mechanism, which protects a page in memory when a safepoint is required. Any thread that accesses that page causes a SIGSEGV, which results in the execution of a stub that brings the thread to a safepoint.

• Unexpected signals. This includes a SIGSEGV when executing in VM code, Java Native Interface (JNI) code, or native code. In these cases the signal is unexpected, so fatal error handling is invoked to create the error log and terminate the process.

The following table lists the signals that are currently used on Solaris OS and Linux.

Signals tagged as "optional" are not used when the -Xrs option is specified to reduce signal usage. With this option fewer signals are used, although the VM installs its own signal handler for essential signals such as SIGSEGV. Specifying this option means that the shutdown hook mechanism will not execute if the process receives a SIGQUIT, SIGTERM, SIGINT, or SIGHUP. Shutdown hooks will execute, as expected, if the VM terminates normally (that is, when the last non-daemon thread completes or the System.exit method is invoked).

On Solaris 8 and 9 OS, signals tagged as "configurable" are substituted when the -XX:+UseAltSigs option is specified to use alternative signals. Starting with Solaris 10 OS, this option is ignored, as the operating system reserves two additional signals (called SIGJVM1 and SIGJVM2).

On Linux, the handler for SIGUSR1 cannot be overridden. SIGUSR2 is used to implement suspend and resume. However it is possible to specify an alternative signal to be used instead of SIGUSR2. This is done by specifying the _JAVA_SR_SIGNUM environment variable. If this environment variable is set, it must be set to a value larger than the maximum of SIGSEGV and SIGBUS.

Exception Handling on Windows

On Windows, an exception is an event that occurs during the execution of a program. There are two kinds of exceptions: hardware exceptions and software exceptions. Hardware exceptions are comparable to signals such as SIGSEGV and SIGKILL on Solaris OS and Linux. Software exceptions are initiated explicitly by applications or the operating system using the RaiseException() API.

On Windows, the mechanism for handling both hardware and software exceptions is called structured exception handling (SEH). This is stack frame-based exception handling similar to the C++ and Java exception handling mechanism. In C++ the __try and __except keywords are used to guard a section of code that might result in an exception, as in the following example:

__try {
     // guarded body of code
 } __except (filter-expression) {
     // exception-handler block
 }

The __except block is filtered by a filter expression that uses the integer exception code returned by the GetExceptionCode() API, exception information returned by the GetExceptionInformation() API, or both.

The filter expression should evaluate to one of the following values:

• EXCEPTION_CONTINUE_EXECUTION = -1

  The filter expression has repaired the situation, and execution continues where the exception occurred. Unlike some exception schemes, SEH supports the resumption model as well. This is much like Unix signal handling in the sense that after the signal handler finishes, the execution continues where the program was interrupted. The difference is that the handler in this case is just the filter expression itself and not the __except block. However, the filter expression might also involve a function call.

• EXCEPTION_CONTINUE_SEARCH = 0

  The current handler cannot handle this exception. Continue the handler search for the next handler. This is similar to the catch block not matching an exception type in C++ and Java.

• EXCEPTION_EXECUTE_HANDLER = 1

  The current handler matches and can handle the exception. The __except block is executed.

The __try and __finally keywords are used to construct a termination handler as follows

__try { 
    // guarded body of code  
} __finally { 
    // __finally block  
}

When control leaves the __try block (after exception or without exception), the __finally block is executed. Inside the __finally block, the AbnormalTermination() API can be called to test whether control continued after the exception or not.

Windows programs can also install a top-level unhandled exception filter function to catch exceptions that are not handled in the __try/__except block. This function is installed on a process-wide basis using the SetUnhandledExceptionFilter() API. If there is no handler for an exception, then UnhandledExceptionFilter() is called, and this will call the top-level unhandled exception filter function, if any, to catch that exception. This function also shows a message box to notify the user about the unhandled exception.

Windows exceptions are comparable to Unix synchronous signals that are attributable to the current execution stream. In Windows, asynchronous events such as console events (for example, the user pressing Control+C at the console) are handled by the console control handler registered using the SetConsoleCtlHandler() API.

If an application uses the signal() API on Windows, then the C runtime library (CRT) maps both Windows exceptions and console events to appropriate signals or C runtime errors. For example, CRT maps Control+C to SIGINT and all other console events to SIGBREAK. Similarly, if you register the SIGSEGV handler, CRT translates the corresponding exception to a signal. CRT startup code implements a __try/__except block around the main() function. The CRT's exception filter function (named _XcptFilter) maps the Win32 exceptions to signals and dispatches signals to their appropriate handlers. If a signal's handler is set to SIG_DFL (default handling), then _XcptFilter calls UnhandledExceptionFilter.

With Windows XP or Windows 2003, the vectored exception handling mechanism can also be used. Vectored handlers are not frame-based handlers. A program can register zero or more vectored exception handlers using the AddVectoredExceptionHandler API. Vectored handlers are invoked before structured exception handlers, if any, are invoked, regardless of where the exception occurred.

Vectored exception handler returns one of the following values:

• EXCEPTION_CONTINUE_EXECUTION: Skip next vectored and SEH handlers.

• EXCEPTION_CONTINUE_SEARCH: Continue next vectored or SEH handler.