I have made this small illustrative code that exhibits the same issues the program I'm writing does: namely, it works fine in debug mode, segfaults in release. The problem seems to be that the ui_context, in release mode, when being called to run the work it has assigned, is nullptr.
Running in Fedora 33, with g++ (GCC) 10.2.1 20201125 (Red Hat 10.2.1-9) and clang version 11.0.0 (Fedora 11.0.0-2.fc33) . Both compilers behave in the same way. Boost version is 1.75 .
Code:
#include <iostream>
#include <vector>
#include <memory>
#include <chrono>
#include <thread>
#include <boost/asio.hpp>
#include <boost/signals2.hpp>
constexpr auto MAX_LOOP_COUNT = 100;
class network_client : public std::enable_shared_from_this<network_client>
{
private:
using Signal = boost::signals2::signal<void(int)>;
public:
network_client(boost::asio::io_context &context) :
strand(boost::asio::make_strand(context))
{
std::cout << "network client created" << std::endl;
}
void doNetworkWork()
{
std::cout << "doing network work" << std::endl;
boost::asio::post(strand,std::bind(&network_client::onWorkComplete,shared_from_this()));
}
void onWorkComplete()
{
std::this_thread::sleep_for(std::chrono::milliseconds(10));
std::cout << "signalling completion" << " from thread id:" << std::this_thread::get_id() << std::endl;
signal(42);
}
void workCompleteHandler(const typename Signal::slot_type &slot)
{
signal.connect(slot);
}
private :
boost::asio::strand<boost::asio::io_context::executor_type> strand;
Signal signal;
};
class network_client_producer
{
public :
network_client_producer() : work(boost::asio::make_work_guard(context))
{
using run_function = boost::asio::io_context::count_type (boost::asio::io_context::*)();
for (int i = 0; i < 2; i++)
{
context_threads.emplace_back(std::bind(static_cast<run_function>(&boost::asio::io_context::run), std::ref(context)));
}
}
~network_client_producer()
{
context.stop();
for(auto&& thread : context_threads)
{
if(thread.joinable())
{
thread.join();
}
}
}
using NetworkClientPtr = std::shared_ptr<network_client>;
NetworkClientPtr makeNetworkClient()
{
return std::make_shared<network_client>(context);
}
private :
boost::asio::io_context context;
std::vector<std::thread> context_threads;
boost::asio::executor_work_guard<boost::asio::io_context::executor_type> work;
};
class desktop : public std::enable_shared_from_this<desktop>
{
public:
desktop(const boost::asio::io_context::executor_type &executor):executor(executor)
{
}
void doSomeNetworkWork()
{
auto client = client_producer.makeNetworkClient();
client->workCompleteHandler([self = shared_from_this()](int i){
//post work into the UI thread
std::cout << "calling into the uiThreadWork with index " << i << " from thread id:" << std::this_thread::get_id() << std::endl;
boost::asio::post(self->executor, std::bind(&desktop::uiThreadWorkComplete, self, i));
});
client->doNetworkWork();
}
void showDesktop()
{
std::this_thread::sleep_for(std::chrono::milliseconds(20));
}
public:
void uiThreadWorkComplete(int i)
{
std::cout << "Called in the UI thread with index:" << i << ", on thread id:" << std::this_thread::get_id() << std::endl;
}
private:
const boost::asio::io_context::executor_type& executor;
network_client_producer client_producer;
};
int main()
{
std::cout << "Starting application. Main thread id:"<<std::this_thread::get_id() << std::endl;
int count = 0;
boost::asio::io_context ui_context;
auto work = boost::asio::make_work_guard(ui_context);
/*auto work = boost::asio::require(ui_context.get_executor(),
boost::asio::execution::outstanding_work.tracked);*/
auto ui_desktop = std::make_shared<desktop>(ui_context.get_executor());
ui_desktop->doSomeNetworkWork();
while(true)
{
ui_context.poll_one();
ui_desktop->showDesktop();
if (count >= MAX_LOOP_COUNT)
break;
count++;
}
ui_context.stop();
std::cout << "Stopping application" << std::endl;
return 0;
}
Compiling it with g++ -std=c++17 -g -o main -pthread -O3 main.cpp and
running it in gdb I get this:
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
Starting application. Main thread id:140737348183872
[New Thread 0x7ffff7a51640 (LWP 27082)]
[New Thread 0x7ffff7250640 (LWP 27083)]
network client created
doing network work
signalling completion from thread id:140737348179520
calling into the uiThreadWork with index 42 from thread id:140737348179520
Thread 2 "main" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff7a51640 (LWP 27082)]
0x000000000040b7b8 in boost::asio::io_context::basic_executor_type<std::allocator<void>, 0u>::execute<std::_Bind<void (desktop::*(std::shared_ptr<desktop>, int))(int)> >(std::_Bind<void (desktop::*(std::shared_ptr<desktop>, int))(int)>&&) const (this=<optimized out>, f=...) at /usr/local/include/boost/asio/impl/io_context.hpp:309
309 io_context_->impl_.post_immediate_completion(p.p,
While compiling without any optimizations g++ -std=c++17 -g -o main -pthread -O0 main.cpp works as expected.
