[llvm-dev] Dealing with llvm::errs() and friends in dynamic libs on Windows

[Reid Kleckner via llvm-dev]

I think the easiest way for you to deal with this will be to redirect FDs 1
and 2 to the NUL device, or to a pipe or wherever you'd like LLVM or any
other third party libraries to send their output.

On Sun, Sep 9, 2018 at 9:30 AM Janus Lynggaard Thorborg via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
>
> outs() and errs() are routinely used around llvm and cannot really be
> disabled. It would seem that in certain types of Windows applications,
> stderr and stdout are not opened by default, which is an assumption of llvm.
>
> This leaves errs() and outs() with an error code, because writing calls
> will fail due to invalid fd handles. Thus, when an llvm lib is unloaded, it
> will terminate the entire process due to a non-zero EC in ~raw_fd_ostream()
> that calls report_fatal_error().
>
> It doesn't help that outs(), errs() etc. are not returning raw_fd_ostream,
> so you cannot clear the error codes manually (without dynamic_cast):
>
> struct LLVMCleanup
> {
>     ~LLVMCleanup()
>     {
>         for (auto stream : {&llvm::errs(), &llvm::outs()})
>         {
>             if (auto fdStream =
> dynamic_cast<llvm::raw_fd_ostream*>(stream))
>                 fdStream->clear_error();
>         }
>     }
> };
>
> Above is my currently working fix (if I sprinkle it in enough places) but
> it is a pretty fragile solution. Is there some special initialization or
> handling that I'm missing? My program is itself a dynamic lib, so I don't
> control state of stderr or stdout. I would rather not terminate my host
> process because diagnostic prints failed.
>
> Thanks - and regards,
> Janus
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