Memory and Lifetimes

Understand Asio’s resource management guarantees.

The Deallocate-Before-Invoke Guarantee

This is one of Asio’s most important guarantees:

If an asynchronous operation requires a temporary resource (such as memory, a file descriptor, or a thread), this resource is released before calling the completion handler.

Why does this matter? Consider a chain of operations:

awaitable<void> chain()
{
    co_await async_op1();  // Uses internal buffer
    co_await async_op2();  // Uses internal buffer
    co_await async_op3();  // Uses internal buffer
}

Without this guarantee, all three internal buffers might exist simultaneously, tripling memory usage. With the guarantee, each operation releases its resources before the next operation starts.

Practical Impact

Memory Efficiency

awaitable<void> echo_loop(tcp::socket& socket)
{
    char buf[1024];
    for (;;)
    {
        // Op 1: internal resources allocated
        std::size_t n = co_await socket.async_read_some(
            asio::buffer(buf), use_awaitable);
        // Op 1: internal resources released before we get here

        // Op 2: can reuse the same memory
        co_await asio::async_write(
            socket, asio::buffer(buf, n), use_awaitable);
        // Op 2: internal resources released
    }
}

Each iteration uses the same peak memory, not accumulating.

Safe Handler Chaining

The guarantee enables patterns like:

awaitable<void> infinite_accept(tcp::acceptor& acceptor)
{
    for (;;)
    {
        // Accept completes, releases internal state
        auto socket = co_await acceptor.async_accept(use_awaitable);

        // Now safe to do the next accept without resource buildup
        co_spawn(socket.get_executor(),
                 handle_session(std::move(socket)),
                 detached);
    }
}

Buffer Lifetimes

Critical: You must keep buffers alive until the operation completes.

// WRONG: buffer destroyed before operation completes
awaitable<void> bad()
{
    auto socket = /* ... */;
    {
        std::string data = "Hello";
        socket.async_write_some(asio::buffer(data), use_awaitable);
    }  // data destroyed here!

    // ... operation still in progress, buffer is gone
}

// CORRECT: buffer outlives operation
awaitable<void> good()
{
    auto socket = /* ... */;
    std::string data = "Hello";
    co_await asio::async_write(socket, asio::buffer(data), use_awaitable);
    // data still valid, operation is complete
}

With co_await, this is natural: the local variable stays alive until the operation completes.

I/O Object Lifetimes

Keep I/O objects alive while operations are pending:

// WRONG: socket destroyed while operation pending
void bad()
{
    auto socket = std::make_unique<tcp::socket>(ctx);
    socket->async_read_some(buffer, handler);
    socket.reset();  // Destroys socket while read is pending!
}

// CORRECT: socket outlives operation
awaitable<void> good()
{
    tcp::socket socket(co_await asio::this_coro::executor);
    co_await socket.async_connect(endpoint, use_awaitable);
    co_await socket.async_read_some(buffer, use_awaitable);
    // socket destroyed here, after operations complete
}

For servers with multiple sessions, use shared_ptr:

class session : public std::enable_shared_from_this<session>
{
    tcp::socket socket_;

public:
    awaitable<void> run()
    {
        // Keep self alive while running
        auto self = shared_from_this();

        co_await do_read();
        co_await do_write();
    }
};

Executor Lifetime

The executor (and its underlying io_context) must outlive all operations:

// WRONG: context destroyed before operation completes
void bad()
{
    asio::io_context ctx;
    tcp::socket socket(ctx);
    socket.async_connect(endpoint, handler);
}  // ctx destroyed before run() called!

// CORRECT: context outlives operations
void good()
{
    asio::io_context ctx;
    tcp::socket socket(ctx);
    socket.async_connect(endpoint, handler);
    ctx.run();  // Keeps ctx alive until complete
}

Cancellation and Resources

When an operation is cancelled, resources are still released before the handler is invoked:

awaitable<void> cancellable()
{
    tcp::socket socket(co_await asio::this_coro::executor);

    try
    {
        co_await socket.async_connect(endpoint, use_awaitable);
    }
    catch (const boost::system::system_error& e)
    {
        if (e.code() == asio::error::operation_aborted)
        {
            // Resources are already released
            // Safe to start new operations
        }
    }
}

The Cost of Safety

The deallocate-before-invoke guarantee has minimal cost:

  • Operations already need to complete before invoking the handler

  • The deallocation happens at a natural point in the flow

  • Modern allocators make this efficient

Debugging Lifetime Issues

Enable buffer debugging:

#define BOOST_ASIO_ENABLE_BUFFER_DEBUGGING
#include <boost/asio.hpp>

This adds runtime checks that detect use of invalidated buffers.

Summary

  1. Internal resources released before handler — Enables efficient chaining

  2. Your buffers must outlive operations — With co_await, this is natural

  3. I/O objects must outlive pending operations — Use shared_ptr when needed

  4. Executors must outlive operations — Don’t destroy io_context early

These guarantees make async code composable and memory-efficient.

Next Steps

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