I tried to keep it as close as I can to the real program that actually does network IO, which is why I have that strand in there.
It's obvious that I'm doing something horribly wrong here. The question is: what is the problem? Thank you for any pointers.
Add the sanitizers -fsanitize=undefined,address:
Starting application. Main thread id:139902898299968
network client created
doing network work
signalling completion from thread id:139902399940352
calling into the uiThreadWork with index 42 from thread id:139902399940352
=================================================================
==29084==ERROR: AddressSanitizer: stack-use-after-scope on address 0x7ffc8393d7c0 at pc 0x000000507ce0 bp 0x7f3d90d9e3d0 sp 0x7f3d90d9e3c8
READ of size 8 at 0x7ffc8393d7c0 thread T1
#0 0x507cdf in boost::asio::io_context::basic_executor_type<std::allocator<void>, ...
#1 0x507cdf in boost::asio::detail::initiate_post_with_executor<boost::asio::io_co...
#2 0x507cdf in auto boost::asio::post<boost::asio::io_context::basic_executor_type...
#3 0x5077cf in desktop::doSomeNetworkWork()::'lambda'(int)::operator()(int) const ...
#4 0x518ce2 in boost::function1<void, int>::operator()(int) const /home/sehe/custo...
#5 0x518481 in boost::signals2::detail::void_type boost::signals2::detail::call_wi...
#6 0x518481 in boost::signals2::detail::void_type boost::signals2::detail::variadi...
#7 0x517f43 in boost::signals2::detail::slot_call_iterator_t<boost::signals2::deta...
#8 0x516397 in void boost::signals2::optional_last_value<void>::operator()<boost::...
#9 0x516397 in void boost::signals2::detail::combiner_invoker<void>::operator()<bo...
#10 0x516397 in boost::signals2::detail::signal_impl<void (int), boost::signals2::...
#11 0x50d9d4 in network_client::onWorkComplete() /home/sehe/Projects/stackoverflow...
#12 0x51021d in void std::_Bind<void (network_client::* (std::shared_ptr<network_c...
#13 0x51021d in void boost::asio::asio_handler_invoke<std::_Bind<void (network_cli...
#14 0x51021d in void boost_asio_handler_invoke_helpers::invoke<std::_Bind<void (ne...
#15 0x51021d in boost::asio::detail::executor_op<std::_Bind<void (network_client::...
#16 0x51188e in boost::asio::detail::strand_executor_service::invoker<boost::asio:...
#17 0x514311 in void boost::asio::asio_handler_invoke<boost::asio::detail::strand_...
#18 0x514311 in void boost_asio_handler_invoke_helpers::invoke<boost::asio::detail...
#19 0x514311 in boost::asio::detail::executor_op<boost::asio::detail::strand_execu...
#20 0x4d8704 in boost::asio::detail::scheduler::do_run_one(boost::asio::detail::co...
#21 0x4d70dc in boost::asio::detail::scheduler::run(boost::system::error_code&) /h...
#22 0x523a6e in boost::asio::io_context::run() /home/sehe/custom/boost_1_75_0/boos...
#23 0x5258ef in unsigned long std::_Bind<unsigned long (boost::asio::io_context::*...
#24 0x5258ef in unsigned long std::__invoke_impl<unsigned long, std::_Bind<unsigne...
