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 //
 // read_at.hpp
 // ~~~~~~~~~~~
 //
 // Copyright (c) 2003-2023 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 //
 // Distributed under the Boost Software License, Version 1.0. (See accompanying
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 //
 
 #ifndef BOOST_ASIO_READ_AT_HPP
 #define BOOST_ASIO_READ_AT_HPP
 
 #if defined(_MSC_VER) && (_MSC_VER >= 1200)
 # pragma once
 #endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
 
 #include <boost/asio/detail/config.hpp>
 #include <cstddef>
 #include <boost/asio/async_result.hpp>
 #include <boost/asio/completion_condition.hpp>
 #include <boost/asio/detail/cstdint.hpp>
 #include <boost/asio/error.hpp>
 
 #if !defined(BOOST_ASIO_NO_EXTENSIONS)
 # include <boost/asio/basic_streambuf_fwd.hpp>
 #endif // !defined(BOOST_ASIO_NO_EXTENSIONS)
 
 #include <boost/asio/detail/push_options.hpp>
 
 namespace boost {
 namespace asio {
 namespace detail {
 
 template <typename> class initiate_async_read_at;
 #if !defined(BOOST_ASIO_NO_IOSTREAM)
 template <typename> class initiate_async_read_at_streambuf;
 #endif // !defined(BOOST_ASIO_NO_IOSTREAM)
 
 } // namespace detail
 
 /**
  * @defgroup read_at boost::asio::read_at
  *
  * @brief The @c read_at function is a composed operation that reads a certain
  * amount of data at the specified offset before returning.
  */
 /*@{*/
 
 /// Attempt to read a certain amount of data at the specified offset before
 /// returning.
 /**
  * This function is used to read a certain number of bytes of data from a
  * random access device at the specified offset. The call will block until one
  * of the following conditions is true:
  *
  * @li The supplied buffers are full. That is, the bytes transferred is equal to
  * the sum of the buffer sizes.
  *
  * @li An error occurred.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * read_some_at function.
  *
  * @param d The device from which the data is to be read. The type must support
  * the SyncRandomAccessReadDevice concept.
  *
  * @param offset The offset at which the data will be read.
  *
  * @param buffers One or more buffers into which the data will be read. The sum
  * of the buffer sizes indicates the maximum number of bytes to read from the
  * device.
  *
  * @returns The number of bytes transferred.
  *
  * @throws boost::system::system_error Thrown on failure.
  *
  * @par Example
  * To read into a single data buffer use the @ref buffer function as follows:
  * @code boost::asio::read_at(d, 42, boost::asio::buffer(data, size)); @endcode
  * See the @ref buffer documentation for information on reading into multiple
  * buffers in one go, and how to use it with arrays, boost::array or
  * std::vector.
  *
  * @note This overload is equivalent to calling:
  * @code boost::asio::read_at(
  *     d, 42, buffers,
  *     boost::asio::transfer_all()); @endcode
  */
 template <typename SyncRandomAccessReadDevice, typename MutableBufferSequence>
 std::size_t read_at(SyncRandomAccessReadDevice& d,
     uint64_t offset, const MutableBufferSequence& buffers);
 
 /// Attempt to read a certain amount of data at the specified offset before
 /// returning.
 /**
  * This function is used to read a certain number of bytes of data from a
  * random access device at the specified offset. The call will block until one
  * of the following conditions is true:
  *
  * @li The supplied buffers are full. That is, the bytes transferred is equal to
  * the sum of the buffer sizes.
  *
  * @li An error occurred.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * read_some_at function.
  *
  * @param d The device from which the data is to be read. The type must support
  * the SyncRandomAccessReadDevice concept.
  *
  * @param offset The offset at which the data will be read.
  *
  * @param buffers One or more buffers into which the data will be read. The sum
  * of the buffer sizes indicates the maximum number of bytes to read from the
  * device.
  *
  * @param ec Set to indicate what error occurred, if any.
  *
  * @returns The number of bytes transferred.
  *
  * @par Example
  * To read into a single data buffer use the @ref buffer function as follows:
  * @code boost::asio::read_at(d, 42,
  *     boost::asio::buffer(data, size), ec); @endcode
  * See the @ref buffer documentation for information on reading into multiple
  * buffers in one go, and how to use it with arrays, boost::array or
  * std::vector.
  *
  * @note This overload is equivalent to calling:
  * @code boost::asio::read_at(
  *     d, 42, buffers,
  *     boost::asio::transfer_all(), ec); @endcode
  */
 template <typename SyncRandomAccessReadDevice, typename MutableBufferSequence>
 std::size_t read_at(SyncRandomAccessReadDevice& d,
     uint64_t offset, const MutableBufferSequence& buffers,
     boost::system::error_code& ec);
 
