cnd_timedwait() functions 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
cnd_broadcast() function invoked by another thread. The
cnd_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
cnd_timedwait(), 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
cnd_broadcast() 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
cnd_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
cnd_broadcast(), which notifies all threads. Notification using
cnd_broadcast() is frequently preferred over using
cnd_signal(). (See CON38-C. 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 != NULL), whereas the condition expression for the
while loop condition is
(list->next == NULL).
This noncompliant code example nests the
cnd_wait() function 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.
This compliant solution calls the
cnd_wait() function from within a
while loop to check the condition both before and after the call to
Failure to enclose calls to the
cnd_timedwait() functions inside a
while loop can lead to indefinite blocking and denial of service (DoS).
Inappropriate Call Outside Loop
Wrap functions that can spuriously wake up in a loop
|Polyspace Bug Finder
|CERT C: Rule CON36-C
|Checks for situations where functions that can spuriously wake up are not wrapped in loop (rule fully covered)
Key here (explains table format and definitions)
|CERT Oracle Secure Coding Standard for Java
|THI03-J. Always invoke wait() and await() methods inside a loop
|Prior to 2018-01-12: CERT: Unspecified Relationship
atomic_compare_exchange Generic Functions"