#25 0x5258ef in std::__invoke_result<std::_Bind<unsigned long (boost::asio::io_con...
#26 0x5258ef in unsigned long std::thread::_Invoker<std::tuple<std::_Bind<unsigned...
#27 0x7f3da660bd7f (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0xd0d7f)
#28 0x7f3da5f856da in start_thread (/lib/x86_64-linux-gnu/libpthread.so.0+0x76da)
#29 0x7f3da5a9671e in clone /build/glibc-S7xCS9/glibc-2.27/misc/../sysdeps/unix/sy...
Address 0x7ffc8393d7c0 is located in stack of thread T0 at offset 224 in frame
#0 0x4cb30f in main /home/sehe/Projects/stackoverflow/test.cpp:109
This frame has 6 object(s):
[32, 40) 'ref.tmp.i85' (line 96)
[64, 80) 'ref.tmp.i'
[96, 112) 'ui_context' (line 113)
[128, 152) 'work' (line 114)
[192, 208) 'ui_desktop' (line 117)
[224, 240) 'ref.tmp' (line 117) <== Memory access at offset 224 is inside this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
(longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-use-after-scope /home/sehe/custom/boost_1_75_0/boost/asio/io_context.hpp:678:25 in boost::asio::io_context::basic_executor_type<std::allocator<void>, 0u>::basic_executor_type(boost::asio::io_context::basic_executor_type<std::allocator<void>, 0u> const&)
Shadow bytes around the buggy address:
0x10001071faa0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10001071fab0: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 f8 f2 f2 f2
0x10001071fac0: f8 f2 f2 f2 00 f2 f2 f2 00 00 f3 f3 00 00 00 00
0x10001071fad0: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
0x10001071fae0: 00 f2 f2 f2 f8 f8 f2 f2 00 00 f2 f2 00 00 00 f2
=>0x10001071faf0: f2 f2 f2 f2 00 00 f2 f2[f8]f8 f3 f3 00 00 00 00
0x10001071fb00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10001071fb10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10001071fb20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10001071fb30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10001071fb40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
Container overflow: fc
Array cookie: ac
Intra object redzone: bb
ASan internal: fe
Left alloca redzone: ca
Right alloca redzone: cb
Shadow gap: cc
Thread T1 created by T0 here:
#0 0x483a6a in pthread_create (/home/sehe/Projects/stackoverflow/sotest+0x483a6a)
#1 0x7f3da660c014 in std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std...
#2 0x5241d4 in void std::vector<std::thread, std::allocator<std::thread> >::_M_realloc_ins...
#3 0x523517 in std::thread& std::vector<std::thread, std::allocator<std::thread> >::emplac...
==29084==ABORTING
There's your culprit.
This frame has 6 object(s):
[32, 40) 'ref.tmp.i85' (line 96)
[64, 80) 'ref.tmp.i'
[96, 112) 'ui_context' (line 113)
[128, 152) 'work' (line 114)
[192, 208) 'ui_desktop' (line 117)
[224, 240) 'ref.tmp' (line 117) <== Memory access at offset 224 is inside this variable
What variable is that? Apparently in the line
auto ui_desktop = std::make_shared<desktop>(ui_context.get_executor());
there's a temporary that is being kept a reference to. It must be ui_context.get_executor() because ui_desktop is named and has "obvious" lifetime.
