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Subject: [boost] [iostreams] libstdc++ vs MSVC exception handling behavior in seekg, seekp
From: James E. King III (jking_at_[hidden])
Date: 2018-07-17 16:17:05

I found a disabled test in Boost.Iostreams called stream_state_test. It
was disabled because it was thought it only tests the standard library
implementation. Partially true, but it also tests some parts of iostreams
so I have been working to re-enable it along with the generic CI upgrades.

It looks like libstdc++ and the standard library that comes with MSVC (is
it still Dinkumware? It used to be...) handle exceptions on stream seekg
and seekp differently. I haven't consulted the standards documents yet,
but I found this reference for exception handling: which states, " Any
exception thrown by an internal operation is caught and handled by the
function, setting badbit <>. "

Sitting in the debugger where the test throws an exception in
error_device::seek, there is no exception handling within iostreams or in
msvcp140d code paths for a call to seekg, while the libstdc++
implementation is able to pass all of the tests. This leads to a couple

1. Are the standards clear in this area?
2. Is someone violating them?

A cursory review (such as this) leads me to believe that the MSVC C++
runtime implementation is not properly handling exceptions here. The
sources for seekg in libstdc++ wrap the internals in a try catch in order
to restore state. The sources for seekg in the MSVCPRT do not.

- Jim

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