CODEWITHSUNDEEP
c++concurrency
WhatABeautifulWorld
WhatABeautifulWorld

Reputation: 3388

Will this be a deadlock?

From C++11 book on detector and reactor: Detector detects something then asks reactor to do the job:

// Detector:
std::condition_variable cv;
std::mutex m;
bool flag(false);

// ... detect event

{
  std::lock_guard<std::mutex> g(m);
  flag = true;
}
cv.notify_one();

On the other hand, reactor:

// Reactor
{
  std::unique_lock<std::mutex> lk(m);
  cv.wait(lk, [] {return flag;} )
  ... ...
}

My question is, if the reactor is triggered first and acquire a lock on the mutex m, will it be deadlock, as the reactor will be blocked on cv.wait and the detector can't get the lock for m?

Upvotes: 0

Views: 83

Answers (1)

Drew Dormann
Drew Dormann

Reputation: 63735

This is a common pattern and use for std::condition_variable.

The function std::condition_variable::wait:

  • Expects a locked unique_lock to be passed to it.
  • Immediately unlocks the unique_lock before waiting.
  • When the function "wakes", the unique_lock is locked again before exiting.

Hopefully, your book will go on to account for spurious wakes. Edit: ... which is not a concern for the predicate form you are using!

Upvotes: 1

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