Senders - A Standard Model for Asynchronous Execution in C++

stdexec is an experimental reference implementation of the Senders model of asynchronous programming proposed by P2300 - std::execution for adoption into the C++ Standard.

Purpose of this Repository:

1. Provide a proof-of-concept implementation of the design proposed in P2300.
2. Provide early access to developers looking to experiment with the Sender model.
3. Collaborate with those interested in participating or contributing to the design of P2300 (contributions welcome!).

Disclaimer

stdexec is experimental in nature and subject to change without warning. The authors and NVIDIA do not guarantee that this code is fit for any purpose whatsoever.

Example

Below is a simple program that executes three senders concurrently on a thread pool. Try it live on godbolt!.

#include <stdexec/execution.hpp>
#include <exec/static_thread_pool.hpp>

int main()
{
    // Declare a pool of 3 worker threads:
    exec::static_thread_pool pool(3);

    // Get a handle to the thread pool:
    auto sched = pool.get_scheduler();

    // Describe some work:
    // Creates 3 sender pipelines that are executed concurrently by passing to `when_all`
    // Each sender is scheduled on `sched` using `on` and starts with `just(n)` that creates a
    // Sender that just forwards `n` to the next sender.
    // After `just(n)`, we chain `then(fun)` which invokes `fun` using the value provided from `just()`
    // Note: No work actually happens here. Everything is lazy and `work` is just an object that statically
    // represents the work to later be executed
    auto fun = [](int i) { return i*i; };
    auto work = stdexec::when_all(
        stdexec::on(sched, stdexec::just(0) | stdexec::then(fun)),
        stdexec::on(sched, stdexec::just(1) | stdexec::then(fun)),
        stdexec::on(sched, stdexec::just(2) | stdexec::then(fun))
    );

    // Launch the work and wait for the result
    auto [i, j, k] = stdexec::sync_wait(std::move(work)).value();

    // Print the results:
    std::printf("%d %d %d\n", i, j, k);
}

Resources

• Working with Asynchrony Generically: A Tour of Executors: Part 1 (Part 2) (Video): A comprehensive introduction to Senders and structured concurrency
• What are Senders Good For, Anyway? (Blog): Demonstrates the value of a standard async programming model by wrapping a C-style async API in a sender
• From Zero to Sender/Receiver in ~60 Minutes (Video): Live-coding a toy sender/receiver implementation from scratch
• A Unifying Abstraction for Async in C++ (Video): A simple introduction to the concepts behind P2300
• A Universal Async Abstraction for C++ (Blog): An introduction to Senders
• A Universal I/O Abstraction for C++ (Blog): A look at how the Senders concepts interact with io_uring on Linux
• Structured Concurrency (Video): An explanation of structured concurrency in C++ and its benefits
• Executors: a Change of Perspective (Article): An article about the computational completeness of Senders
• Structured Concurrency in C++ (Article): An article about how Senders manifest the principles of structured concurrency
• Structured Networking in C++ (Video): A look at what a P2300-style networking library could look like
• HPCWire Article: Provides a high-level overview of the Sender model and its benefits
• NVIDIA HPC SDK Documentation: Documentation for the NVIDIA HPC SDK
• P2300 - std::execution: Senders proposal to C++ Standard

Structure

This library is header-only, so all the source code can be found in the include/ directory. The physical and logical structure of the code can be summarized by the following table:

Kind	Path	Namespace
Things approved for the C++ standard	<stdexec/...>	::stdexec
Generic additions and extensions	<exec/...>	::exec
NVIDIA-specific extensions and customizations	<nvexec/...>	::nvexec

How to get stdexec

There are a few ways to get stdexec:

1. Clone from GitHub
   • git clone https://github.com/NVIDIA/stdexec.git
2. Download the NVIDIA HPC SDK starting with 22.11
3. (Recommended) Use CMake Package Manager (CPM) to automatically pull stdexec as part of your CMake project. See below for more information.

You can also try it directly on godbolt.org where it is available as a C++ library or via the nvc++ compiler starting with version 22.11 (see below for more details).

