Попытайтесь прочитать определенное количество данных из потока перед возвращением.

template<
    typename SyncReadStream,
    typename MutableBufferSequence>
std::size_t read(
    SyncReadStream & s,
    const MutableBufferSequence & buffers);
  » more...
template<
    typename SyncReadStream,
    typename MutableBufferSequence>
std::size_t read(
    SyncReadStream & s,
    const MutableBufferSequence & buffers,
    boost::system::error_code & ec);
  » more...
template<
    typename SyncReadStream,
    typename MutableBufferSequence,
    typename CompletionCondition>
std::size_t read(
    SyncReadStream & s,
    const MutableBufferSequence & buffers,
    CompletionCondition completion_condition);
  » more...
template<
    typename SyncReadStream,
    typename MutableBufferSequence,
    typename CompletionCondition>
std::size_t read(
    SyncReadStream & s,
    const MutableBufferSequence & buffers,
    CompletionCondition completion_condition,
    boost::system::error_code & ec);
  » more...
template<
    typename SyncReadStream,
    typename Allocator>
std::size_t read(
    SyncReadStream & s,
    basic_streambuf< Allocator > & b);
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template<
    typename SyncReadStream,
    typename Allocator>
std::size_t read(
    SyncReadStream & s,
    basic_streambuf< Allocator > & b,
    boost::system::error_code & ec);
  » more...
template<
    typename SyncReadStream,
    typename Allocator,
    typename CompletionCondition>
std::size_t read(
    SyncReadStream & s,
    basic_streambuf< Allocator > & b,
    CompletionCondition completion_condition);
  » more...
template<
    typename SyncReadStream,
    typename Allocator,
    typename CompletionCondition>
std::size_t read(
    SyncReadStream & s,
    basic_streambuf< Allocator > & b,
    CompletionCondition completion_condition,
    boost::system::error_code & ec);
  » more...

Requirements

Заголовок:<boost/asio/read.hpp>

Заголовок удобства:<boost/asio.hpp>