[libcxx-dev] system_error in shared_future::wait_for

Louis Dionne via libcxx-dev libcxx-dev at lists.llvm.org
Tue May 14 14:57:48 PDT 2019

> On May 14, 2019, at 11:49, b via libcxx-dev <libcxx-dev at lists.llvm.org> wrote:
> Hello everyone,
> I am new to this list. I am writing because I think I might have caught a bug in libc++. What I hope to get is a quick assessment of my issue so that I know if I should file this as a bug or some hints in how I should proceed to gather more information or even rectify the issue on my end.
> The issue I am seeing popped up in a macOS project (Xcode 10.2.1) I am working on but I was able to condense it into a very small test program, see below for the code.
> The issue presents itself as a crash in shared_future::wait_for due to an uncatchable exception. The crash does not occur immediately but only after a (huge) number of calls. Console output in Xcode reads  "libc++abi.dylib: terminating with uncaught exception of type std::__1::system_error: condition_variable timed_wait failed: Invalid argument“. See backtrace further down for details. Unfortunately I cannot step into code of the method where the exception originates which is condition_variable::__do_timed_wait. I browsed the code in the libcxx repository and found that the error must be the result code EINVAL from a call to pthread_cond_timedwait.
> The interesting bit is that the issue can be reproduced in each test run within (mostly) less than 10 minutes on an iMac18,3/i7 and on a MacBookPro15,1/i7 but never on a MacBookPro15,2/i5. I only have one machine of each of those available so I cannot be sure how these results hold up.
> 1. Is there something fundamentally wrong with my code, i.e. how I use the shared_future?
> 2. Is this likely to be a bug in libc++ or is it more likely to be an issue with the BSD level API and/or the hardware? In case of the latter option where should I seek contact?
> 3. I would like to be able to step into the code for condition_variable::__do_timed_wait and get the debugger info for the local variables. Would this be simply a matter of pulling the libcxx repository, building and linking it? I am not a command line compilation guy so I was hoping for some good documentation on how to do this with and for Xcode. However, if must be, I am willing to accept my fate.

In order to do that with 100% reproduceability, you'd have to build libc++ exactly the same way it was built for the platform you're running on, which is not easily achievable at the moment. I'll take a look.

I'll try running this program overnight to reproduce:

   cat <<EOF | clang++ -xc++ - -std=c++17 && ./a.out
   #include <future>
   #include <thread>

   using namespace std;

   int main(int argc, const char * argv[]) {
      promise<void> thePromise = promise<void> {};
      shared_future<void> sharedFuture = thePromise.get_future().share();

      thread anotherThread = thread( [sharedFuture]
         int debugCount = 0;
         while (sharedFuture.wait_for(100ns) == future_status::timeout)


      return 0;

However, it would be useful for me to know the OSes you've been able to reproduce it on.


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