How to use custom allocator when using boost::asio/beast with C++20 coroutines

Question

I'm trying to find a way to hook a custom allocator when using boost::beast with asio::use_awaitable and C++ 20 coroutines. Given this function sketch:

asio::awaitable<void> process_request(asio::ip::tcp::socket sock) 
{                                                                           
    // Prepare the beast http buffer and request stuff using the custom allocator
    ...  
                      
    fmt::print(stdout, "Reading\n");                                        
    co_await http::async_read(sock, buffer, req, asio::use_awaitable); 
    fmt::print(stdout, "Done reading\n"); 

    // Prepare the beast http buffer and response stuff using the custom allocator
    ...         
                                                                                                                             
    fmt::print(stdout, "Writing\n");                                        
    co_await http::async_write(sock, res, asio::use_awaitable);            
    fmt::print(stdout, "Done!\n\n");                    
}

and overwriting the global operator new/delete I did some tracing of the allocations and hooked most of them to the custom allocator. Here are my questions:

How to hook a custom allocator for the read/write handlers when using use_awaitable? I see that there is an allocation of 288 bytes in between the reading prints and 408 bytes allocation in between the writing prints. I suppose these are the allocations that asio internally does for the handlers. I've read here about how to provide custom allocator for callback hadnlers but I can't figure a way to do it for this kind of Completion Token. Asio seems to provide a way for hooking an allocator with use_future but not with use_awaitable.
How to give the custom allocator to the asio machinery so that it uses it for the coroutine frame allocation? As far as I know, I can overwrite operator new/delete for the coroutine promise type but this type is hidden inside the asio machinery. In addition to that I'm not able to see the allocation for the above coroutine in my printing. I've 1024 bytes stack buffer inside the above function and I don't see so big allocations in my printing.

Thanks.

score 1 · Answer 1 · answered Apr 13 '22 at 12:12

I asked myself the same question (just out of curiosity, so I have not tested the following myself) and found an answer within some comment (from the creator of ASIO) to a github issue in the original ASIO repository. The example code of the comment can be found below.

awaitable<void> foo()
{
  my_pre_allocatated_storage_type my_storage;
  my_custom_allocator my_alloc(my_storage);
  ...
  co_await my_operation(/*...*/, bind_allocator(my_alloc, use_awaitable));
}

However, the used bind_allocator call was added on 24. Feb. 2022 and does currently not appear in Boost.Asio (Boost 1.78.0). If you want to use this feature via Boost you might just have to wait a bit for it.

How to use custom allocator when using boost::asio/beast with C++20 coroutines

1 Answers1