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+//
+// read_at.hpp
+// ~~~~~~~~~~~
+//
+// Copyright (c) 2003-2024 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 ASIO_READ_AT_HPP
+#define ASIO_READ_AT_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/config.hpp"
+#include <cstddef>
+#include "asio/async_result.hpp"
+#include "asio/completion_condition.hpp"
+#include "asio/detail/cstdint.hpp"
+#include "asio/error.hpp"
+
+#if !defined(ASIO_NO_EXTENSIONS)
+# include "asio/basic_streambuf_fwd.hpp"
+#endif // !defined(ASIO_NO_EXTENSIONS)
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+namespace detail {
+
+template <typename> class initiate_async_read_at;
+#if !defined(ASIO_NO_IOSTREAM)
+template <typename> class initiate_async_read_at_streambuf;
+#endif // !defined(ASIO_NO_IOSTREAM)
+
+} // namespace detail
+
+/**
+ * @defgroup read_at 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 asio::system_error Thrown on failure.
+ *
+ * @par Example
+ * To read into a single data buffer use the @ref buffer function as follows:
+ * @code asio::read_at(d, 42, 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 asio::read_at(
+ * d, 42, buffers,
+ * 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 asio::read_at(d, 42,
+ * 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 asio::read_at(
+ * d, 42, buffers,
+ * asio::transfer_all(), ec); @endcode
+ */
+template <typename SyncRandomAccessReadDevice, typename MutableBufferSequence>
+std::size_t read_at(SyncRandomAccessReadDevice& d,
+ uint64_t offset, const MutableBufferSequence& buffers,
+ asio::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 asio::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 asio::system_error Thrown on failure.
+ *
+ * @par Example
+ * To read into a single data buffer use the @ref buffer function as follows:
+ * @code asio::read_at(d, 42, asio::buffer(data, size),
+ * 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 asio::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, asio::error_code& ec);
+
+#if !defined(ASIO_NO_EXTENSIONS)
+#if !defined(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 asio::system_error Thrown on failure.
+ *
+ * @note This overload is equivalent to calling:
+ * @code asio::read_at(
+ * d, 42, b,
+ * 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 asio::read_at(
+ * d, 42, b,
+ * asio::transfer_all(), ec); @endcode
+ */
+template <typename SyncRandomAccessReadDevice, typename Allocator>
+std::size_t read_at(SyncRandomAccessReadDevice& d,
+ uint64_t offset, basic_streambuf<Allocator>& b,
+ asio::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 asio::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 asio::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 asio::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, asio::error_code& ec);
+
+#endif // !defined(ASIO_NO_IOSTREAM)
+#endif // !defined(ASIO_NO_EXTENSIONS)
+
+/*@}*/
+/**
+ * @defgroup async_read_at 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 asio::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 asio::post().
+ *
+ * @par Completion Signature
+ * @code void(asio::error_code, std::size_t) @endcode
+ *
+ * @par Example
+ * To read into a single data buffer use the @ref buffer function as follows:
+ * @code
+ * asio::async_read_at(d, 42, 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 asio::async_read_at(
+ * d, 42, buffers,
+ * asio::transfer_all(),
+ * handler); @endcode
+ *
+ * @par Per-Operation Cancellation
+ * This asynchronous operation supports cancellation for the following
+ * 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,
+ ASIO_COMPLETION_TOKEN_FOR(void (asio::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 (asio::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 asio::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 asio::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 asio::post().
+ *
+ * @par Completion Signature
+ * @code void(asio::error_code, std::size_t) @endcode
+ *
+ * @par Example
+ * To read into a single data buffer use the @ref buffer function as follows:
+ * @code asio::async_read_at(d, 42,
+ * asio::buffer(data, size),
+ * 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
+ * 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,
+ ASIO_COMPLETION_TOKEN_FOR(void (asio::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 (asio::error_code, std::size_t)>(
+ declval<detail::initiate_async_read_at<AsyncRandomAccessReadDevice>>(),
+ token, offset, buffers,
+ static_cast<CompletionCondition&&>(completion_condition)));
+
+#if !defined(ASIO_NO_EXTENSIONS)
+#if !defined(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 asio::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 asio::post().
+ *
+ * @par Completion Signature
+ * @code void(asio::error_code, std::size_t) @endcode
+ *
+ * @note This overload is equivalent to calling:
+ * @code asio::async_read_at(
+ * d, 42, b,
+ * asio::transfer_all(),
+ * handler); @endcode
+ *
+ * @par Per-Operation Cancellation
+ * This asynchronous operation supports cancellation for the following
+ * 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,
+ ASIO_COMPLETION_TOKEN_FOR(void (asio::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 (asio::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 asio::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 asio::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 asio::post().
+ *
+ * @par Completion Signature
+ * @code void(asio::error_code, std::size_t) @endcode
+ *
+ * @par Per-Operation Cancellation
+ * This asynchronous operation supports cancellation for the following
+ * 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,
+ ASIO_COMPLETION_TOKEN_FOR(void (asio::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 (asio::error_code, std::size_t)>(
+ declval<detail::initiate_async_read_at_streambuf<
+ AsyncRandomAccessReadDevice>>(),
+ token, offset, &b,
+ static_cast<CompletionCondition&&>(completion_condition)));
+
+#endif // !defined(ASIO_NO_IOSTREAM)
+#endif // !defined(ASIO_NO_EXTENSIONS)
+
+/*@}*/
+
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#include "asio/impl/read_at.hpp"
+
+#endif // ASIO_READ_AT_HPP