Related
Afte reading the asio's documentation, it's clear to me that the completion handlers are called by one of the threads that called the io_service's io.run() method. However, something that's is not clear to me is which thread the read/write async methods take place. Is it the thread that I call the methods or is it in one of the threads that called the io.run() method? Or, in last case, does the library create another thread behind the scenes and performs the operation?
The I/O operation will be attempted within the initiating async_* function. If either the operation's completion condition is satisfied or an error occurs, then the operation is complete and the completion handler will be posted into the io_service. Otherwise, the operation is not complete, and it will be enqueued into the io_service, where an application thread running the io_service's function poll(), poll_one(), run(), or run_one() performs the underlying I/O operation. In both cases, the completion handler is invoked by a thread processing the io_service.
The async_write() documentation notes that the asynchronous operation may be completed immediately:
Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using boost::asio::io_service::post().
This behavior is also noted in the Requirements on Asynchronous Operations documentation:
When an asynchronous operation is complete, the handler for the operation will be invoked as if by:
Constructing a bound completion handler bch for the handler ...
Calling ios.post(bch) to schedule the handler for deferred invocation ...
This implies that the handler must not be called directly from within the initiating function, even if the asynchronous operation completes immediately.
Here is a complete example demonstrating this behavior. In it, socket1 and socket2 are connected. Initially, socket2 has no data available. However, after invoking async_write(socket1, ...), socket2 has data even though the io_service has not been ran:
#include <boost/asio.hpp>
constexpr auto noop = [](auto&& ...){};
int main()
{
using boost::asio::ip::tcp;
boost::asio::io_service io_service;
// Create all I/O objects.
tcp::acceptor acceptor{io_service, {{}, 0}};
tcp::socket socket1{io_service};
tcp::socket socket2{io_service};
// Connect sockets.
acceptor.async_accept(socket1, noop);
socket2.async_connect(acceptor.local_endpoint(), noop);
io_service.run();
io_service.reset();
// Verify socket2 has no data.
assert(0 == socket2.available());
// Initiate an asynchronous write. However, do not run
// the `io_service`.
std::string data{"example"};
async_write(socket1, boost::asio::buffer(data), noop);
// Verify socket2 has data.
assert(0 < socket2.available());
}
For instance, you want to send some data to a remote-partner - asynchronous.
boost::asio::async_write(_socket, boost::asio::buffer(msg.data(), msg.size()),
std::bind(&Socket::WriteHandlerInternal, this->shared_from_this(), std::placeholders::_1, std::placeholders::_2));
//Where 'this' is the class Socket
Before that, you probably have created a thread which called ioService.run(). The async_write function will take the same ioService you have used to create your socket. It puts it into the queue of your ioService to execute the write operation and the handler - on the thread your ioService runs on, as the async_ already suggests.
I have a method that launches a new std::thread for new connections so that I can read data and do other things.
The method the thread invokes runs the reads in an asynchronous way(using boost functions) and it returns once it calls async_read_some, my question is:
What thread handles the call-back? Is it the same thread that made the call to the async_read_some or did that thread die after it called it and returned and now the main thread is handling the reads?
Here's a code snippet:
connection::connection_thread = std::thread(&connection::read_header,
this);
connection::connection_thread.detach();
.
.
.
void connection::read_header() {
socket_.async_read_some(boost::asio::buffer(headbuf_),
strand_.wrap(
boost::bind(&connection::on_header_read, shared_from_this(),
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred)));
begin_timeout();
}
What thread handles the call-back?
The thread (or one of the threads, if there are more than one) which polls or runs the associated io_service. The handler is passed to the service to be called on completion.
Is is the same thread that made the call to the async_read_some
No, the async functions never call the handler directly; it is always called by the io_service, even if the operation completes immediately.
or did that thread die after it called it and returned and now the main thread is handling the reads?
That entirely depends on how you're managing the threads. The thread that calls async may die, if you don't need it any more; you'll need some other thread or threads (possibly the main thread, possibly others) to process the io_service and complete the asynchronous operation.
