How to list handlers registered in boost::asio::io_service under debugger?

Question

Given running application, I would like to extract information about currently registered complete handlers.

Handler has been registered by class A. For example:

boost::asio::async_read(s, b, boost::bind(&A::F, this->shared_from_this(), boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));

Under debugger I have access to appropriate io_service variable. How to figure out (A::F, this, s, b) for operations, which have not yet completed.

Answer 1

I would like to slightly expand the scope of the question to cover alternatives for what I believe is the final goal: debugging asynchronous handlers.

---

Sample Program

To show debugging via examples, lets start with a basic UDP echo server that listens to port 4321:

#include <boost/array.hpp>
#include <boost/asio.hpp>
#include <boost/bind.hpp>

using boost::asio::ip::udp;

class udp_echo
{
public:
  udp_echo(boost::asio::io_service& service,
           unsigned int port)
    : socket_(service, udp::endpoint(udp::v4(), port))
  {
    socket_.async_receive_from(
      boost::asio::buffer(buffer_), sender_,
      boost::bind(&udp_echo::handle_receive, this,
                  boost::asio::placeholders::error,
                  boost::asio::placeholders::bytes_transferred));
  }

  void handle_receive(const boost::system::error_code& error,
                      std::size_t bytes_transferred)
  {
    socket_.async_send_to(
      boost::asio::buffer(buffer_, bytes_transferred), sender_,
      boost::bind(&udp_echo::handle_send, this,
                  boost::asio::placeholders::error,
                  boost::asio::placeholders::bytes_transferred));
  }

  void handle_send(const boost::system::error_code& error,
                   std::size_t bytes_transferred)
  {
    socket_.close();  
  }

private:
  udp::socket socket_;
  boost::array<char, 128> buffer_;
  udp::endpoint sender_;
};

int main()
{
  boost::asio::io_service service;
  udp_echo echo(service, 4321);
  service.run();
}

This simple program has a single asynchronous call chain:

udp_echo::udp_echo()
{
  socket_.async_receive_from(...); --.
}                                    |
             .-----------------------'
             v
void udp_echo::handle_receive(...)
{
  socket_.async_send_to(...);  ------.
}                                    |
             .-----------------------'
             v
void udp_echo::handle_send()
{
  socket_.close(); 
}

---

Handler Tracking

Boost 1.47 introduced handler tracking. Simply define BOOST_ASIO_ENABLE_HANDLER_TRACKING and Boost.Asio will write debug output, including timestamps, to the standard error stream. Running the programming, and sending "hello world" via UDP produced the following output:

@asio|1363273821.846895|0*1|[email protected]_receive_from // 1
@asio|1363273829.288883|>1|ec=system:0,bytes_transferred=12      // 2
@asio|1363273829.288931|1*2|[email protected]_send_to      // 3
@asio|1363273829.289013|<1|                                      // 4
@asio|1363273829.289026|>2|ec=system:0,bytes_transferred=12      // 5
@asio|1363273829.289035|2|[email protected]                // 6
@asio|1363273829.289075|<2|                                      // 7

It can be read line-by-line as:

1. Non-handler (0) invoked socket.async_receive_from(), creating handler 1.
2. Enter handler 1 socket.async_receive_from() with no error and 12 bytes have been received.
3. Handler 1 socket.async_receive_from() invoked socket.async_send_to(), creating handler 2.
4. Exit handler 1 socket.async_receive_from().
5. Enter handler 2 socket.async_send_to() with no error and 12 bytes have been sent.
6. Handler 2 invoked socket.close().
7. Exit handler 2 socket.async_send_to().

And visually maps to the following:

udp_echo::udp_echo()
{
  socket_.async_receive_from(...); --. // 1
}                                    |
             .-----------------------'
             v
void udp_echo::handle_receive(...)
{                                      // 2
  socket_.async_send_to(...);  ------. // 3
}                                    | // 4
             .-----------------------'
             v
void udp_echo::handle_send()
{                                      // 5
  socket_.close();                     // 6
}                                      // 7

---

GDB

Debugging through GDB will require digging through multiple layers. It helps to have an understanding of some of the implementation details of Boost.Asio. Here are a few concepts:

• io_service only contains handlers that are ready to run.
• The reactor will generally contain work operations, and a handle to completion handlers that are not ready to run.
• The reactor will register itself with the io_service.

