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async_read_until (10 of 12 overloads)

Start an asynchronous operation to read data into a dynamic buffer sequence until it contains a specified delimiter.

    typename AsyncReadStream,
    typename DynamicBuffer_v2,
    typename ReadToken = default_completion_token_t<        typename AsyncReadStream::executor_type>>
DEDUCED async_read_until(
    AsyncReadStream & s,
    DynamicBuffer_v2 buffers,
    string_view delim,
    ReadToken && token = default_completion_token_t< typename AsyncReadStream::executor_type >(),
    constraint_t< is_dynamic_buffer_v2< DynamicBuffer_v2 >::value >  = 0);

This function is used to asynchronously read data into the specified dynamic buffer sequence until the dynamic buffer sequence's get area contains the specified delimiter. It is an initiating function for an asynchronous operation, and always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

This operation is implemented in terms of zero or more calls to the stream's async_read_some function, and is known as a composed operation. If the dynamic buffer sequence's get area already contains the delimiter, this asynchronous operation completes immediately. The program must ensure that the stream performs no other read operations (such as async_read, async_read_until, the stream's async_read_some function, or any other composed operations that perform reads) until this operation completes.



The stream from which the data is to be read. The type must support the AsyncReadStream concept.


The dynamic buffer sequence into which the data will be read. 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.


The delimiter string.


The completion token that will be used to produce a completion handler, which will be called when the read completes. Potential completion tokens include use_future, use_awaitable, yield_context, or a function object with the correct completion signature. The function signature of the completion handler must be:

void handler(
  // Result of operation.
  const boost::system::error_code& error,

  // The number of bytes in the dynamic buffer sequence's
  // get area up to and including the delimiter.
  std::size_t bytes_transferred

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 post.

Completion Signature
void(boost::system::error_code, std::size_t)

After a successful async_read_until operation, the dynamic buffer sequence may contain additional data beyond the delimiter. An application will typically leave that data in the dynamic buffer sequence for a subsequent async_read_until operation to examine.


To asynchronously read data into a std::string until a CR-LF sequence is encountered:

std::string data;
void handler(const boost::system::error_code& e, std::size_t size)
  if (!e)
    std::string line = data.substr(0, n);
    data.erase(0, n);
boost::asio::async_read_until(s, data, "\r\n", handler);

After the async_read_until operation completes successfully, the string data contains the delimiter:

{ 'a', 'b', ..., 'c', '\r', '\n', 'd', 'e', ... }

The call to substr then extracts the data up to and including the delimiter, so that the string line contains:

{ 'a', 'b', ..., 'c', '\r', '\n' }

After the call to erase, the remaining data is left in the string data as follows:

{ 'd', 'e', ... }

This data may be the start of a new line, to be extracted by a subsequent async_read_until operation.

Per-Operation Cancellation

This asynchronous operation supports cancellation for the following cancellation_type values:

if they are also supported by the AsyncReadStream type's async_read_some operation.