C++ Boost ASIO: how to read/write with a timeout?

Question

From reading other Stack Overflow entries and the boost::asio documentation, I've confirmed that there is no synchronous ASIO read/write calls that also provide an easy-to-use timeout as a parameter to the call.

I'm in the middle of converting an old-school Linux socket application with select(2) calls that employs timeouts, and I need to do more-or-less the same.

So what is the best way to do this in boost::asio? Looking at the asio documentation, there are many confusing examples of various things to do with timers, but I'm quite confused.

I'd love to see a simple-to-read example of this: Read from a socket, but wait for a maximum of X seconds after which the function either returns with nothing, or returns with whatever it was able to read from the socket before the timeout expired.

Answer 1

This has been brought up on the asio mailing lists, there's a ticket requesting the feature as well. To summarize, it is suggested to use asynchronous methods if you desire timeouts and cancellability.

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If you cannot convert to asynchronous methods, you might try the SO_RCVTIMEO and SO_SNDTIMEO socket options. They can be set with setsockopt, the descriptor can be obtained with the boost::asio::ip::tcp::socket::native method. The man 7 socket man page says

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.

Answer 2

I used some asio docs to produce this:

class TimeoutAdjust { public:   TimeoutAdjust(unsigned int dwTimeout) : m_dwTimeout(dwTimeout) {};    template<class Protocol>   int level(const Protocol& p) const {return SOL_SOCKET;}    template<class Protocol>   int name(const Protocol& p) const {return SO_SNDTIMEO;}    template<class Protocol>   const void* data(const Protocol& p) const {return &m_dwTimeout;}    template<class Protocol>   size_t size(const Protocol& p) const {return sizeof(m_dwTimeout);} private:   unsigned int m_dwTimeout; }; 

Usage:

TimeoutAdjust adjust(5000); sSocket.set_option(adjust); 

I debugged it, and it appears to do what it is supposed to.