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write_at (7 of 8 overloads)

Write a certain amount of data at a specified offset before returning.

    typename SyncRandomAccessWriteDevice,
    typename Allocator,
    typename CompletionCondition>
std::size_t write_at(
    SyncRandomAccessWriteDevice & d,
    uint64_t offset,
    basic_streambuf< Allocator > & b,
    CompletionCondition completion_condition);

This function is used to write a certain number of bytes of data to a random access device at a specified offset. The call will block until one of the following conditions is true:

This operation is implemented in terms of zero or more calls to the device's write_some_at function.



The device to which the data is to be written. The type must support the SyncRandomAccessWriteDevice concept.


The offset at which the data will be written.


The basic_streambuf object from which data will be written.


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

std::size_t completion_condition(
  // Result of latest write_some_at operation.
  const boost::system::error_code& error,

  // Number of bytes transferred so far.
  std::size_t bytes_transferred

A return value of 0 indicates that the write operation is complete. A non-zero return value indicates the maximum number of bytes to be written on the next call to the device's write_some_at function.

Return Value

The number of bytes transferred.



Thrown on failure.