 /// Attempt to read a certain amount of data at the specified offset before
 /// returning.
 /**
  * This function is used to read a certain number of bytes of data from a
  * random access device at the specified offset. The call will block until one
  * of the following conditions is true:
  *
  * @li The supplied buffers are full. That is, the bytes transferred is equal to
  * the sum of the buffer sizes.
  *
  * @li The completion_condition function object returns 0.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * read_some_at function.
  *
  * @param d The device from which the data is to be read. The type must support
  * the SyncRandomAccessReadDevice concept.
  *
  * @param offset The offset at which the data will be read.
  *
  * @param buffers One or more buffers into which the data will be read. The sum
  * of the buffer sizes indicates the maximum number of bytes to read from the
  * device.
  *
  * @param completion_condition The function object to be called to determine
  * whether the read operation is complete. The signature of the function object
  * must be:
  * @code std::size_t completion_condition(
  *   // Result of latest read_some_at operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes transferred so far.
  *   std::size_t bytes_transferred
  * ); @endcode
  * A return value of 0 indicates that the read operation is complete. A non-zero
  * return value indicates the maximum number of bytes to be read on the next
  * call to the device's read_some_at function.
  *
  * @returns The number of bytes transferred.
  *
  * @throws boost::system::system_error Thrown on failure.
  *
  * @par Example
  * To read into a single data buffer use the @ref buffer function as follows:
  * @code boost::asio::read_at(d, 42, boost::asio::buffer(data, size),
  *     boost::asio::transfer_at_least(32)); @endcode
  * See the @ref buffer documentation for information on reading into multiple
  * buffers in one go, and how to use it with arrays, boost::array or
  * std::vector.
  */
 template <typename SyncRandomAccessReadDevice, typename MutableBufferSequence,
     typename CompletionCondition>
 std::size_t read_at(SyncRandomAccessReadDevice& d,
     uint64_t offset, const MutableBufferSequence& buffers,
     CompletionCondition completion_condition);
 
 /// Attempt to read a certain amount of data at the specified offset before
 /// returning.
 /**
  * This function is used to read a certain number of bytes of data from a
  * random access device at the specified offset. The call will block until one
  * of the following conditions is true:
  *
  * @li The supplied buffers are full. That is, the bytes transferred is equal to
  * the sum of the buffer sizes.
  *
  * @li The completion_condition function object returns 0.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * read_some_at function.
  *
  * @param d The device from which the data is to be read. The type must support
  * the SyncRandomAccessReadDevice concept.
  *
  * @param offset The offset at which the data will be read.
  *
  * @param buffers One or more buffers into which the data will be read. The sum
  * of the buffer sizes indicates the maximum number of bytes to read from the
  * device.
  *
  * @param completion_condition The function object to be called to determine
  * whether the read operation is complete. The signature of the function object
  * must be:
  * @code std::size_t completion_condition(
  *   // Result of latest read_some_at operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes transferred so far.
  *   std::size_t bytes_transferred
  * ); @endcode
  * A return value of 0 indicates that the read operation is complete. A non-zero
  * return value indicates the maximum number of bytes to be read on the next
  * call to the device's read_some_at function.
  *
  * @param ec Set to indicate what error occurred, if any.
  *
  * @returns The number of bytes read. If an error occurs, returns the total
  * number of bytes successfully transferred prior to the error.
  */
 template <typename SyncRandomAccessReadDevice, typename MutableBufferSequence,
     typename CompletionCondition>
 std::size_t read_at(SyncRandomAccessReadDevice& d,
     uint64_t offset, const MutableBufferSequence& buffers,
     CompletionCondition completion_condition, boost::system::error_code& ec);
 