Here is a debug session:

(gdb) bt
#0  0x00ab1402 in __kernel_vsyscall ()
#1  0x00237ab8 in __epoll_wait_nocancel () from /lib/libc.so.6
#2  0x080519c3 in boost::asio::detail::epoll_reactor::run (this=0x80560b0, 
    block=true, ops=...)
    at /opt/boost/include/boost/asio/detail/impl/epoll_reactor.ipp:392
#3  0x08051c2d in boost::asio::detail::task_io_service::do_run_one (
    this=0x8056030, lock=..., this_thread=..., ec=...)
    at /opt/boost/include/boost/asio/detail/impl/task_io_service.ipp:396
#4  0x08051e8a in boost::asio::detail::task_io_service::run (this=0x8056030, 
    ec=...)
    at /opt/boost/include/boost/asio/detail/impl/task_io_service.ipp:153
#5  0x08051f50 in boost::asio::io_service::run (this=0xbfffe818)
    at /opt/boost/include/boost/asio/impl/io_service.ipp:59
#6  0x08049a44 in main () at example.cpp:48
(gdb) frame 6
#6  0x08049a44 in main () at example.cpp:48
48        service.run();

First, the reactor service will need to be located. Downcasting will need to occur, so lets use the debugger to locate some types:

(gdb) p service.service_registry_.init_keytab
init_key
init_key<boost::asio::datagram_socket_service<boost::asio::ip::udp> >
init_key<boost::asio::detail::epoll_reactor>
init_key<boost::asio::detail::task_io_service>

Each key is associated with a specific service, and all services are maintained in a linked list within service.service_registry_. Type information is associated with them, allowing us to identify the desired service.

(gdb) set $service = service.service_registry_.first_service_
(gdb) p $service.key_.type_info_.__name
$1 = 0x8052b60
"N5boost4asio6detail14typeid_wrapperINS0_23datagram_socket_serviceINS0_2ip3udpEEEEE"

That is the boost::asio::datagram_socket_service<boost::asio::ip::udp>, so continue to the next:

(gdb) set $service = $service.next_
(gdb) p $service.key_.type_info_.__name
$2 = 0x8052cc0 "N5boost4asio6detail14typeid_wrapperINS1_13epoll_reactorEEE"

$service now points to the reactor service. Downcast the service, based on the init_key type argument:

(gdb) set $service = *('boost::asio::detail::epoll_reactor'*) $service

The outstanding handlers with work are in an linked list of operations within the reactor:

(gdb) set $ops = $service.registered_descriptors_.live_list_.op_queue_
(gdb) set $op = $ops.front_
(gdb) p *$op
$3 = {<boost::asio::detail::task_io_service_operation> = {next_ = 0x0, 
    func_ = 0x804c256
    <boost::asio::detail::reactive_socket_recvfrom_op<
    boost::asio::mutable_buffers_1, boost::asio::ip::basic_endpoint<
    boost::asio::ip::udp>, boost::_bi::bind_t<void, 
    boost::_mfi::mf2<void, udp_echo, boost::system::error_code const&, 
    unsigned int>, boost::_bi::list3<boost::_bi::value<udp_echo*>,
    boost::arg<1> (*)(), boost::arg<2> (*)()> > >::
    do_complete(boost::asio::io_service::io_service_impl*, 
    boost::asio::detail::epoll_reactor::descriptor_state::operation*, 
    boost::system::error_code const&, size_t)>, task_result_ = 0}, ec_ = {
    m_val = 11, m_cat = 0x13b2c8}, bytes_transferred_ = 0, perform_func_ =
    0x80514c8 <boost::asio::detail::reactive_socket_recvfrom_op_base<
    boost::asio::mutable_buffers_1,  
    boost::asio::ip::basic_endpoint<boost::asio::ip::udp> 
    >::do_perform(boost::asio::detail::reactor_op*)>}

Another downcast is required. Cast $op to the class for which the func_ member function pointer belongs.

(gdb) set $op = *('boost::asio::detail::reactive_socket_recvfrom_op<
boost::asio::mutable_buffers_1, boost::asio::ip::basic_endpoint<
boost::asio::ip::udp>, boost::_bi::bind_t<void, boost::_mfi::mf2<
void, udp_echo, boost::system::error_code const&, unsigned int>,
 boost::_bi::list3<boost::_bi::value<udp_echo*>, 
 boost::arg<1> (*)(), boost::arg<2> (*)()> > >'*) $op

This operation contains the desired information.

The buffer:

(gdb) p $op.buffers_ 
$4 = {<boost::asio::mutable_buffer> = {data_ = 0xbfffe77c, 
    size_ = 128}, <No data fields>}
(gdb) p &echo.buffer_
$5 = (boost::array<char, 128u> *) 0xbfffe77c

The this instance:

(gdb) p $op.handler_.l_.a1_.t_ 
$6 = (udp_echo *) 0xbfffe768
(gdb) p &echo
$7 = (udp_echo *) 0xbfffe768

The member function pointer:

(gdb) p $op.handler_.f_.f_
$8 = (void (udp_echo::*)(udp_echo *, const boost::system::error_code &, 
    unsigned int)) 0x80505b0 <
    udp_echo::handle_receive(boost::system::error_code const&, size_t)>

Socket information:

(gdb) p $op.socket_ 
$9 = 10
(gdb) p echo.socket_.implementation.socket_ 
$10 = 10

In this case, the operation only knows about the native socket representation (the file descriptor). One helpful way to determine what socket it is, is to query lsof.

$/usr/sbin/lsof -i -P | grep a.out 
a.out     4265 ghost   10u  IPv4 1166143       UDP *:4321

Thus, file descriptor 10 is listening on UDP 4321.