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The wait(), wait_for(), and wait_until() member functions of the std::condition_variable class temporarily cede possession of a mutex so that other threads that may be requesting the mutex can proceed. These functions must always be called from code that is protected by locking a mutex. The waiting thread resumes execution only after it has been notified, generally as the result of the invocation of the notify_one() or notify_all() member functions invoked by another thread. The wait() function must be invoked from a loop that checks whether a condition predicate holds. A condition predicate is an expression constructed from the variables of a function that must be true for a thread to be allowed to continue execution. The thread pauses execution, via wait(), wait_for(), wait_until(), or some other mechanism, and is resumed later, presumably when the condition predicate is true and the thread is notified.

#include <condition_variable>
#include <mutex>
 
extern bool until_finish(void);
extern std::mutex m;
extern std::condition_variable condition;
 
void func(void) {
  std::unique_lock<std::mutex> lk(m);

  while (until_finish()) {  //* Predicate does not hold */.
    condition.wait(lk);
  }
 
  /*/ Resume when condition holds */.

  /*/ . . . */
}

The notification mechanism notifies the waiting thread and allows it to check its condition predicate. The invocation of notify_all() in another thread cannot precisely determine which waiting thread will be resumed. Condition predicate statements allow notified threads to determine whether they should resume upon receiving the notification. 

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Note that a condition predicate is typically the negation of the condition expression in the loop. In this noncompliant code example, the condition predicate for removing an element from a linked list is (list->next != NULLnullptr), whereas the condition expression for the while loop condition is (list->next == NULLnullptr).

This noncompliant code example nests the call to wait() inside an if block and consequently fails to check the condition predicate after the notification is received. If the notification was spurious or malicious, the thread would wake up prematurely.

#include <condition_variable>
#include <mutex>
 
struct node_t {
  void *node;
  struct node_t *next;
};
  
structstatic node_t list;
static std::mutex m;
static std::condition_variable condition;
  
void consume_list_element(voidstd::condition_variable &condition) {
  std::unique_lock<std::mutex> lk(m);
  
  if (list.next == NULL) {
    condition.wait(lk);
  }
 
  /*/ Proceed when condition holds */.
 
}

Compliant Solution (Explicit loop with predicate)

This compliant solution calls the wait() member function from within a while loop to check the condition both before and after the call to wait():

#include <condition_variable>
#include <mutex>
 
struct node_t {
  void *node;
  struct node_t *next;
};
  
structstatic node_t list;
static std::mutex m;
static std::condition_variable condition;
  
void consume_list_element(void) {
  std::unique_lock<std::mutex> lk(m);
  
  while (list.next == NULL) {
    condition.wait(lk);
  }
 
  /*/ Proceed when condition holds.
 
}

Compliant Solution (Implicit loop with lambda predicate)

The std::condition_variable::wait() function has an overload which accepts a function object representing the predicate. This overload of wait() behaves as if it were implemented as: while (!pred()) wait(lock);. This compliant solution uses a lambda as a predicate and passes it to the wait() function. Note that the predicate is expected to return true when it is safe to proceed, which reverses the predicate logic from the compliant solution using an explicit loop predicate.

#include <condition_variable>
#include <mutex>
 
struct node_t {
  void *node;
  struct node_t *next;
};
  
static node_t list;
static std::mutex m;
static std::condition_variable condition;
  
void consume_list_element() {
  std::unique_lock<std::mutex> lk(m);
 
  condition.wait(lk, []{ return !list.next; });
  // Proceed when condition holds.
 
}

Failure to enclose calls to the wait(), wait_for(), or wait_until() member functions inside a while loop can lead to indefinite blocking and denial of service (DoS).

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