Monday, October 13, 2014

process-wide exception handling, across lang

label - not sure...

I blogged before that one of the "services" a managed environment offers is (uncaught) exception handling.

By design, try/catch exception handling is per-thread. However, an innovative feature is the app-domain-wide exception handler that handles any "unhandled" exception on any thread. JVM and CLR both support this feature. (Actually they aren't bullet-proof. They can't handle stack overflow for example. I think it all boils down to the design of the VM.)

Q: Why doesn't c++ have this feature (except set_terminate())?  No VM, but exactly how?

Now I think any VM (CLR, JVM etc) offers "containers" to contain your applications. A CLR can contain 2 app domains. When some thread in an app domain throws an exception and you don't catch it, the container can "notice" it and fire the event. At least it has a chance to print the stack trace, which is sometimes impossible in c++.

I think the CLR is the "container" or "host" to the app-domain. The OS doesn't see the code running in the app-domain; OS sees the CLR only. If app-domain generates an uncaught exception then the host/container VM won't let it bring down entire VM. VM will print the exception and continue to live and host the other app-domains.

In contrast, in a traditional compiled application like c++, the uncaught exception would bring down entire process, as if the CLR itself throws an exception.

In a crude analogy, suppose I write a container (like a CLR) in C that evaluates a byte array containing some byte code. If the byte code throws exception, my container can print the error and continue to live.

I feel an OS is also a container. Without the OS, a program (compiled into assembly) can run directly on a CPU. Any error would crash the entire CPU. The OS isolates one such program from other programs, so one crasher doesn't bring down entire OS.

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New York (Time Square), NY, United States
http://www.linkedin.com/in/tanbin