However, there's no point launching a thread to start an asynchronous operation, since that will complete immediately. Either move the call to async_read_some to where you're currently launching the thread; or use the thread to perform a synchronous operation. If you opt for a multithreaded synchronous design, then you won't need a thread to poll the io_service for asynchronous operations.
Being a total beginner to Boost.Asio, I am confused with io_service::run(). I would appreciate it if someone could explain to me when this method blocks/unblocks. The documentations states:
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.
A normal exit from the run() function implies that the io_service object is stopped (the stopped() function returns true). Subsequent calls to run(), run_one(), poll() or poll_one() will return immediately unless there is a prior call to reset().
What does the following statement mean?
[...] no more handlers to be dispatched [...]
While trying to understand the behavior of io_service::run(), I came across this example (example 3a). Within it, I observe that io_service->run() blocks and waits for work orders.
// WorkerThread invines io_service->run()
void WorkerThread(boost::shared_ptr<boost::asio::io_service> io_service);
void CalculateFib(size_t);
boost::shared_ptr<boost::asio::io_service> io_service(
new boost::asio::io_service);
boost::shared_ptr<boost::asio::io_service::work> work(
new boost::asio::io_service::work(*io_service));
// ...
boost::thread_group worker_threads;
for(int x = 0; x < 2; ++x)
{
worker_threads.create_thread(boost::bind(&WorkerThread, io_service));
}
io_service->post( boost::bind(CalculateFib, 3));
io_service->post( boost::bind(CalculateFib, 4));
io_service->post( boost::bind(CalculateFib, 5));
work.reset();
worker_threads.join_all();
However, in the following code that I was working on, the client connects using TCP/IP and the run method blocks until data is asynchronously received.
typedef boost::asio::ip::tcp tcp;
boost::shared_ptr<boost::asio::io_service> io_service(
new boost::asio::io_service);
boost::shared_ptr<tcp::socket> socket(new tcp::socket(*io_service));
// Connect to 127.0.0.1:9100.
tcp::resolver resolver(*io_service);
tcp::resolver::query query("127.0.0.1",
boost::lexical_cast< std::string >(9100));
tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);
socket->connect(endpoint_iterator->endpoint());
// Just blocks here until a message is received.
socket->async_receive(boost::asio::buffer(buf_client, 3000), 0,
ClientReceiveEvent);
io_service->run();
// Write response.
boost::system::error_code ignored_error;
std::cout << "Sending message \n";
boost::asio::write(*socket, boost::asio::buffer("some data"), ignored_error);
Any explanation of run() that describes its behavior in the two examples below would be appreciated.
Foundation
Lets start with a simplified example and examine the relevant Boost.Asio pieces:
void handle_async_receive(...) { ... }
void print() { ... }
...
boost::asio::io_service io_service;
boost::asio::ip::tcp::socket socket(io_service);
...
io_service.post(&print); // 1
socket.connect(endpoint); // 2
socket.async_receive(buffer, &handle_async_receive); // 3
io_service.post(&print); // 4
io_service.run(); // 5
What Is A Handler?
A handler is nothing more than a callback. In the example code, there are 3 handlers:
The print handler (1).
The handle_async_receive handler (3).
The print handler (4).
Even though the same print() function is used twice, each use is considered to create its own uniquely identifiable handler. Handlers can come in many shapes and sizes, ranging from basic functions like the ones above to more complex constructs such as functors generated from boost::bind() and lambdas. Regardless of the complexity, the handler still remains nothing more than a callback.
What Is Work?
Work is some processing that Boost.Asio has been requested to do on behalf of the application code. Sometimes Boost.Asio may start some of the work as soon as it has been told about it, and other times it may wait to do the work at a later point in time. Once it has finished the work, Boost.Asio will inform the application by invoking the supplied handler.
Boost.Asio guarantees that handlers will only run within a thread that is currently calling run(), run_one(), poll(), or poll_one(). These are the threads that will do work and call handlers. Therefore, in above example, print() is not invoked when it is posted into the io_service (1). Instead, it is added to the io_service and will be invoked at a later point in time. In this case, it within io_service.run() (5).
