PTHREAD_COND_DESTROY(P) POSIX Programmer's Manual PTHREAD_COND_DESTROY(P)NAME
pthread_cond_destroy, pthread_cond_init - destroy and initialize condi‐
tion variables
SYNOPSIS
#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;
DESCRIPTION
The pthread_cond_destroy() function shall destroy the given condition
variable specified by cond; the object becomes, in effect, uninitial‐
ized. An implementation may cause pthread_cond_destroy() to set the
object referenced by cond to an invalid value. A destroyed condition
variable object can be reinitialized using pthread_cond_init(); the
results of otherwise referencing the object after it has been destroyed
are undefined.
It shall be safe to destroy an initialized condition variable upon
which no threads are currently blocked. Attempting to destroy a condi‐
tion variable upon which other threads are currently blocked results in
undefined behavior.
The pthread_cond_init() function shall initialize the condition vari‐
able 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 vari‐
able attributes object. Upon successful initialization, the state of
the condition variable shall become initialized.
Only cond itself may be used for performing synchronization. The
result of referring to copies of cond in calls to pthread_cond_wait(),
pthread_cond_timedwait(), pthread_cond_signal(), pthread_cond_broad‐
cast(), and pthread_cond_destroy() is undefined.
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 that are statically allocated. The effect shall be equivalent
to dynamic initialization by a call to pthread_cond_init() with parame‐
ter attr specified as NULL, except that no error checks are performed.
RETURN VALUE
If successful, the pthread_cond_destroy() and pthread_cond_init() func‐
tions shall return zero; otherwise, an error number shall be returned
to indicate the error.
The [EBUSY] and [EINVAL] error checks, if implemented, shall act as if
they were performed immediately at the beginning of processing for the
function and caused an error return prior to modifying the state of the
condition variable specified by cond.
ERRORS
The pthread_cond_destroy() function may fail if:
EBUSY The implementation has detected an attempt to destroy the object
referenced by cond while it is referenced (for example, while
being used in a pthread_cond_wait() or pthread_cond_timedwait())
by another thread.
EINVAL The value specified by cond is invalid.
The pthread_cond_init() function shall fail if:
EAGAIN The system lacked the necessary resources (other than memory) to
initialize another condition variable.
ENOMEM Insufficient memory exists to initialize the condition variable.
The pthread_cond_init() function may fail if:
EBUSY The implementation has detected an attempt to reinitialize the
object referenced by cond, a previously initialized, but not yet
destroyed, condition variable.
EINVAL The value specified by attr is invalid.
These functions shall not return an error code of [EINTR].
The following sections are informative.
EXAMPLES
A condition variable can be destroyed immediately after all the threads
that are blocked on it are awakened. For example, consider the follow‐
ing code:
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(&rp->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 awak‐
ened (line A), since the mutex and the code ensure that no other thread
can touch the element to be deleted.
APPLICATION USAGE
None.
RATIONALE
See pthread_mutex_init() ; a similar rationale applies to condition
variables.
FUTURE DIRECTIONS
None.
SEE ALSOpthread_cond_broadcast() , pthread_cond_signal() , pthread_cond_timed‐
wait() , the Base Definitions volume of IEEE Std 1003.1-2001,
<pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 PTHREAD_COND_DESTROY(P)