 #if !defined(BOOST_ASIO_NO_EXTENSIONS)
 #if !defined(BOOST_ASIO_NO_IOSTREAM)
 
 /// Attempt to read a certain amount of data at the specified offset before
 /// returning.
 /**
  * This function is used to read a certain number of bytes of data from a
  * random access device at the specified offset. The call will block until one
  * of the following conditions is true:
  *
  * @li An error occurred.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * read_some_at function.
  *
  * @param d The device from which the data is to be read. The type must support
  * the SyncRandomAccessReadDevice concept.
  *
  * @param offset The offset at which the data will be read.
  *
  * @param b The basic_streambuf object into which the data will be read.
  *
  * @returns The number of bytes transferred.
  *
  * @throws boost::system::system_error Thrown on failure.
  *
  * @note This overload is equivalent to calling:
  * @code boost::asio::read_at(
  *     d, 42, b,
  *     boost::asio::transfer_all()); @endcode
  */
 template <typename SyncRandomAccessReadDevice, typename Allocator>
 std::size_t read_at(SyncRandomAccessReadDevice& d,
     uint64_t offset, basic_streambuf<Allocator>& b);
 
 /// Attempt to read a certain amount of data at the specified offset before
 /// returning.
 /**
  * This function is used to read a certain number of bytes of data from a
  * random access device at the specified offset. The call will block until one
  * of the following conditions is true:
  *
  * @li An error occurred.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * read_some_at function.
  *
  * @param d The device from which the data is to be read. The type must support
  * the SyncRandomAccessReadDevice concept.
  *
  * @param offset The offset at which the data will be read.
  *
  * @param b The basic_streambuf object into which the data will be read.
  *
  * @param ec Set to indicate what error occurred, if any.
  *
  * @returns The number of bytes transferred.
  *
  * @note This overload is equivalent to calling:
  * @code boost::asio::read_at(
  *     d, 42, b,
  *     boost::asio::transfer_all(), ec); @endcode
  */
 template <typename SyncRandomAccessReadDevice, typename Allocator>
 std::size_t read_at(SyncRandomAccessReadDevice& d,
     uint64_t offset, basic_streambuf<Allocator>& b,
     boost::system::error_code& ec);
 
 /// Attempt to read a certain amount of data at the specified offset before
 /// returning.
 /**
  * This function is used to read a certain number of bytes of data from a
  * random access device at the specified offset. The call will block until one
  * of the following conditions is true:
  *
  * @li The completion_condition function object returns 0.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * read_some_at function.
  *
  * @param d The device from which the data is to be read. The type must support
  * the SyncRandomAccessReadDevice concept.
  *
  * @param offset The offset at which the data will be read.
  *
  * @param b The basic_streambuf object into which the data will be read.
  *
  * @param completion_condition The function object to be called to determine
  * whether the read operation is complete. The signature of the function object
  * must be:
  * @code std::size_t completion_condition(
  *   // Result of latest read_some_at operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes transferred so far.
  *   std::size_t bytes_transferred
  * ); @endcode
  * A return value of 0 indicates that the read operation is complete. A non-zero
  * return value indicates the maximum number of bytes to be read on the next
  * call to the device's read_some_at function.
  *
  * @returns The number of bytes transferred.
  *
  * @throws boost::system::system_error Thrown on failure.
  */
 template <typename SyncRandomAccessReadDevice, typename Allocator,
     typename CompletionCondition>
 std::size_t read_at(SyncRandomAccessReadDevice& d,
     uint64_t offset, basic_streambuf<Allocator>& b,
     CompletionCondition completion_condition);
 
