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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.
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.
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.
ERRORS
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.
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(&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.
APPLICATION USAGE
None.
RATIONALE
If an implementation detects that the value specified by the cond argument to
pthread_cond_destroy() does not refer to an initialized condition variable, it is
recommended that the function should fail and report an [EINVAL] error.
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().
FUTURE DIRECTIONS
None.
SEE ALSO
pthread_cond_broadcast(), pthread_cond_timedwait(), pthread_mutex_destroy()
The Base Definitions volume of POSIX.1‐2017, <pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .