Boost C++ Libraries

A stream socket wrapper with timeouts, an executor, and a rate limit policy.

Synopsis

Defined in header <boost/beast/core/basic_stream.hpp>

template<
    class Protocol,
    class Executor = net::any_io_executor,
    class RatePolicy = unlimited_rate_policy>
class basic_stream

Types

Member Functions

Description

This stream wraps a net::basic_stream_socket to provide the following features:

Although the stream supports multiple concurrent outstanding asynchronous operations, the stream object is not thread-safe. The caller is responsible for ensuring that the stream is accessed from only one thread at a time. This includes the times when the stream, and its underlying socket, are accessed by the networking implementation. To meet this thread safety requirement, all asynchronous operations must be performed by the stream within the same implicit strand (only one thread net::io_context::run) or within the same explicit strand, such as an instance of net::strand.

Completion handlers with explicit associated executors (such as those arising from use of net::bind_executor) will be invoked by the stream using the associated executor. Otherwise, the completion handler will be invoked by the executor associated with the stream upon construction. The type of executor used with this stream must meet the following requirements:

The executor type net::strand meets these requirements. Use of a strand as the executor in the stream class template offers an additional notational convenience: the strand does not need to be specified in each individual initiating function call.

Unlike other stream wrappers, the underlying socket is accessed through the socket member function instead of next_layer. This causes the basic_stream to be returned in calls to get_lowest_layer.

Usage

To use this stream declare an instance of the class. Then, before each logical operation for which a timeout is desired, call expires_after with a duration, or call expires_at with a time point. Alternatively, call expires_never to disable the timeout for subsequent logical operations. A logical operation is any series of one or more direct or indirect calls to the timeout stream's asynchronous read, asynchronous write, or asynchronous connect functions.

When a timeout is set and a mixed operation is performed (one that includes both reads and writes, for example) the timeout applies to all of the intermediate asynchronous operations used in the enclosing operation. This allows timeouts to be applied to stream algorithms which were not written specifically to allow for timeouts, when those algorithms are passed a timeout stream with a timeout set.

When a timeout occurs the socket will be closed, canceling any pending I/O operations. The completion handlers for these canceled operations will be invoked with the error beast::error::timeout.

Examples

This function reads an HTTP request with a timeout, then sends the HTTP response with a different timeout.

void process_http_1 (tcp_stream& stream, net::yield_context yield)
{
    flat_buffer buffer;
    http::request<http::empty_body> req;

    // Read the request, with a 15 second timeout
    stream.expires_after(std::chrono::seconds(15));
    http::async_read(stream, buffer, req, yield);

    // Calculate the response
    http::response<http::string_body> res = make_response(req);

    // Send the response, with a 30 second timeout.
    stream.expires_after (std::chrono::seconds(30));
    http::async_write (stream, res, yield);
}

The example above could be expressed using a single timeout with a simple modification. The function that follows first reads an HTTP request then sends the HTTP response, with a single timeout that applies to the entire combined operation of reading and writing:

void process_http_2 (tcp_stream& stream, net::yield_context yield)
{
    flat_buffer buffer;
    http::request<http::empty_body> req;

    // Require that the read and write combined take no longer than 30 seconds
    stream.expires_after(std::chrono::seconds(30));

    http::async_read(stream, buffer, req, yield);

    http::response<http::string_body> res = make_response(req);
    http::async_write (stream, res, yield);
}

Some stream algorithms, such as ssl::stream::async_handshake perform both reads and writes. A timeout set before calling the initiating function of such composite stream algorithms will apply to the entire composite operation. For example, a timeout may be set on performing the SSL handshake thusly:

void do_ssl_handshake (net::ssl::stream<tcp_stream>& stream, net::yield_context yield)
{
    // Require that the SSL handshake take no longer than 10 seconds
    stream.expires_after(std::chrono::seconds(10));

    stream.async_handshake(net::ssl::stream_base::client, yield);
}

Blocking I/O

Synchronous functions behave identically as that of the wrapped net::basic_stream_socket. Timeouts are not available when performing blocking calls.

Template Parameters

Thread Safety

Distinct objects: Safe.

Shared objects: Unsafe. The application must also ensure that all asynchronous operations are performed within the same implicit or explicit strand.

See Also