asio之读写

简介

asio读写支持同步和异步

读操作

主要在read.hpp,read_at.hpp和read_until.hpp文件中
异步读函数对象有

• read_until_delim_op
• read_until_delim_string_op
• read_until_expr_op
• read_until_match_op
• read_streambuf_op
• read_op
• read_at_op
• read_at_streambuf_op

写操作

主要在write.hpp,write_at.hpp文件中
异步写函数对象有

• write_at_op
• write_at_streambuf_op
• write_op
• write_streambuf_handler

异步操作分析

异步操作可能在调用线程和io线程中调用，以read_until_delim_op为例，其第三个参数为默认参数，区分不同线程的调用。在switch语句中，使用循环，同时循环中使用了default分支。

void operator()(const boost::system::error_code& ec,std::size_t bytes_transferred, int start = 0){const std::size_t not_found = (std::numeric_limits<std::size_t>::max)();std::size_t bytes_to_read;switch (start_ = start){case 1:for (;;){{// Determine the range of the data to be searched.typedef typename boost::asio::basic_streambuf<Allocator>::const_buffers_type const_buffers_type;typedef boost::asio::buffers_iterator<const_buffers_type> iterator;const_buffers_type buffers = streambuf_.data();iterator begin = iterator::begin(buffers);iterator start_pos = begin + search_position_;iterator end = iterator::end(buffers);// Look for a match.iterator iter = std::find(start_pos, end, delim_);if (iter != end){// Found a match. We're done.search_position_ = iter - begin + 1;bytes_to_read = 0;}// No match yet. Check if buffer is full.else if (streambuf_.size() == streambuf_.max_size()){search_position_ = not_found;bytes_to_read = 0;}// Need to read some more data.else{// Next search can start with the new data.search_position_ = end - begin;bytes_to_read = read_size_helper(streambuf_, 65536);}}// Check if we're done.if (!start && bytes_to_read == 0)break;// Start a new asynchronous read operation to obtain more data.stream_.async_read_some(streambuf_.prepare(bytes_to_read),BOOST_ASIO_MOVE_CAST(read_until_delim_op)(*this));return; default:streambuf_.commit(bytes_transferred);if (ec || bytes_transferred == 0)break;}const boost::system::error_code result_ec =(search_position_ == not_found)? error::not_found : ec;const std::size_t result_n =(ec || search_position_ == not_found)? 0 : search_position_;handler_(result_ec, result_n);}}

在发起异步调用时直接return了

stream_.async_read_some(streambuf_.prepare(bytes_to_read),BOOST_ASIO_MOVE_CAST(read_until_delim_op)(*this));
return; default:

当在io线程调用时，会进入default分支，如果数据没有读完，会进入for循环，继续发起异步读取返回。直到将数据读取完毕， 才会调用自定义的handler

handler_(result_ec, result_n);