What Are Asynchronous Operations?
An asynchronous operation creates work and Boost.Asio will invoke a handler to inform the application when the work has completed. Asynchronous operations are created by calling a function that has a name with the prefix async_. These functions are also known as initiating functions.
Asynchronous operations can be decomposed into three unique steps:
Initiating, or informing, the associated io_service that works needs to be done. The async_receive operation (3) informs the io_service that it will need to asynchronously read data from the socket, then async_receive returns immediately.
Doing the actual work. In this case, when socket receives data, bytes will be read and copied into buffer. The actual work will be done in either:
The initiating function (3), if Boost.Asio can determine that it will not block.
When the application explicitly run the io_service (5).
Invoking the handle_async_receive ReadHandler. Once again, handlers are only invoked within threads running the io_service. Thus, regardless of when the work is done (3 or 5), it is guaranteed that handle_async_receive() will only be invoked within io_service.run() (5).
The separation in time and space between these three steps is known as control flow inversion. It is one of the complexities that makes asynchronous programming difficult. However, there are techniques that can help mitigate this, such as by using coroutines.
What Does io_service.run() Do?
When a thread calls io_service.run(), work and handlers will be invoked from within this thread. In the above example, io_service.run() (5) will block until either:
It has invoked and returned from both print handlers, the receive operation completes with success or failure, and its handle_async_receive handler has been invoked and returned.
The io_service is explicitly stopped via io_service::stop().
An exception is thrown from within a handler.
One potential psuedo-ish flow could be described as the following:
create io_service
create socket
add print handler to io_service (1)
wait for socket to connect (2)
add an asynchronous read work request to the io_service (3)
add print handler to io_service (4)
run the io_service (5)
is there work or handlers?
yes, there is 1 work and 2 handlers
does socket have data? no, do nothing
run print handler (1)
is there work or handlers?
yes, there is 1 work and 1 handler
does socket have data? no, do nothing
run print handler (4)
is there work or handlers?
yes, there is 1 work
does socket have data? no, continue waiting
-- socket receives data --
socket has data, read it into buffer
add handle_async_receive handler to io_service
is there work or handlers?
yes, there is 1 handler
run handle_async_receive handler (3)
is there work or handlers?
no, set io_service as stopped and return
Notice how when the read finished, it added another handler to the io_service. This subtle detail is an important feature of asynchronous programming. It allows for handlers to be chained together. For instance, if handle_async_receive did not get all the data it expected, then its implementation could post another asynchronous read operation, resulting in io_service having more work, and thus not returning from io_service.run().
Do note that when the io_service has ran out of work, the application must reset() the io_service before running it again.
Example Question and Example 3a code
Now, lets examine the two pieces of code referenced in the question.
Question Code
socket->async_receive adds work to the io_service. Thus, io_service->run() will block until the read operation completes with success or error, and ClientReceiveEvent has either finished running or throws an exception.
Example 3a Code
In hopes of making it easier to understand, here is a smaller annotated Example 3a:
void CalculateFib(std::size_t n);
int main()
{
boost::asio::io_service io_service;
boost::optional<boost::asio::io_service::work> work = // '. 1
boost::in_place(boost::ref(io_service)); // .'
boost::thread_group worker_threads; // -.
for(int x = 0; x < 2; ++x) // :
{ // '.
worker_threads.create_thread( // :- 2
boost::bind(&boost::asio::io_service::run, &io_service) // .'
); // :
} // -'
io_service.post(boost::bind(CalculateFib, 3)); // '.
io_service.post(boost::bind(CalculateFib, 4)); // :- 3
io_service.post(boost::bind(CalculateFib, 5)); // .'
work = boost::none; // 4
worker_threads.join_all(); // 5
}
At a high-level, the program will create 2 threads that will process the io_service's event loop (2). This results in a simple thread pool that will calculate Fibonacci numbers (3).
