How to set a timeout on blocking sockets in boost asio?

Is there a way to cancel a pending operation (without disconnect) or set a timeout for the boost library functions?

I.e. I want to set a timeout on blocking socket in boost asio?

socket.read_some(boost::asio::buffer(pData, maxSize), error_);

Example: I want to read some from the socket, but I want to throw an error if 10 seconds have passed.

Solution 1:

TL;DR

socket.set_option(boost::asio::detail::socket_option::integer<SOL_SOCKET, SO_RCVTIMEO>{ 200 });

FULL ANSWER This question keep being asked over and over again for many years. Answers I saw so far are quite poor. I'll add this info right here in one of the first occurrences of this question.

Everybody trying to use ASIO to simplify their networking code would be perfectly happy if the author would just add an optional parameter timeout to all sync and async io functions. Unfortunately, this is unlikely to happen (in my humble opinion, just for ideological reasons, after all, AS in ASIO is for a reason).

So these are the ways to skin this poor cat available so far, none of them especially appetizing. Let's say we need 200ms timeout.

1) Good (bad) old socket API:

const int timeout = 200;
::setsockopt(socket.native_handle(), SOL_SOCKET, SO_RCVTIMEO, (const char *)&timeout, sizeof timeout);//SO_SNDTIMEO for send ops

Please note those peculiarities: - const int for timeout - on Windows the required type is actually DWORD, but the current set of compilers luckily has it the same, so const int will work both in Win and Posix world. - (const char*) for value. On Windows const char* is required, Posix requires const void*, in C++ const char* will convert to const void* silently while the opposite is not true.

Advantages: works and probably will always work as the socket API is old and stable. Simple enough. Fast. Disadvantages: technically might require appropriate header files (different on Win and even different UNIX flavors) for setsockopt and the macros, but current implementation of ASIO pollutes global namespace with them anyway. Requires a variable for timeout. Not type-safe. On Windows, requires that the socket is in overlapped mode to work (which current ASIO implementation luckily uses, but it is still an implementation detail). UGLY!

2) Custom ASIO socket option:

typedef boost::asio::detail::socket_option::integer<SOL_SOCKET, SO_RCVTIMEO> rcv_timeout_option; //somewhere in your headers to be used everywhere you need it
//...
socket.set_option(rcv_timeout_option{ 200 });

Advantages: Simple enough. Fast. Beautiful (with typedef). Disadvantages: Depends on ASIO implementation detail, which might change (but OTOH everything will change eventually, and such detail is less likely to change then public APIs subject to standardization). But in case this happens, you'll have to either write a class according to https://www.boost.org/doc/libs/1_68_0/doc/html/boost_asio/reference/SettableSocketOption.html (which is of course a major PITA thanks to obvious overengineering of this part of ASIO) or better yet revert to 1.

3) Use C++ async/future facilities.

#include <future>
#include <chrono>
//...
auto status = std::async(std::launch::async, [&] (){ /*your stream ops*/ })
    .wait_for(std::chrono::milliseconds{ 200 });
switch (status)
    {
    case std::future_status::deferred:
    //... should never happen with std::launch::async
        break;
    case std::future_status::ready:
    //...
        break;
    case std::future_status::timeout:
    //...
        break;
    }

Advantages: standard. Disadvantages: always starts a new thread (in practice), which is relatively slow (might be good enough for clients, but will lead to DoS vulnerability for servers as threads and sockets are "expensive" resources). Don't try to use std::launch::deferred instead of std::launch::async to avoid new thread launch as wait_for will always return future_status::deferred without trying to run the code.

4) The method prescribed by ASIO - use async operations only (which is not really the answer to the question).

Advantages: good enough for servers too if huge scalability for short transactions is not required. Disadvantages: quite wordy (so I will not even include examples - see ASIO examples). Requires very careful lifetime management of all your objects used both by async operations and their completion handlers, which in practice requires all classes containing and using such data in async operations be derived from enable_shared_from_this, which requires all such classes allocated on heap, which means (at least for short operations) that scalability will start taper down after about 16 threads as every heap alloc/dealloc will use a memory barrier.

Solution 2:

When this question was asked, I guess ASIO did not have any example on how to accomplish what the OP needed, that is to timeout a blocking operation such as a blocking socket operation. Now there exists examples to show you exactly how to do this. the example seems long, but that is because it is WELL commented. It shows how to use the ioservice in a 'one shot' kind of mode.

I think the example is a great solution. The other solutions here break portability and don't take advantage of ioservice. if portability is not important and the ioservice seems like to much overhead --THEN-- you should not be using ASIO. No matter what, you will have an ioservice created (almost all ASIO functionality depends on it, even sync sockets) so, take advantage of it.

Timeout a blocking asio tcp operation

Timeout a blocking asio udp operation

The ASIO documentation has been updated, so check it out for new examples on how to overcome some of the 'gotchas' ASIO use to have.

Solution 3:

You could do an async_read and also set a timer for your desired time out. Then if the timer fires, call cancel on your socket object. Otherwise if your read happens, you can cancel your timer. This requires you to use an io_service object of course.

edit: Found a code snippet for you that does this

http://lists.boost.org/Archives/boost/2007/04/120339.php

Solution 4:

Under Linux/BSD the timeout on I/O operations on sockets is directly supported by the operating system. The option can be enabled via setsocktopt(). I don't know if boost::asio provides a method for setting it or exposes the socket scriptor to allow you to directly set it -- the latter case is not really portable.

For a sake of completeness here's the description from the man page:

SO_RCVTIMEO and SO_SNDTIMEO

          Specify the receiving or sending  timeouts  until  reporting  an
          error.  The argument is a struct timeval.  If an input or output
          function blocks for this period of time, and data has been  sent
          or  received,  the  return  value  of  that function will be the
          amount of data transferred; if no data has been transferred  and
          the  timeout has been reached then -1 is returned with errno set
          to EAGAIN or EWOULDBLOCK just as if the socket was specified  to
          be  non-blocking.   If  the timeout is set to zero (the default)
          then the operation  will  never  timeout.   Timeouts  only  have
          effect  for system calls that perform socket I/O (e.g., read(2),
          recvmsg(2), send(2), sendmsg(2)); timeouts have  no  effect  for
          select(2), poll(2), epoll_wait(2), etc.