 /// Attempt to read a certain amount of data at the specified offset before
 /// returning.
 /**
  * This function is used to read a certain number of bytes of data from a
  * random access device at the specified offset. The call will block until one
  * of the following conditions is true:
  *
  * @li The completion_condition function object returns 0.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * read_some_at function.
  *
  * @param d The device from which the data is to be read. The type must support
  * the SyncRandomAccessReadDevice concept.
  *
  * @param offset The offset at which the data will be read.
  *
  * @param b The basic_streambuf object into which the data will be read.
  *
  * @param completion_condition The function object to be called to determine
  * whether the read operation is complete. The signature of the function object
  * must be:
  * @code std::size_t completion_condition(
  *   // Result of latest read_some_at operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes transferred so far.
  *   std::size_t bytes_transferred
  * ); @endcode
  * A return value of 0 indicates that the read operation is complete. A non-zero
  * return value indicates the maximum number of bytes to be read on the next
  * call to the device's read_some_at function.
  *
  * @param ec Set to indicate what error occurred, if any.
  *
  * @returns The number of bytes read. If an error occurs, returns the total
  * number of bytes successfully transferred prior to the error.
  */
 template <typename SyncRandomAccessReadDevice, typename Allocator,
     typename CompletionCondition>
 std::size_t read_at(SyncRandomAccessReadDevice& d,
     uint64_t offset, basic_streambuf<Allocator>& b,
     CompletionCondition completion_condition, boost::system::error_code& ec);
 
 #endif // !defined(BOOST_ASIO_NO_IOSTREAM)
 #endif // !defined(BOOST_ASIO_NO_EXTENSIONS)
 
 /*@}*/
 /**
  * @defgroup async_read_at boost::asio::async_read_at
  *
  * @brief The @c async_read_at function is a composed asynchronous operation
  * that reads a certain amount of data at the specified offset.
  */
 /*@{*/
 
 /// Start an asynchronous operation to read a certain amount of data at the
 /// specified offset.
 /**
  * This function is used to asynchronously read a certain number of bytes of
  * data from a random access device at the specified offset. It is an
  * initiating function for an @ref asynchronous_operation, and always returns
  * immediately. The asynchronous operation will continue until one of the
  * following conditions is true:
  *
  * @li The supplied buffers are full. That is, the bytes transferred is equal to
  * the sum of the buffer sizes.
  *
  * @li An error occurred.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * async_read_some_at function.
  *
  * @param d The device from which the data is to be read. The type must support
  * the AsyncRandomAccessReadDevice concept.
  *
  * @param offset The offset at which the data will be read.
  *
  * @param buffers One or more buffers into which the data will be read. The sum
  * of the buffer sizes indicates the maximum number of bytes to read from the
  * device. Although the buffers object may be copied as necessary, ownership of
  * the underlying memory blocks is retained by the caller, which must guarantee
  * that they remain valid until the completion handler is called.
  *
  * @param token The @ref completion_token that will be used to produce a
  * completion handler, which will be called when the read completes.
  * Potential completion tokens include @ref use_future, @ref use_awaitable,
  * @ref yield_context, or a function object with the correct completion
  * signature. The function signature of the completion handler must be:
  * @code void handler(
  *   // Result of operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes copied into the buffers. If an error
  *   // occurred, this will be the number of bytes successfully
  *   // transferred prior to the error.
  *   std::size_t bytes_transferred
  * ); @endcode
  * Regardless of whether the asynchronous operation completes immediately or
  * not, the completion handler will not be invoked from within this function.
  * On immediate completion, invocation of the handler will be performed in a
  * manner equivalent to using boost::asio::post().
  *
  * @par Completion Signature
  * @code void(boost::system::error_code, std::size_t) @endcode
  *
  * @par Example
  * To read into a single data buffer use the @ref buffer function as follows:
  * @code
  * boost::asio::async_read_at(d, 42, boost::asio::buffer(data, size), handler);
  * @endcode
  * See the @ref buffer documentation for information on reading into multiple
  * buffers in one go, and how to use it with arrays, boost::array or
  * std::vector.
  *
  * @note This overload is equivalent to calling:
  * @code boost::asio::async_read_at(
  *     d, 42, buffers,
  *     boost::asio::transfer_all(),
  *     handler); @endcode
  *
  * @par Per-Operation Cancellation
  * This asynchronous operation supports cancellation for the following
  * boost::asio::cancellation_type values:
  *
  * @li @c cancellation_type::terminal
  *
  * @li @c cancellation_type::partial
  *
  * if they are also supported by the @c AsyncRandomAccessReadDevice type's
  * async_read_some_at operation.
  */
 template <typename AsyncRandomAccessReadDevice, typename MutableBufferSequence,
     BOOST_ASIO_COMPLETION_TOKEN_FOR(void (boost::system::error_code,
       std::size_t)) ReadToken = default_completion_token_t<
         typename AsyncRandomAccessReadDevice::executor_type>>
 auto async_read_at(AsyncRandomAccessReadDevice& d,
     uint64_t offset, const MutableBufferSequence& buffers,
     ReadToken&& token = default_completion_token_t<
       typename AsyncRandomAccessReadDevice::executor_type>())
   -> decltype(
     async_initiate<ReadToken,
       void (boost::system::error_code, std::size_t)>(
         declval<detail::initiate_async_read_at<AsyncRandomAccessReadDevice>>(),
         token, offset, buffers, transfer_all()));
 
