Updated: October 26, 2022

Wait on a condition variable, with a time limit


#include <threads.h>
#include <time.h>

int cnd_timedwait( cnd_t *cond,
                   mtx_t *mutex,
                   const struct timespec* ts );


The condition variable on which to block the thread.
The mutex associated with the condition variable.
A pointer to a timespec structure that specifies the absolute time at which the time-out is to expire.



Use the -l c option to qcc to link against this library. This library is usually included automatically.


The cnd_timedwait() function blocks the calling thread on the condition variable cond, and unlocks the associated mutex mutex. The calling thread must have locked mutex before waiting on the condition variable. Upon return from the function, the mutex is again locked and owned by the calling thread.

The calling thread is blocked until either another thread performs a signal or broadcast on the condition variable, the absolute time specified by ts has passed, a signal is delivered to the thread, or the thread is canceled (waiting on a condition variable is a cancellation point). In all cases, the thread reacquires the mutex before being unblocked.

Note: Don't use a recursive mutex with condition variables.

If a thread that's blocked on a condition variable is canceled, the thread reacquires the mutex that's guarding the condition variable, so that the thread's cleanup handlers run in the same state as the critical code before and after the call to this function. If some other thread owns the lock, the canceled thread blocks until the mutex is available.

  • Make sure that the thread's cleanup handlers unlock the mutex.
  • To specify a timeout of less than a second, obtain the current time using clock_gettime(), and then add the number of nanoseconds to the ts.tv_nsec field.
  • You can use nsec2timespec() to convert times in nanoseconds to a timespec structure (and timespec2nsec() to convert them back again).


Success, or the call was interrupted by a signal.
An error occurred.
The time specified by ts has passed.



Cancellation point Yes
Interrupt handler No
Signal handler Yes
Thread Yes