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_all() member functions invoked by another thread.
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_until(), or some other mechanism, and is resumed later, presumably when the condition predicate is true and the thread is notified.
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.
Noncompliant Code Example
This noncompliant code example monitors a linked list and assigns one thread to consume list elements when the list is nonempty.
This thread pauses execution using
wait() and resumes when notified, presumably when the list has elements to be consumed. It is possible for the thread to be notified even if the list is still empty, perhaps because the notifying thread used
notify_all(), which notifies all threads. Notification using
notify_all() is frequently preferred over using
notify_one(). (See CON55-CPP. Preserve thread safety and liveness when using condition variables for more information.)
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 != nullptr), whereas the condition expression for the
while loop condition is
(list->next == nullptr).
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.
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
Compliant Solution (Implicit loop with lambda predicate)
std::condition_variable::wait() function has an overloaded form that accepts a function object representing the predicate. This form 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. 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.
Failure to enclose calls to the
wait_until() member functions inside a
while loop can lead to indefinite blocking and denial of service (DoS).
Wrap functions that can spuriously wake up in a loop
|Polyspace Bug Finder|
|CERT C++: CON54-CPP||Checks for situations where functions that can spuriously wake up are not wrapped in loop|
|[ISO/IEC 9899:2011]||220.127.116.11, "The |