boost::asio::socket thread safety

Question

( This is a simplified version of my original question )

I have several threads that write to a boost asio socket. This seems to work very well, with no problems.

The documentation says a shared socket is not thread safe( here, way down at the bottom ) so I am wondering if I should protect the socket with mutex, or something.

This question insists that protection is necessary, but gives no advice on how to do so.

All the answers to my original question also insisted that what I was doing dangerous, and most urged me to replace my writes with async_writes or even more complicated things. However, I am reluctant to do this, since it would complicate code that is already working and none of the answerers convinced me they knew what they ware talking about - they seemed to have read the same documentation as I and were guessing, just as I was.

So, I wrote a simple program to stress test writing to a shared socket from two threads.

Here is the server, which simply writes out whatever it receives from the client

int main()
{
    boost::asio::io_service io_service;

    tcp::acceptor acceptor(io_service, tcp::endpoint(tcp::v4(), 3001));

    tcp::socket socket(io_service);
    acceptor.accept(socket);

    for (;;)
    {
        char mybuffer[1256];
        int len = socket.read_some(boost::asio::buffer(mybuffer,1256));
        mybuffer[len] = '\0';
        std::cout << mybuffer;
        std::cout.flush();

    }

  return 0;
}

Here is the client, which creates two threads that write to a shared socket as fast as they can

boost::asio::ip::tcp::socket * psocket;

void speaker1()
{
    string msg("speaker1: hello, server, how are you running?\n");
    for( int k = 0; k < 1000; k++ ) {
        boost::asio::write(
            *psocket,boost::asio::buffer(msg,msg.length()));
    }

}
void speaker2()
{
    string msg("speaker2: hello, server, how are you running?\n");
    for( int k = 0; k < 1000; k++ ) {
        boost::asio::write(
            *psocket,boost::asio::buffer(msg,msg.length()));
    }

}

int main(int argc, char* argv[])
{

    boost::asio::io_service io_service;

  // connect to server

    tcp::resolver resolver(io_service);
    tcp::resolver::query query("localhost", "3001");
    tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);
    tcp::resolver::iterator end;
    psocket = new tcp::socket(io_service);
    boost::system::error_code error = boost::asio::error::host_not_found;
    while (error && endpoint_iterator != end)
    {
        psocket->close();
        psocket->connect(*endpoint_iterator++, error);
    }


    boost::thread t1( speaker1 );
    boost::thread t2( speaker2 );

    Sleep(50000);

}

This works! Perfectly, as far as I can tell. The client does not crash. The messages arrive at the server without garbles. They usually arrive alternately, one from each thread. Sometimes one thread get two or three messages in before the other, but I do not think this is a problem so long as there are no garbles and all the messages arrive.

My conclusion: the socket may not be thread safe in some theoretical sense, but it is so hard to make it fail that I am not going to worry about it.

Secondhand

There is little to no value in using io_service::post() with a handler that invokes write(). You are overcomplicating this, use async_write(). – Parlour

Your implementation does not make sense from ASIO perspective. There is no need to asio in that code style. – Leucopoiesis

Why do you think you need multiple threads? – Babu

Answer 1

After restudying the code for async_write I am now convinced that any write operation is thread safe if and only if the packet size is smaller than

default_max_transfer_size = 65536;

What happens is that as soon as an async_write is called an async_write_some is called in the same thread. Any threads in the pool calling some form of io_service::run will keep on calling async_write_some for that write operation until it completes.

These async_write_some calls can and will interleave if it has to be called more than once (the packets are larger than 65536).

ASIO does not queue writes to a socket as you would expect, one finishing after the other. In order to ensure both thread and interleave safe writes consider the following piece of code:

    void my_connection::async_serialized_write(
            boost::shared_ptr<transmission> outpacket) {
        m_tx_mutex.lock();
        bool in_progress = !m_pending_transmissions.empty();
        m_pending_transmissions.push(outpacket);
        if (!in_progress) {
            if (m_pending_transmissions.front()->scatter_buffers.size() > 0) {
                boost::asio::async_write(m_socket,
                    m_pending_transmissions.front()->scatter_buffers,
                        boost::asio::transfer_all(),
            boost::bind(&my_connection::handle_async_serialized_write,
                        shared_from_this(),
                        boost::asio::placeholders::error,
                                       boost::asio::placeholders::bytes_transferred));
            } else { // Send single buffer
                boost::asio::async_write(m_socket,
                                    boost::asio::buffer(
                                           m_pending_transmissions.front()->buffer_references.front(),                          m_pending_transmissions.front()->num_bytes_left),
                boost::asio::transfer_all(),
                boost::bind(
                        &my_connection::handle_async_serialized_write,
                        shared_from_this(),
                        boost::asio::placeholders::error,
                        boost::asio::placeholders::bytes_transferred));
            }
        }
        m_tx_mutex.unlock();
    }

