NAME
pthread_cond_destroy, pthread_cond_init - destroy and initialize
condition 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,
uninitialized. 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
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.
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_broadcast(), 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
parameter attr specified as NULL, except that no error checks are
performed.
RETURN VALUE
If successful, the pthread_cond_destroy() and pthread_cond_init()
functions 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
following 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
awakened (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 ALSO
pthread_cond_broadcast() , pthread_cond_signal() ,
pthread_cond_timedwait() , 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 .