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### transfer_at_least

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.

##### Example

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.
    }
    else
    {
      // n >= 64 && n <= 128
    }