    void my_connection::handle_async_serialized_write(
    const boost::system::error_code& e, size_t bytes_transferred) {
        if (!e) {
            boost::shared_ptr<transmission> transmission;
            m_tx_mutex.lock();
            transmission = m_pending_transmissions.front();
            m_pending_transmissions.pop();
            if (!m_pending_transmissions.empty()) {
                if (m_pending_transmissions.front()->scatter_buffers.size() > 0) {
            boost::asio::async_write(m_socket,
                    m_pending_transmissions.front()->scatter_buffers,
                    boost::asio::transfer_exactly(
                            m_pending_transmissions.front()->num_bytes_left),
                    boost::bind(
                            &chreosis_connection::handle_async_serialized_write,
                            shared_from_this(),
                            boost::asio::placeholders::error,
                            boost::asio::placeholders::bytes_transferred));
                } else { // Send single buffer
                    boost::asio::async_write(m_socket,
                    boost::asio::buffer(
                            m_pending_transmissions.front()->buffer_references.front(),
                            m_pending_transmissions.front()->num_bytes_left),
                    boost::asio::transfer_all(),
                    boost::bind(
                            &my_connection::handle_async_serialized_write,
                            shared_from_this(),
                            boost::asio::placeholders::error,
                            boost::asio::placeholders::bytes_transferred));
                }
            }
            m_tx_mutex.unlock();
            transmission->handler(e, bytes_transferred, transmission);
        } else {
            MYLOG_ERROR(
            m_connection_oid.toString() << " " << "handle_async_serialized_write: " << e.message());
            stop(connection_stop_reasons::stop_async_handler_error);
        }
    }

This basically makes a queue for sending one packet at a time. async_write is called only after the first write succeeds which then calls the original handler for the first write.

It would have been easier if asio made write queues automatic per socket/stream.

Phagy

Interesting! Since I never would think of sending such large packets, seems like I will be OK. – Secondhand

This may also happen when the socket's internal buffer is full(er) – Phagy

I always was doubtful that strands where relevant to this. – Secondhand

The observation on interleaving is important. And it is also important for people to recognize that a strand (non-concurrent invocation) can do nothing to guard against concurrent access to the socket by asynchronous methods (async_read and async_write). The asio design calls for reads and writes to be invoked in the handlers of reads and writes. This results in (1) non-concurrent invocation of the asynchronous methods, (2) non-concurrent execution of the asynchronous code they launch, and (3) avoidance of interleaving. Unfortunately this does not speak to the needs of true concurrency. – Eonism

If I don't call async_write ( some_huge_data_packet ) directly from worker thread, instead I 'post' the write operation to my single io_service::run thread, will I get guarantee that my huge data packet will not be interleaved? – Yuen

@Yuen not in my opinion and experience. the asio interface has changed a lot in recent years. perhaps this should be re-evaluated – Phagy

2024 update. I've confirmed even in the latest asio this is still an issue. If anyone is interested in my test code I'll gladly share it – Phagy

gist.github.com/Climax777/ecea9f6021647b92ead1eadc438125fe – Phagy

Answer 2

Use a boost::asio::io_service::strand for asynchronous handlers that are not thread safe.

A strand is defined as a strictly sequential invocation of event handlers (i.e. no concurrent invocation). Use of strands allows execution of code in a multithreaded program without the need for explicit locking (e.g. using mutexes).

The timer tutorial is probably the easiest way to wrap your head around strands.