 /// Start an asynchronous operation to read a certain amount of data at the
 /// specified offset.
 /**
  * This function is used to asynchronously read a certain number of bytes of
  * data from a random access device at the specified offset. It is an
  * initiating function for an @ref asynchronous_operation, and always returns
  * immediately. The asynchronous operation will continue until one of the
  * following conditions is true:
  *
  * @li The supplied buffers are full. That is, the bytes transferred is equal to
  * the sum of the buffer sizes.
  *
  * @li The completion_condition function object returns 0.
  *
  * @param d The device from which the data is to be read. The type must support
  * the AsyncRandomAccessReadDevice concept.
  *
  * @param offset The offset at which the data will be read.
  *
  * @param buffers One or more buffers into which the data will be read. The sum
  * of the buffer sizes indicates the maximum number of bytes to read from the
  * device. Although the buffers object may be copied as necessary, ownership of
  * the underlying memory blocks is retained by the caller, which must guarantee
  * that they remain valid until the completion handler is called.
  *
  * @param completion_condition The function object to be called to determine
  * whether the read operation is complete. The signature of the function object
  * must be:
  * @code std::size_t completion_condition(
  *   // Result of latest async_read_some_at operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes transferred so far.
  *   std::size_t bytes_transferred
  * ); @endcode
  * A return value of 0 indicates that the read operation is complete. A non-zero
  * return value indicates the maximum number of bytes to be read on the next
  * call to the device's async_read_some_at function.
  *
  * @param token The @ref completion_token that will be used to produce a
  * completion handler, which will be called when the read completes.
  * Potential completion tokens include @ref use_future, @ref use_awaitable,
  * @ref yield_context, or a function object with the correct completion
  * signature. The function signature of the completion handler must be:
  * @code void handler(
  *   // Result of operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes copied into the buffers. If an error
  *   // occurred, this will be the number of bytes successfully
  *   // transferred prior to the error.
  *   std::size_t bytes_transferred
  * ); @endcode
  * Regardless of whether the asynchronous operation completes immediately or
  * not, the completion handler will not be invoked from within this function.
  * On immediate completion, invocation of the handler will be performed in a
  * manner equivalent to using boost::asio::post().
  *
  * @par Completion Signature
  * @code void(boost::system::error_code, std::size_t) @endcode
  *
  * @par Example
  * To read into a single data buffer use the @ref buffer function as follows:
  * @code boost::asio::async_read_at(d, 42,
  *     boost::asio::buffer(data, size),
  *     boost::asio::transfer_at_least(32),
  *     handler); @endcode
  * See the @ref buffer documentation for information on reading into multiple
  * buffers in one go, and how to use it with arrays, boost::array or
  * std::vector.
  *
  * @par Per-Operation Cancellation
  * This asynchronous operation supports cancellation for the following
  * boost::asio::cancellation_type values:
  *
  * @li @c cancellation_type::terminal
  *
  * @li @c cancellation_type::partial
  *
  * if they are also supported by the @c AsyncRandomAccessReadDevice type's
  * async_read_some_at operation.
  */
 template <typename AsyncRandomAccessReadDevice,
     typename MutableBufferSequence, typename CompletionCondition,
     BOOST_ASIO_COMPLETION_TOKEN_FOR(void (boost::system::error_code,
       std::size_t)) ReadToken = default_completion_token_t<
         typename AsyncRandomAccessReadDevice::executor_type>>
 auto async_read_at(AsyncRandomAccessReadDevice& d,
     uint64_t offset, const MutableBufferSequence& buffers,
     CompletionCondition completion_condition,
     ReadToken&& token = default_completion_token_t<
       typename AsyncRandomAccessReadDevice::executor_type>())
   -> decltype(
     async_initiate<ReadToken,
       void (boost::system::error_code, std::size_t)>(
         declval<detail::initiate_async_read_at<AsyncRandomAccessReadDevice>>(),
         token, offset, buffers,
         static_cast<CompletionCondition&&>(completion_condition)));
 
