Structured concurency

Mircea Baja - 7 June 2025

# Structured concurency

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---

# Structured programming

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[Nathaniel J. Smith: Notes on structured concurrency, or: Go statement
considered
harmful](https://vorpus.org/blog/notes-on-structured-concurrency-or-go-statement-considered-harmful/)

- `goto` corresponds to low level assembly `jump` instructions
- ends in spagetti code: graphs that do not obviously reduce to sequential
  (i.e. linear)
- replaced with constructs that reduce to sequential
- in modern languages even `goto` is tamed (only jumps within the same
  function)
  - but kept for the three cases where it's needed once every 20 years or so

---

# Low level concurrency APIs

```cpp
int WSAAPI WSARecv(
  [in]      SOCKET                             s,
  [in, out] LPWSABUF                           lpBuffers,
  [in]      DWORD                              dwBufferCount,
  [out]     LPDWORD                            lpNumberOfBytesRecvd,
  [in, out] LPDWORD                            lpFlags,
  [in]      LPWSAOVERLAPPED                    lpOverlapped,
  [in]      LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
);
```

- Can be called with a pointer to the completion routine (a callback)
- Until that routine is called (or cancelled) the socket, buffer, overlapped
  structure (often a derived type holding more data)
  - need to be kept alive
  - they might be accessed during or after the call to `WSARecv`

---

# Unstructured concurrency

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[Nathaniel J. Smith: Notes on structured concurrency, or: Go statement
considered
harmful](https://vorpus.org/blog/notes-on-structured-concurrency-or-go-statement-considered-harmful/)

Low level C APIs expose a detached behaviour which on one side is necessary,
like `jump` statements in assembly, but we want that wrapped in structured
concurrency primitives that are easier to use correctly.

Easy to say, but what are some examples of structured concurrency primitives?

---

# An example: wait_any

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- we started three tasks
- the second completed first
- the other two are cancelled
- continues when all three are completed (cancelled or otherwise)
- returns the value of winner (second task in this case)

---

- continuation
- chains
- sane principles
  - a child coroutine does not outlives its parent
  - a coroutine runs uninterrupted until a co_await
- discuss issue with reference parameters, lambda captures, pointers, string_views
  - the generator problems with scope
- discuss the issue of detach/fire and forget
  - sharing data e.g. by value
  - lifetime
    - discuss bad advice that uses detached models
    - https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rcoro-capture
    - more bad advice https://devblogs.microsoft.com/oldnewthing/20211103-00/?p=105870
    - though they really like detached in embedded environments, explain why
  - error propagation
- sync start a chain
- wait_any, wait_all
- wait_for
  - stop_when (e.g. on user interrupt instead of timer)
- nursery

---

# Questions?