Parlour

A strand looks like a very interesting and useful beastie. However, at first glance it does not seem helpful in my particular case. The writes to the socket are not controlled by the same io_service, but occur as the result of a completely independant process. ( +1 vote for usefulness, on other occasions. ( – Secondhand

@Secondhand a single socket is serviced by a single I/O service, it takes a reference in the constructor. If you use multiple io_service objects, that is an entirely different problem. – Parlour

I do not understand your previous comment. I have edited my question to clarify, I hope, what I need to do. – Secondhand

@Secondhand you are mixing asynchronous methods with synchronous methods. Don't do that. A strand is only useful in an asynchronous context. – Parlour

This does not hold if the methods invoked by the strand are not synchronous. Only the invocation will be synchronous, which will not prevent concurrent execution of the code initiated by the asynchronous method(s). – Eonism

@Eonism Actually, it will do that. There's a bunch of boost magic that makes that happen. (It's important to understand exactly how it works before you rely on it and there's no room to explain it here. But you can punch "asio_handler_invoke" into your favorite search engine or read this.) – Glean

@DavidSchwartz There is no amount of boost magic that is going to cause a thread created from a handler invoked on a strand to execute on the strand. – Eonism

@Eonism That's exactly what asio_handler_invoke does and it's built into the strand class. Essentially, the "handler" also includes an "invoker" that's used to invoke both the handler and any intermediary composed operations prior to it. The "invoker" dispatches on the strand. This is how strands work with async operations. – Glean

@DavidSchwartz You are making assumptions about how threads are created. – Eonism

@Eonism Actually, you are. I don't have to make assumptions because I know how the code works. The asynchronous methods invoke their handlers (and even the code that invokes the handlers if the operation is composed) using asio_handler_invoke which dispatches to the strand. This is how strands and asynchronous operations are designed to interact. Without this, it would be impossible to use async I/O with SSL if more than one thread ran the io_service (because the SSL code that handles the intermediate composed operations is not thread safe and requires an implicit or explicit strand). – Glean

Do you believe that any std::thread that is created inside of an asio handler posted to a strand is going to be somehow captured by the strand? That would be a neat trick. – Eonism

@DavidSchwartz Sorry, forgot to tag you on the previous comment. I'd also be interested in whether you believe posting async write operations from a strand handler prevents interleaving of multiple composed writes when the payload size exceeds 64k. If this is true it would save me a lot of trouble, but it is not what I have experienced. – Eonism

@Eonism You can't post more than one async operation in the same direction on the same socket. I don't know what you mean by creating a thread inside of an asio handler -- if you're using a strand, why are you creating a thread? (Are you talking about invocation of a handler or code called manually by non-boost code?) – Glean

@DavidSchwartz This was the assumption you were making - that one was not creating a thread. That was what I was warning about. Why one would do it was not my concern. They may call an API that they didn't know was doing it (hence the warning). Also, as far as I can tell, a strand cannot prevent interleaving of sends via async_write. This is not the same as concurrent writing to the socket. Async writes have multiple parts (if over 64kb), posted internally. This allows another such write (from the strand) to do the same while the first writer's posts are still being posted. – Eonism

Answer 3

It sounds like this question boils down to:

what happens when async_write_some() is called simultaneously on a single socket from two different threads

I believe this is exactly the operation that's not thread safe. The order those buffers will go out on the wire is undefined, and they may even be interleaved. Especially if you use the convenience function async_write(), since it's implemented as a series of calls to async_write_some() underneath, until the whole buffer has been sent. In this case each fragment that's sent from the two threads may be interleaved randomly.

The only way to protect you from hitting this case is to build your program to avoid situations like this.

One way to do that is by writing an application layer send buffer which a single thread is responsible for pushing onto the socket. That way you could protect the send buffer itself only. Keep in mind though that a simple std::vector won't work, since adding bytes to the end may end up re-allocating it, possibly while there is an outstanding async_write_some() referencing it. Instead, it's probably a good idea to use a linked list of buffers, and make use of the scatter/gather feature of asio.

Octonary

I am not using aysnc_write_some nor async_write. I am using write ( boost.org/doc/libs/1_47_0/doc/html/boost_asio/reference/… ) – Secondhand

Answer 4

The key to understanding ASIO is to realize that completion handlers only run in the context of a thread that has called io_service.run() no matter which thread called the asynchronous method. If you've only called io_service.run() in one thread then all completion handlers will execute serially in the context of that thread. If you've called io_service.run() in more than one thread then completion handlers will execute in the context of one of those threads. You can think of this as a thread pool where the threads in the pool are those threads that have called io_service.run() on the same io_service object.

If you have multiple threads call io_service.run() then you can force completion handlers to be serialized by putting them in a strand.

