Asynchronous request using wininet

Question

I have already used wininet to send some synchronous HTTP requests. Now, I want to go one step further and want to request some content asynchronously.

The goal is to get something "reverse proxy"-like. I send an HTTP request which gets answered delayed - as soon as someone wants to contact me. My thread should continue as if there was nothing in the meanwhile, and a callback should be called in this thread as soon as the response arrives. Note that I don't want a second thread which handles the reply (if it is necessary, it should only provide some mechanism which interrupts the main thread to invoke the callback there)!

Update: Maybe, the best way to describe what I want is a behaviour like in JavaScript where you have only one thread but can send AJAX requests which then result in a callback being invoked in this main thread.

Since I want to understand how it works, I don't want library solutions. Does anybody know some good tutorial which explains me how to achieve my wanted behavior?

Answer 1

My thread should continue as if there was nothing in the meanwhile, and a callback should be called in this thread as soon as the response arrives.

What you're asking for here is basically COME FROM (as opposed to GO TO). This is a mythical instruction which doesn't really exist. The only way you can get your code called is to either poll in the issuing thread, or to have a separate thread which is performing the synchronous IO and then executing the callback (in that thread, or in yet another spawned thread) with the results.

When I was working in C++ with sockets I set up a dedicated thread to iterate over all the open sockets, poll for data which would be available without blocking, take the data and stuff it in a buffer, sending the buffer to a callback on a given circumstance (EOL, EOF, that sort of thing).

Answer 2

Unless your main thread is listening to something like a message queue there isn't really a way to just hijack it and start it executing code other than what it is currently doing.

Take a look at how boost::asio works, it basically lets you asyncronously do connects, reads, writes, etc... For example you start an async read with the primary (or any) thread, asio then uses overlapped IO to ask the OS to notify it of IO completion. When the async read completes your callback will be executed by one of the worker threads.

All you need to do is to be sure to call io_service::run() with either your main thread or a worker thread to handle the IO completion queue. Any threads that you call run with will be the ones that execute the callback.

Asio has some guarantees that make this method of multithreading fairly robust if you follow the rules.

Take a look at the documentation for asio even if you don't plan to use it, a lot of the patterns and ideas are quite interesting if this is something you want to tackle yourself.

If you don't want to look at it, remember, on Windows the method of doing async IO is called "Overlapped IO".