Sure enough, desktop declares its executor member by reference:
const boost::asio::io_context::executor_type& executor;
This is a clear error. Executors are not services nor execution contexts, and are designed to be cheaply copyable and passed by value. The fix is trivial:
boost::asio::io_context::executor_type executor;
As a bonus, here's a simplified version that runs the demo for half a second. Notes:
.stop() on execution contexts, or forgetting about the redundant word guards?#include <iostream>
#include <chrono>
#include <iomanip>
#include <memory>
#include <thread>
#include <boost/asio.hpp>
#include <boost/signals2.hpp>
namespace {
using namespace std::chrono_literals;
auto now = std::chrono::high_resolution_clock::now;
auto elapsed = [start=now()] { return (now()-start)/1ms; };
inline std::string thread_hash() {
static constexpr std::hash<std::thread::id> h{};
std::ostringstream oss;
oss << std::hex << std::setw(2) << std::setfill('0')
<< h(std::this_thread::get_id()) % 0xff;
return oss.str();
}
auto trace = [](auto const&... args) {
std::cout << "thread #" << thread_hash() << " at t+" << std::setw(3) << elapsed() << "ms\t";
(std::cout << ... << args) << std::endl;
};
} // namespace
struct network_client : std::enable_shared_from_this<network_client> {
explicit network_client(const boost::asio::any_io_executor& context) : strand(make_strand(context)) {
trace("network client created");
}
void doNetworkWork() {
trace("doing network work");
post(strand, std::bind(&network_client::onWorkComplete, shared_from_this()));
}
void onWorkComplete() {
std::this_thread::sleep_for(10ms);
trace("signalling completion");
signal(42);
}
template <typename F> void workCompleteHandler(F slot) {
signal.connect(std::move(slot));
}
private:
boost::asio::strand<boost::asio::any_io_executor> strand;
using Signal = boost::signals2::signal<void(int)>;
Signal signal;
};
struct network_client_producer {
auto makeNetworkClient() {
return std::make_shared<network_client>(context_threads.get_executor());
}
private :
boost::asio::thread_pool context_threads {2};
};
struct desktop : std::enable_shared_from_this<desktop> {
explicit desktop(boost::asio::io_context::executor_type executor) : executor(std::move(executor)) {}
void doSomeNetworkWork() {
auto client = client_producer.makeNetworkClient();
client->workCompleteHandler([this, self = shared_from_this()](int i) {
// post work into the UI thread
trace("calling into the uiThreadWork with index ", i);
post(executor, std::bind(&desktop::uiThreadWorkComplete, self, i));
});
client->doNetworkWork();
}
static void showDesktop() {
trace("showDesktop");
std::this_thread::sleep_for(20ms);
}
void uiThreadWorkComplete(int i) const {
trace("Called in the UI thread with index:", i);
}
private:
boost::asio::io_context::executor_type executor;
network_client_producer client_producer;
};
int main() {
trace("Starting application. Main thread is #", thread_hash());
boost::asio::io_context ui_context;
auto work = boost::asio::make_work_guard(ui_context);
/*auto work = boost::asio::require(ui_context.get_executor(),
boost::asio::execution::outstanding_work.tracked);*/
auto ui_desktop = std::make_shared<desktop>(ui_context.get_executor());
ui_desktop->doSomeNetworkWork();
for (auto deadline = now() + 0.5s; now() < deadline;) {
ui_context.poll_one();
ui_desktop->showDesktop();
}
trace("Stopping application");
work.reset();
ui_context.run();
// ui_context.stop();
trace("Bye\n");
}
Prints
thread #97 at t+ 0ms Starting application. Main thread is #97
thread #97 at t+ 0ms network client created
thread #97 at t+ 1ms doing network work
thread #97 at t+ 1ms showDesktop
thread #2d at t+ 11ms signalling completion
thread #2d at t+ 11ms calling into the uiThreadWork with index 42
thread #97 at t+ 21ms Called in the UI thread with index:42
thread #97 at t+ 21ms showDesktop
thread #97 at t+ 41ms showDesktop
thread #97 at t+ 61ms showDesktop
thread #97 at t+ 81ms showDesktop
thread #97 at t+101ms showDesktop
thread #97 at t+122ms showDesktop
thread #97 at t+142ms showDesktop
thread #97 at t+162ms showDesktop
thread #97 at t+182ms showDesktop
thread #97 at t+202ms showDesktop
thread #97 at t+222ms showDesktop
thread #97 at t+242ms showDesktop
thread #97 at t+262ms showDesktop
thread #97 at t+282ms showDesktop
thread #97 at t+302ms showDesktop
thread #97 at t+323ms showDesktop
thread #97 at t+343ms showDesktop
thread #97 at t+363ms showDesktop
thread #97 at t+383ms showDesktop
thread #97 at t+403ms showDesktop
thread #97 at t+423ms showDesktop
thread #97 at t+443ms showDesktop
thread #97 at t+463ms showDesktop
thread #97 at t+483ms showDesktop
thread #97 at t+503ms Stopping application
thread #97 at t+504ms Bye