Using stdexec

Requirements

stdexec requires compiling with C++20 (-std=c++20) but otherwise does not have any dependencies and only requires a sufficiently new compiler:

• clang 13+
• gcc 11+
• nvc++ 22.11+ (required for GPU support). If using stdexec from GitHub, then nvc++ 23.3+ is required.

How you configure your environment to use stdexec depends on how you got stdexec.

NVHPC SDK

Starting with the 22.11 release of the NVHPC SDK, stdexec is available as an experimental, opt-in feature. Specifying the --experimental-stdpar flag to nvc++ makes the stdexec headers available on the include path. You can then include any stdexec header as normal: #include <stdexec/...>, #include <nvexec/...>. See godbolt example.

GPU features additionally require specifying -stdpar=gpu. For more details, see GPU Support.

GitHub

As a header-only C++ library, technically all one needs to do is add the stdexec include/ directory to your include path as -I<stdexec root>/include in addition to specifying any necessary compile options.

For simplicity, we recommend using the CMake targets that stdexec provides as they encapsulate the necessary configuration.

cmake

If your project uses CMake, then after cloning stdexec simply add the following to your CMakeLists.txt:

add_subdirectory(<stdexec root>)

This will make the STDEXEC::stdexec target available to link with your project:

target_link_libraries(my_project PRIVATE STDEXEC::stdexec)

This target encapsulates all of the necessary configuration and compiler flags for using stdexec.

CMake Package Manager (CPM)

To further simplify obtaining and including stdexec in your CMake project, we recommend using CMake Package Manager (CPM) to fetch and configure stdexec.

Complete example:

cmake_minimum_required(VERSION 3.14 FATAL_ERROR)

project(stdexecExample)

# Get CPM
# For more information on how to add CPM to your project, see: https://github.com/cpm-cmake/CPM.cmake#adding-cpm
include(CPM.cmake)

CPMAddPackage(
  NAME stdexec
  GITHUB_REPOSITORY NVIDIA/stdexec
  GIT_TAG main # This will always pull the latest code from the `main` branch. You may also use a specific release version or tag
)

add_executable(main example.cpp)

target_link_libraries(main STDEXEC::stdexec)

GPU Support

stdexec provides schedulers that enable execution on NVIDIA GPUs:

• nvexec::stream_scheduler
  • Single GPU scheduler that executes on the first available GPU (device 0)
  • Defined in <nvexec/stream_context.cuh>
• nvexec::multi_gpu_stream_scheduler
  • Executes on all visible GPUs
  • Defined in <nvexec/multi_gpu_context.cuh>

These schedulers are only supported when using the nvc++ compiler with -stdpar=gpu.

Example: https://godbolt.org/z/4cEMqY8r9

Building

stdexec is a header-only library and does not require building anything.

This section is only relevant if you wish to build the stdexec tests or examples.

The following tools are needed:

• CMake
• One of the following supported C++ compilers:
  • GCC 11+
  • clang 12+
  • nvc++ 22.11 (nvc++ 23.3+ for stdexec from GitHub)

Perform the following actions:

# Configure the project
cmake -S . -B build -G<gen>
# Build the project
cmake --build build

Here, <gen> can be Ninja, "Unix Makefiles", XCode, "Visual Studio 15 Win64", etc.

Specifying the compiler

You can set the C++ compiler via -D CMAKE_CXX_COMPILER:

# Use GCC:
cmake -S . -B build/g++ -DCMAKE_CXX_COMPILER=$(which g++)
cmake --build build/g++

# Or clang:
cmake -S . -B build/clang++ -DCMAKE_CXX_COMPILER=$(which clang++)
cmake --build build/clang++

Specifying the stdlib

If you want to use libc++ with clang instead of libstdc++, you can specify the standard library as follows:

# Do the actual build
cmake -S . -B build/clang++ -G<gen> \
    -DCMAKE_CXX_FLAGS=-stdlib=libc++ \
    -DCMAKE_CXX_COMPILER=$(which clang++)

cmake --build build/clang++

Tooling

For users of VSCode, stdexec provides a VSCode extension that colorizes compiler output. The highlighter recognizes the diagnostics generated by the stdexec library, styling them to make them easier to pick out. Details about how to configure the extension can be found here.