The one major difference between the Question Code and this code is that this code invokes io_service::run() (2) before actual work and handlers are added to the io_service (3). To prevent the io_service::run() from returning immediately, an io_service::work object is created (1). This object prevents the io_service from running out of work; therefore, io_service::run() will not return as a result of no work.
The overall flow is as follows:
Create and add the io_service::work object added to the io_service.
Thread pool created that invokes io_service::run(). These worker threads will not return from io_service because of the io_service::work object.
Add 3 handlers that calculate Fibonacci numbers to the io_service, and return immediately. The worker threads, not the main thread, may start running these handlers immediately.
Delete the io_service::work object.
Wait for worker threads to finish running. This will only occur once all 3 handlers have finished execution, as the io_service neither has handlers nor work.
The code could be written differently, in the same manner as the Original Code, where handlers are added to the io_service, and then the io_service event loop is processed. This removes the need to use io_service::work, and results in the following code:
int main()
{
boost::asio::io_service io_service;
io_service.post(boost::bind(CalculateFib, 3)); // '.
io_service.post(boost::bind(CalculateFib, 4)); // :- 3
io_service.post(boost::bind(CalculateFib, 5)); // .'
boost::thread_group worker_threads; // -.
for(int x = 0; x < 2; ++x) // :
{ // '.
worker_threads.create_thread( // :- 2
boost::bind(&boost::asio::io_service::run, &io_service) // .'
); // :
} // -'
worker_threads.join_all(); // 5
}
Synchronous vs. Asynchronous
Although the code in the question is using an asynchronous operation, it is effectively functioning synchronously, as it is waiting for the asynchronous operation to complete:
socket.async_receive(buffer, handler)
io_service.run();
is equivalent to:
boost::asio::error_code error;
std::size_t bytes_transferred = socket.receive(buffer, 0, error);
handler(error, bytes_transferred);
As a general rule of thumb, try to avoid mixing synchronous and asynchronous operations. Often times, it can turn a complex system into a complicated system. This answer highlights advantages of asynchronous programming, some of which are also covered in the Boost.Asio documentation.
To simplify how what run does, think of it as an employee that must process a pile of paper; it takes one sheet, does what the sheet tells, throws the sheet away and takes the next one; when he runs out of sheets, it leaves the office. On each sheet there can be any kind of instruction, even adding a new sheet to the pile.
Back to asio: you can give to an io_service work in two ways, essentially: by using post on it as in the sample you linked, or by using other objects that internally call post on the io_service, like the socket and its async_* methods.
This is my scenario. Several async writes are requested on a socket from a single thread. The same thread calls io_service::run. So I assume all completion handlers are executed in the same order as the corresponding async writes were initiated. Is it OK to shutdown and close the socket in the last completion handler?
Edit: I am using TCP protocol.
You will want to invoke async_write() from within the completion handler of the first async_write(). Pseudo code is below
boost::asio::io_service ios;
boost::asio::ip::tcp::socket socket( ios );
void
handler2()
{
socket.shutdown( boost::asio::ip::tcp::socket::shutdown_both );
socket.close();
}
void
handler1()
{
async_write( socket, boost::bind(&handler2) );
}
int
main()
{
async_write( socket, boost::bind(&handler1) );
}
Note that the documentation explicitly states a single outstanding operation can be in-flight:
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.
Which is why invoking multiple async_write() operations on a single socket is a bad idea. Alternatively, use a scatter operation to send both buffers in one async_write().
I assume you have a TCP socket (because UDP protocol is connectionless).
If you do something like this:
start_async_write(first)
start_async_write(second) -> close socket on complete
io_service.run()
everything will be OK, your writes will be serialized, run() method will return after close method of the socket will be called by the completion handler.
( 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.
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.
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.
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.
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.
Consider first that the socket is a stream and is not internally guarded against concurrent read and/or write. There are three distinct considerations.
Concurrent execution of functions that access the same socket.
Concurrent execution of delegates that enclose the same socket.
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?
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.
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).
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.
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.
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 ;-)
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.
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.
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.
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.