 #if !defined(BOOST_ASIO_NO_EXTENSIONS)
 #if !defined(BOOST_ASIO_NO_IOSTREAM)
 
 /// Start an asynchronous operation to read a certain amount of data at the
 /// specified offset.
 /**
  * This function is used to asynchronously read a certain number of bytes of
  * data from a random access device at the specified offset. It is an
  * initiating function for an @ref asynchronous_operation, and always returns
  * immediately. The asynchronous operation will continue until one of the
  * following conditions is true:
  *
  * @li An error occurred.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * async_read_some_at function.
  *
  * @param d The device from which the data is to be read. The type must support
  * the AsyncRandomAccessReadDevice concept.
  *
  * @param offset The offset at which the data will be read.
  *
  * @param b A basic_streambuf object into which the data will be read. Ownership
  * of the streambuf is retained by the caller, which must guarantee that it
  * remains valid until the completion handler is called.
  *
  * @param token The @ref completion_token that will be used to produce a
  * completion handler, which will be called when the read completes.
  * Potential completion tokens include @ref use_future, @ref use_awaitable,
  * @ref yield_context, or a function object with the correct completion
  * signature. The function signature of the completion handler must be:
  * @code void handler(
  *   // Result of operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes copied into the buffers. If an error
  *   // occurred, this will be the number of bytes successfully
  *   // transferred prior to the error.
  *   std::size_t bytes_transferred
  * ); @endcode
  * Regardless of whether the asynchronous operation completes immediately or
  * not, the completion handler will not be invoked from within this function.
  * On immediate completion, invocation of the handler will be performed in a
  * manner equivalent to using boost::asio::post().
  *
  * @par Completion Signature
  * @code void(boost::system::error_code, std::size_t) @endcode
  *
  * @note This overload is equivalent to calling:
  * @code boost::asio::async_read_at(
  *     d, 42, b,
  *     boost::asio::transfer_all(),
  *     handler); @endcode
  *
  * @par Per-Operation Cancellation
  * This asynchronous operation supports cancellation for the following
  * boost::asio::cancellation_type values:
  *
  * @li @c cancellation_type::terminal
  *
  * @li @c cancellation_type::partial
  *
  * if they are also supported by the @c AsyncRandomAccessReadDevice type's
  * async_read_some_at operation.
  */
 template <typename AsyncRandomAccessReadDevice, typename Allocator,
     BOOST_ASIO_COMPLETION_TOKEN_FOR(void (boost::system::error_code,
       std::size_t)) ReadToken = default_completion_token_t<
         typename AsyncRandomAccessReadDevice::executor_type>>
 auto async_read_at(AsyncRandomAccessReadDevice& d,
     uint64_t offset, basic_streambuf<Allocator>& b,
     ReadToken&& token = default_completion_token_t<
       typename AsyncRandomAccessReadDevice::executor_type>())
   -> decltype(
     async_initiate<ReadToken,
       void (boost::system::error_code, std::size_t)>(
         declval<detail::initiate_async_read_at_streambuf<
           AsyncRandomAccessReadDevice>>(),
         token, offset, &b, transfer_all()));
 
