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Re: [RFC / musing] Scoped exception handling in Linux userspace?

On 07/18/2013 08:29 PM, Andy Lutomirski wrote:
On Thu, Jul 18, 2013 at 6:17 PM, David Daney <> wrote:
On 07/18/2013 05:50 PM, Andy Lutomirski wrote:

On Thu, Jul 18, 2013 at 5:40 PM, David Daney <>

On 07/18/2013 05:26 PM, Andy Lutomirski wrote:

How is this different than throwing exceptions from a signal handler?

Two ways.  First, exceptions thrown from a signal handler can't be


s/retries/retried, by which I mean that you can't do things like
implementing virtual memory in userspace by catching SIGSEGV, calling
mmap, and resuming.

Second, and more importantly, installing a signal handler in
a library is a terrible idea.

The signal handler would be installed by main() before calling into the
library.  You have to have a small amount of boiler plate code to set it up,
but the libraries wouldn't have to be modified if they were already
exception safe.

FWIW the libgcj java runtime environment uses this strategy for handling
NullPointerExceptions and DivideByZeroError(sp?).  Since all that code for
the most part follows the standard C++ ABIs, it is an example of this
technique that has been deployed in many environments.

Other way around: a *library* that wants to use exception handling
can't do so safely without the cooperation, or at least understanding,
of the main program and every other library that wants to do something
similar.  Suppose my library installs a SIGFPE handler and throws
my_sigfpe_exception and your library installs a SIGFPE handler and
throws your_sigfpe_exception.  The result: one wins and the other
crashes due to an unhandled exception.

In my particular usecase, I have code (known to the main program) that
catches all kinds of fatal signals to log nice error messages before
dying.  That means that I can't use a library that handles signals for
any other purpose.  Right now I want to have a small snippet of code
handle SIGBUS, but now I need to coordinate it with everything else.

If this stuff were unified, then everything would just work.

That's right. But I think the Linux kernel already supplies all the needed functionality to do this. It is really a matter of choosing a userspace implementation and standardizing your entire system around it. In the realm of GNU/GLibc/Linux, it is really more of social/political exercise rather than a technical problem.

David Daney

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