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Return a completion condition function object that indicates that a read or write operation should continue until a minimum number of bytes has been transferred, or until an error occurs.

unspecified transfer_at_least(
    std::size_t minimum);

This function is used to create an object, of unspecified type, that meets CompletionCondition requirements.


Reading until a buffer is full or contains at least 64 bytes:

boost::array<char, 128> buf;
boost::system::error_code ec;
std::size_t n = boost::asio::read(
    sock, boost::asio::buffer(buf),
    boost::asio::transfer_at_least(64), ec);
if (ec)
  // An error occurred.
  // n >= 64 && n <= 128

Header: boost/asio/completion_condition.hpp

Convenience header: boost/asio.hpp