 /// Start an asynchronous operation to read a certain amount of data at the
 /// specified offset.
 /**
  * This function is used to asynchronously read a certain number of bytes of
  * data from a random access device at the specified offset. It is an
  * initiating function for an @ref asynchronous_operation, and always returns
  * immediately. The asynchronous operation will continue until one of the
  * following conditions is true:
  *
  * @li The completion_condition function object returns 0.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * async_read_some_at function.
  *
  * @param d The device from which the data is to be read. The type must support
  * the AsyncRandomAccessReadDevice concept.
  *
  * @param offset The offset at which the data will be read.
  *
  * @param b A basic_streambuf object into which the data will be read. Ownership
  * of the streambuf is retained by the caller, which must guarantee that it
  * remains valid until the completion handler is called.
  *
  * @param completion_condition The function object to be called to determine
  * whether the read operation is complete. The signature of the function object
  * must be:
  * @code std::size_t completion_condition(
  *   // Result of latest async_read_some_at operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes transferred so far.
  *   std::size_t bytes_transferred
  * ); @endcode
  * A return value of 0 indicates that the read operation is complete. A non-zero
  * return value indicates the maximum number of bytes to be read on the next
  * call to the device's async_read_some_at function.
  *
  * @param token The @ref completion_token that will be used to produce a
  * completion handler, which will be called when the read completes.
  * Potential completion tokens include @ref use_future, @ref use_awaitable,
  * @ref yield_context, or a function object with the correct completion
  * signature. The function signature of the completion handler must be:
  * @code void handler(
  *   // Result of operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes copied into the buffers. If an error
  *   // occurred, this will be the number of bytes successfully
  *   // transferred prior to the error.
  *   std::size_t bytes_transferred
  * ); @endcode
  * Regardless of whether the asynchronous operation completes immediately or
  * not, the completion handler will not be invoked from within this function.
  * On immediate completion, invocation of the handler will be performed in a
  * manner equivalent to using boost::asio::post().
  *
  * @par Completion Signature
  * @code void(boost::system::error_code, std::size_t) @endcode
  *
  * @par Per-Operation Cancellation
  * This asynchronous operation supports cancellation for the following
  * boost::asio::cancellation_type values:
  *
  * @li @c cancellation_type::terminal
  *
  * @li @c cancellation_type::partial
  *
  * if they are also supported by the @c AsyncRandomAccessReadDevice type's
  * async_read_some_at operation.
  */
 template <typename AsyncRandomAccessReadDevice,
     typename Allocator, typename CompletionCondition,
     BOOST_ASIO_COMPLETION_TOKEN_FOR(void (boost::system::error_code,
       std::size_t)) ReadToken = default_completion_token_t<
         typename AsyncRandomAccessReadDevice::executor_type>>
 auto async_read_at(AsyncRandomAccessReadDevice& d, uint64_t offset,
     basic_streambuf<Allocator>& b, CompletionCondition completion_condition,
     ReadToken&& token = default_completion_token_t<
       typename AsyncRandomAccessReadDevice::executor_type>())
   -> decltype(
     async_initiate<ReadToken,
       void (boost::system::error_code, std::size_t)>(
         declval<detail::initiate_async_read_at_streambuf<
           AsyncRandomAccessReadDevice>>(),
         token, offset, &b,
         static_cast<CompletionCondition&&>(completion_condition)));
 
 #endif // !defined(BOOST_ASIO_NO_IOSTREAM)
 #endif // !defined(BOOST_ASIO_NO_EXTENSIONS)
 
 /*@}*/
 
 } // namespace asio
 } // namespace boost
 
 #include <boost/asio/detail/pop_options.hpp>
 
 #include <boost/asio/impl/read_at.hpp>
 
 #endif // BOOST_ASIO_READ_AT_HPP

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