To answer the last part of your question, you should call boost::async_write(). This will dispatch the write operation onto a thread that has called io_service.run() and will invoked the completion handler when the write is done. If you need to serialize this operation then it's a little more complicated and you should read the documentation on strands here.

Whalebone

A big part of my difficulty is that I do not know what is and is not a 'completion handler'. Is boost::asio::write() a completion handler? Will it 'magically' queue the write operation into the ( different ) thread which called the run() method of the io_service which was passed to the socket constructor? If the answer to both is yes, then my original code is correct and I can relax! – Secondhand

@ravenspoint: boost::asio::write() is a synchronous call so it gets executed as soon as you call it in the context of the thread where the call takes place. boost::asio::async_write() takes a function object as one of it's arguments. When you call boost::asio::async_write() the write operation is 'magically' queued onto the thread that called run(). – Whalebone

@ravenspoint: If you're running on linux it works something like this: When you call io_service::run() the underlying code calls epoll() which blocks the current thread until there's activity on one of it's file descriptors. When you call async_write() it adds the socket file descriptor to the epoll set and associates the completion handler with that file descriptor. When epoll returns the underlying code writes the data and invokes the handler. This all happens in the thread that called run(). – Whalebone

I am prepared to accept that async_write() works as you describe. The snag is that now I have to prevent the writing thread from executing another async_write() call before the first is complete. Sigh. Perhaps posting the functor is easier? At least I have already written and tested the code. – Secondhand

Still struggling to understand this. You say that for ':asio::async_write() the write operation is 'magically' queued onto the thread that called run().' Fine. Why does not the same thing happen with asio:write()? The only difference should be that one blocks and one doesn't. – Secondhand

@ravenspoint: The difference isn't that one blocks and the other doesn't. The difference is that one is synchronous and the other one is asynchronous. In order to execute an asynchronous operation, the operation has to be queued. You can execute a synchronous operation without queuing it, which is what ASIO does. – Whalebone

async_write() does not dispatch the write operation unless the write operation exceeds a certain size (64kb, hard coded in default_max_transfer_size). The write operation is invoked on the calling thread and the completion handler is invoked by posting to the io_service. If the write is large it is broken up and subsequent write operations are invoked within generated completion handlers. These also execute on threads of the io_service. Since you do not control these generated completion handlers they cannot be strand-ed. This makes it hard to prevent write interleaving. – Eonism

Answer 5

Consider first that the socket is a stream and is not internally guarded against concurrent read and/or write. There are three distinct considerations.

1. Concurrent execution of functions that access the same socket.
2. Concurrent execution of delegates that enclose the same socket.
3. Interleaved execution of delegates that write to the same socket.

The chat example is asynchronous but not concurrent. The io_service is run from a single thread, making all chat client operations non-concurrent. In other words, it avoids all of these problems. Even the async_write must internally complete sending all parts of a message before any other work can proceed, avoiding the interleaving problem.

Handlers are invoked only by a thread that is currently calling any overload of run(), run_one(), poll() or poll_one() for the io_service.

By posting work to the single thread io_service other threads can safely avoid both concurrency and blocking by queuing up work in the io_service. If however your scenario precludes you from buffering all work for a given socket, things get more complicated. You may need to block the socket communication (but not threads) as opposed to queuing up work indefinately. Also, the work queue can be very difficult to manage as it's entirely opaque.

If your io_service runs more than one thread you can still easily avoid the above problems, but you can only invoke reads or writes from the handlers of other reads or writes (and at startup). This sequences all access to the socket while remaining non-blocking. The safety arises from the fact that the pattern is using only one thread at any given time. But posting work from an independent thread is problematic - even if you don't mind buffering it.

A strand is an asio class that posts work to an io_service in a way that ensures non-concurrent invocation. However using a strand to invoke async_read and/or async_write solves only the first of the three problems. These functions internally post work to the io_service of the socket. If that service is running multiple threads the work can be exectuted concurrently.

So how do you, for a given socket, safely invoke async_read and/or async_write concurrently?

1. With concurrent callers the first problem can be resolved with a mutex or a strand, using the former if you don't want to buffer the work and the latter if you do. This protects the socket during the function invocations but does nothing for the other problems.

2. The second problem seems hardest, because it's difficult to see what's going on inside of the code executing asynchronously from the two functions. The async functions both post work to the io_service of the socket.

