read_at (7 of 8 overloads)

Attempt to read a certain amount of data at the specified offset before returning.

template<
    typename SyncRandomAccessReadDevice,
    typename Allocator,
    typename CompletionCondition>
std::size_t read_at(
    SyncRandomAccessReadDevice & d,
    uint64_t offset,
    basic_streambuf< Allocator > & b,
    CompletionCondition completion_condition);

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:

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.

Parameters

d

The device from which the data is to be read. The type must support the SyncRandomAccessReadDevice concept.

offset

The offset at which the data will be read.

b

The basic_streambuf object into which the data will be read.

completion_condition

The function object to be called to determine whether the read operation is complete. The signature of the function object must be:

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
);

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.

Return Value

The number of bytes transferred.

Exceptions

asio::system_error: Thrown on failure.