Function __cxa_decrement_exception_refcount

src/from_exception.cpp:282–332

// __cxa_decrement_exception_refcount has an inlined call to
// __cxa_free_exception. Overriding libc++ specific function
extern "C" BOOST_SYMBOL_EXPORT
void __cxa_decrement_exception_refcount(void *thrown_object) throw() {
  BOOST_ASSERT(is_libcpp_runtime());

#if !defined(BOOST_STACKTRACE_LIBCXX_RUNTIME_MAY_CAUSE_MEMORY_LEAK) && defined(BOOST_HAS_THREADS)
  static const char* leaks_are_fine = std::getenv("BOOST_STACKTRACE_LIBCXX_RUNTIME_MAY_CAUSE_MEMORY_LEAK");
  if (!leaks_are_fine || leaks_are_fine[0] != '1') {
        const char* const warning =
            "\n\n"
            "=======================================================================================\n"
            "\n"
            "On this platform, memory leaks may occur if capturing stacktrace from exceptions is\n"
            "enabled and exceptions are thrown concurrently by libc++ runtime (libc++abi).\n"
            "\n"
            "A proper workaround is to use libstdc++ runtime (libgcc_s) instead.\n"
            "\n"
            "Alternatively, if you are willing to accept potential MEMORY LEAKS, set the environment\n"
            "variable `BOOST_STACKTRACE_LIBCXX_RUNTIME_MAY_CAUSE_MEMORY_LEAK=1` to proceed. You can\n"
            "also define the `BOOST_STACKTRACE_LIBCXX_RUNTIME_MAY_CAUSE_MEMORY_LEAK` macro when\n"
            "building the `boost_stacktrace_from_exception` library to disable this warning and to\n"
            "get the MEMORY LEAKS silently on libc++ runtime.\n"
            "\n"
            "=======================================================================================\n"
        ;
        write(STDERR_FILENO, warning, std::strlen(warning));
        std::abort();
  }
#endif

  if (!thrown_object) {
    return;
  }

  static const auto orig_decrement_refcount = []() {
    void* const ptr = ::dlsym(RTLD_NEXT, "__cxa_decrement_exception_refcount");
    BOOST_ASSERT_MSG(ptr, "Failed to find '__cxa_decrement_exception_refcount'");
    return reinterpret_cast<void(*)(void*)>(ptr);
  }();

  const auto* exception_header = exception_begin_llvm_ptr(thrown_object);

  // The following line has a race and could give false positives and false
  // negatives. In first case we remove the trace earlier, in the second case
  // we get a memory leak.
  if (exception_header->referenceCount == 1) {
    const std::lock_guard<std::mutex> guard{g_mapping_mutex};
    g_exception_to_dump_mapping.erase(thrown_object);
  }

  orig_decrement_refcount(thrown_object);
}

#endif