From the boost socket source:

/**
 * This constructor creates a stream socket without opening it. The socket
 * needs to be opened and then connected or accepted before data can be sent
 * or received on it.
 *
 * @param io_service The io_service object that the stream socket will use to
 * dispatch handlers for any asynchronous operations performed on the socket.
 */
explicit basic_stream_socket(boost::asio::io_service& io_service)
: basic_socket<Protocol, StreamSocketService>(io_service)
{
}

And from the io_service::run()

/**
 * The run() function blocks until all work has finished and there are no
 * more handlers to be dispatched, or until the io_service has been stopped.
 *
 * Multiple threads may call the run() function to set up a pool of threads
 * from which the io_service may execute handlers. All threads that are
 * waiting in the pool are equivalent and the io_service may choose any one
 * of them to invoke a handler.
 *
 * ...
 */
BOOST_ASIO_DECL std::size_t run();

So if you give a socket multiple threads, it has no choice but to utilize multiple threads - despite not being thread safe. The only way to avoid this problem (apart from replacing the socket implementation) is to give the socket only one thread to work with. For a single socket this is what you want anyway (so don't bother running off to write a replacement).

3. The third problem can be resolved by using a (different) mutex that is locked before the async_write, passed into the completion handler and unlocked at that point. This will prevent any caller from beginning a write until all parts of the preceding write are complete.

Note that the async_write posts work to a queue - that's how it is able to return almost immediately. If you throw too much work at it you may have to deal with some consequences. Despite using a single io_service thread for the socket, you may have any number of threads posting work via concurrent or non-concurrent calls to async_write.

On the other hand, async_read is straightforward. There is no interleaving problem and you simply loop back from the handler of the previous call. You may or may not want to dispatch the resulting work to another thread or queue, but if you perform it on the completion handler thread you are simply blocking all reads and writes on your single-threaded socket.

UPDATE

I did some more digging into the implementation of the underlying implementation of the socket stream (for one platform). It appears to be the case that the socket consistently executes platform socket calls on the invoking thread, not the delegate posted to the io_service. In other words, despite the fact that async_read and async_write appear to return immediately, they do in fact execute all socket operations before returning. Only the handlers are posted to the io_service. This is neither documented nor exposed by the exaple code I've reviewed, but assuming it is guaranteed behavior, it significantly impacts the second problem above.

Assuming that the work posted to the io_service does not incorporate socket operations, there is no need to limit the io_service to a single thread. It does however reinforce the importance of guarding against concurrent execution of the async functions. So, for example, if one follows the chat example but instead adds another thread to the io_service, there becomes a problem. With async function invocations executing within function handlers, you have concurrent function execution. This would require either a mutex, or all async function invocations to be reposted for execution on a strand.

UPDATE 2

With respect to the third problem (interleaving), if the data size exceeds 65536 bytes, the work is broken up internal to async_write and sent in parts. But it is critical to understand that, if there is more than one thread in the io_service, chunks of work other than the first will be posted to different threads. This all happens internal in the async_write function before your completion handler is called. The implementation creates its own intermediate completion handlers and uses them to execute all but the first socket operation.

This means any guard around the async_write call (mutex or strand) will not protect the socket if there are multiple io_service threads and more than 64kb of data to post (by default, this may possibly vary). Therefore, in this case, the interleave guard is necessary not only for interleave safety, but also thread safety of the socket. I verified all of this in a debugger.

THE MUTEX OPTION

The async_read and async_write functions internally use the io_service in order to obtain threads on which to post completion handlers, blocking until threads are available. This makes them hazardous to guard with mutex locks. When a mutex is used to guard these functions a deadlock will occur when threads back up against the lock, starving the io_service. Given that there is no other way to guard async_write when sending > 64k with a multithread io_service, it effectively locks us into a single thread in that scenario - which of course resolves the concurrency question.

Eonism

Answer 6

According to Nov. 2008 boost 1.37 asio updates, certain synchronous operations including writes "are now thread safe" allowing "concurrent synchronous operations on an individual socket, if supported by the OS" boost 1.37.0 history. This would seem to support what you are seeing but the oversimplification "Shared objects: Unsafe" clause remains in the boost docs for ip::tcp::socket.

Aliphatic

Thanks for checking. It's a pity that no-one has come by to give an definite answer - just other readers of the docs. BTW your XML editor, foxe, rocks! – Secondhand

Answer 7

Another comment on an old post...

