terminate called after throwing an instance of 'Poco::SystemException'

Question

Sometimes (about 1 out of 100 runs), my program terminates with this message:

terminate called after throwing an instance of 'Poco::SystemException'
  what():  System exception

my code is not the one catching the exception (all my catches are more verbose), and I am not sure where it's caught. it's very likely that the exception does contain a useful message, but it's not returned through the what() method but by the displayText() method.

The string "terminate called after throwing an instance of" has ~600k in Google, so it's probably printed by code inserted by the compiler or by some common library (pthread?). I only seen this error message when the program ran on Linux (never on Windows).

anyone knows in what code this uncaught exception is caught?

Answer 1

An uncaught exception is— by definition —not caught anywhere.

If an exception cannot be handled, the C++ exception mechanism will call std::terminate() (see include header <exception>), which will call a customizable termination handler. On your platform, the standard termination handler prints the output of std::exception::what() (which Poco's exceptions inherit from). Unfortunately, the way Poco's exceptions are designed, this will not contain any useful information.

There are multiple ways an exception cannot be handled:

• No suitable catch() handler is found and the unwinding mechanism exits main(). You can try wrapping your main() code in try...catch to print the exception's displayText().
• A function exists with an exception that does not match its exception specification (... functionname(...) throw(...)). This will call std::unexpected() which will by default call std::terminate().
• An exception is thrown from within a destructor that is called during the unwinding process of another exception. Never throw exceptions in destructors!
• An exception is thrown while trying to create the original exception object. Never throw exceptions in custom exception classes!

When using Poco threads and a thread is terminated by an unhandled exception, Poco will invoke its internal ErrorHandler and the program will not exit, so I doubt that this is a threading issue.