async_read_until (2 of 8 overloads)

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

template<
    typename AsyncReadStream,
    typename DynamicBuffer,
    typename ReadHandler>
DEDUCED async_read_until(
    AsyncReadStream & s,
    DynamicBuffer && buffers,
    string_view delim,
    ReadHandler && handler);

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. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

The get area of the dynamic buffer sequence contains the specified delimiter.
An error occurred.

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.

Parameters

s

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

buffers

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 handler is called.

delim

The delimiter string.

handler

The handler to be called when the read operation completes. Copies will be made of the handler as required. The function signature of the handler must be:

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

  // The number of bytes in the dynamic buffer sequence's
  // get area up to and including the delimiter.
  // 0 if an error occurred.
  std::size_t bytes_transferred
);

Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using asio::io_context::post().

Remarks

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.

Example

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

std::string data;
...
void handler(const asio::error_code& e, std::size_t size)
{
  if (!e)
  {
    std::string line = data.substr(0, n);
    data.erase(0, n);
    ...
  }
}
...
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.