I think the key sentence in the asio documentation for asio::async_write() overloads is the following:

This operation is implemented in terms of zero or more calls to the stream's async_write_some function, and is known as a composed operation. The program must ensure that the stream performs no other write operations (such as async_write, the stream's async_write_some function, or any other composed operations that perform writes) until this operation completes.

As I understand it, this documents what was assumed in many of the above answers: Data from calls to asio::async_write may be interleaved if multiple threads execute io_context.run().

Maybe this helps someone ;-)

Sialkot

Answer 8

It depends if you access same socket object from several threads. Let's say you have two threads running same io_service::run() function.

If for example you do reading and writing simultaneously or may be perform cancel operation from other thread. Then it is not safe.

However if your protocol does only one operation in a time.

1. If only one thread runs the io_service run then there is no problem. If you want to execute something on the socket from other thread you may call io_service::post() with handler that does this operation on socket so it would be executed in the same thread.
2. If you have several threads executing io_service::run and you try to do operations simultaneously - let's say cancel and read operation then you should use strands. There is a tutorial for this in Boost.Asio documentation.

Adali

The socket is accessed from several threads. Not all the threads use the io_service. – Secondhand

Answer 9

I have been running extensive tests and haven't been able to break asio. Even without locking any mutex.

I would nevertheless advise that you use async_read and async_write with a mutex around each of those calls.

I believe the only draw back is that your completion handlers could be called concurrently if you have more than one thread calling io_service::run.

In my case this has not been an issue. Here is my test code:

#include <boost/thread.hpp>
#include <boost/date_time.hpp>
#include <boost/asio.hpp>
#include <vector>

using namespace std;
char databuffer[256];
vector<boost::asio::const_buffer> scatter_buffer;
boost::mutex my_test_mutex;
void my_test_func(boost::asio::ip::tcp::socket* socket, boost::asio::io_service *io) {
while(1) {
    boost::this_thread::sleep(boost::posix_time::microsec(rand()%1000));

    //my_test_mutex.lock(); // It would be safer 
    socket->async_send(scatter_buffer, boost::bind(&mycallback));
    //my_test_mutex.unlock(); // It would be safer
}
}
int main(int argc, char **argv) {

for(int i = 0; i < 256; ++i)
    databuffer[i] = i;

for(int i = 0; i < 4*90; ++i)
    scatter_buffer.push_back(boost::asio::buffer(databuffer));
boost::asio::io_service my_test_ioservice;
boost::asio::ip::tcp::socket my_test_socket(my_test_ioservice);
boost::asio::ip::tcp::resolver my_test_tcp_resolver(my_test_ioservice);
boost::asio::ip::tcp::resolver::query  my_test_tcp_query("192.168.1.10", "40000");
boost::asio::ip::tcp::resolver::iterator my_test_tcp_iterator = my_test_tcp_resolver.resolve(my_test_tcp_query);
boost::asio::connect(my_test_socket, my_test_tcp_iterator);
for (size_t i = 0; i < 8; ++i) {
    boost::shared_ptr<boost::thread> thread(
            new boost::thread(my_test_func, &my_test_socket, &my_test_ioservice));
}

while(1) {
    my_test_ioservice.run_one();
    boost::this_thread::sleep(boost::posix_time::microsec(rand()%1000));
}
return 0;

}

And here is my makeshift server in python:

import socket
def main():
    mysocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    mysocket.bind((socket.gethostname(), 40000))
    mysocket.listen(1)

    while 1:
        (clientsocket, address) = mysocket.accept()
        print("Connection from: " + str(address))
        i = 0
        count = 0
        while i == ord(clientsocket.recv(1)):
            i += 1
            i %= 256

            count+=1
            if count % 1000 == 0:
                print(count/1000)
        print("Error!")
return 0

if __name__ == '__main__':
    main()

Please note that running this code can cause your computer to thrash.

Phagy

This code does not really test it correctly. It should send packets far larger than 65536 in order to illustrate that asio is not inherently thread safe for interleaving async_write calls. – Phagy

A mutex does not guard code that is invoked asynchronously from the critical section. – Eonism

But I agree that using a mutex around otherwise concurrent executions of async_read and async_write is a good idea, because the socket is not thread safe and it would otherwise be concurrently accessed. – Eonism