[libcxx-dev] Question about the no-exceptions mode of libc++

[Louis Dionne via libcxx-dev]

> On Jan 29, 2019, at 13:03, Marshall Clow <mclow.lists at gmail.com> wrote:
> 
> 
> 
> On Mon, Jan 28, 2019 at 10:44 AM Louis Dionne via libcxx-dev <libcxx-dev at lists.llvm.org <mailto:libcxx-dev at lists.llvm.org>> wrote:
> Hi,
> 
> My understanding is that libc++ has a no-exceptions mode under which we never throw and never use try-catch blocks. This mode is enabled by defining _LIBCPP_NO_EXCEPTIONS, or by passing LIBCXX_ENABLE_EXCEPTIONS=OFF when configuring CMake.
> 
> However, I'd like to understand the design of this mode a bit better. It seems like we sometimes call abort() instead of throwing, but sometimes we just swallow the would-be exception and carry on. For example, instead of throwing std::bad_optional_access, we call std::abort():
> 
>     void __throw_bad_optional_access() {
>     #ifndef _LIBCPP_NO_EXCEPTIONS
>             throw bad_optional_access();
>     #else
>             _VSTD::abort();
>     #endif
>     }
> 
> This makes sense to me. However, in other cases, we just swallow the exception. For example, in std::map::at when we can't find the key, we just don't throw an exception, and we dereference a null pointer unless I'm mistaken:
> 
>     template <class _Key, class _Tp, class _Compare, class _Allocator>
>     const _Tp& map<_Key, _Tp, _Compare, _Allocator>::at(const key_type& __k) const {
>         __parent_pointer __parent;
>         __node_base_pointer __child = __tree_.__find_equal(__parent, __k);
>     #ifndef _LIBCPP_NO_EXCEPTIONS
>         if (__child == nullptr)
>             throw out_of_range("map::at:  key not found");
>     #endif  // _LIBCPP_NO_EXCEPTIONS
>         return static_cast<__node_pointer>(__child)->__value_.__get_value().second;
>     }
> 
> Here, when __child == nullptr, we do NOT throw an exception and we end up dereferencing it, which is UB. I haven't made a full survey of libc++, but I'd like to know whether this is by design, and if so, what is the rationale for it. Otherwise, I'd like for us to agree on what the behaviour should be (I suggest std::abort()) so that we can fix places that don't behave correctly.
> 
> 
> std::abort is the "correct" behavior.
> We have a "__throw_out_of_range" call that does what we want.
> Change the code to:
> 
> - #ifndef _LIBCPP_NO_EXCEPTIONS
>         if (__child == nullptr)
> -            throw out_of_range("map::at:  key not found");
> +           __throw_out_of_range("map::at:  key not found");
> - #endif  // _LIBCPP_NO_EXCEPTIONS
> 
> AND it gets rid of ifdefs!
> 
> -- Marshall

I surveyed libc++ and I fixed the occurrences that seemed wrong to me: https://reviews.llvm.org/D57761 <https://reviews.llvm.org/D57761>.

Louis

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/libcxx-dev/attachments/20190205/b709941d/attachment-0001.html>