#include <pthread.h> int pthread_cond_destroy(pthread_cond_t *cond); int pthread_cond_init(pthread_cond_t *restrict cond, const pthread_condattr_t *restrict attr); pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
It shall be safe to destroy an initialized condition variable upon which no threads are currently blocked. Attempting to destroy a condition variable upon which other threads are currently blocked results in undefined behavior.
The pthread_cond_init() function shall initialize the condition variable referenced by cond with attributes referenced by attr. If attr is NULL, the default condition variable attributes shall be used; the effect is the same as passing the address of a default condition variable attributes object. Upon successful initialization, the state of the condition variable shall become initialized.
See Section 2.9.9, Synchronization Object Copies and Alternative Mappings for further requirements.
Attempting to initialize an already initialized condition variable results in undefined behavior.
In cases where default condition variable attributes are appropriate, the macro PTHREAD_COND_INITIALIZER can be used to initialize condition variables. The effect shall be equivalent to dynamic initialization by a call to pthread_cond_init() with parameter attr specified as NULL, except that no error checks are performed.
The behavior is undefined if the value specified by the cond argument to pthread_cond_destroy() does not refer to an initialized condition variable.
The behavior is undefined if the value specified by the attr argument to pthread_cond_init() does not refer to an initialized condition variable attributes object.
These functions shall not return an error code of [EINTR].
The following sections are informative.
struct list {
pthread_mutex_t lm;
...
}
struct elt {
key k;
int busy;
pthread_cond_t notbusy;
...
}
/* Find a list element and reserve it. */
struct elt *
list_find(struct list *lp, key k)
{
struct elt *ep;
pthread_mutex_lock(&lp->lm);
while ((ep = find_elt(l, k) != NULL) && ep->busy)
pthread_cond_wait(&ep->notbusy, &lp->lm);
if (ep != NULL)
ep->busy = 1;
pthread_mutex_unlock(&lp->lm);
return(ep);
}
delete_elt(struct list *lp, struct elt *ep)
{
pthread_mutex_lock(&lp->lm);
assert(ep->busy);
... remove ep from list ...
ep->busy = 0; /* Paranoid. */
(A) pthread_cond_broadcast(&ep->notbusy);
pthread_mutex_unlock(&lp->lm);
(B) pthread_cond_destroy(&ep->notbusy);
free(ep);
}
In this example, the condition variable and its list element may be freed (line B) immediately after all threads waiting for it are awakened (line A), since the mutex and the code ensure that no other thread can touch the element to be deleted.
If an implementation detects that the value specified by the cond argument to pthread_cond_destroy() or pthread_cond_init() refers to a condition variable that is in use (for example, in a pthread_cond_wait() call) by another thread, or detects that the value specified by the cond argument to pthread_cond_init() refers to an already initialized condition variable, it is recommended that the function should fail and report an [EBUSY] error.
If an implementation detects that the value specified by the attr argument to pthread_cond_init() does not refer to an initialized condition variable attributes object, it is recommended that the function should fail and report an [EINVAL] error.
See also pthread_mutex_destroy().
The Base Definitions volume of POSIX.1-2017, <pthread.h>
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