man-pages/man3p/pthread_cond_init.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "PTHREAD_COND_DESTROY" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" pthread_cond_destroy 
.SH NAME
pthread_cond_destroy, pthread_cond_init \- destroy and initialize condition
variables
.SH SYNOPSIS
.LP
\fB#include <pthread.h>
.br
.sp
int pthread_cond_destroy(pthread_cond_t *\fP\fIcond\fP\fB);
.br
int pthread_cond_init(pthread_cond_t *restrict\fP \fIcond\fP\fB,
.br
\ \ \ \ \ \  const pthread_condattr_t *restrict\fP \fIattr\fP\fB);
.br
pthread_cond_t\fP \fIcond\fP \fB= PTHREAD_COND_INITIALIZER; \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIpthread_cond_destroy\fP() function shall destroy the given
condition variable specified by \fIcond\fP; the object
becomes, in effect, uninitialized. An implementation may cause \fIpthread_cond_destroy\fP()
to set the object referenced by
\fIcond\fP to an invalid value. A destroyed condition variable object
can be reinitialized using \fIpthread_cond_init\fP(); the
results of otherwise referencing the object after it has been destroyed
are undefined.
.LP
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.
.LP
The \fIpthread_cond_init\fP() function shall initialize the condition
variable referenced by \fIcond\fP with attributes
referenced by \fIattr\fP. If \fIattr\fP 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.
.LP
Only \fIcond\fP itself may be used for performing synchronization.
The result of referring to copies of \fIcond\fP in calls to
\fIpthread_cond_wait\fP(), \fIpthread_cond_timedwait\fP(), \fIpthread_cond_signal\fP(),
\fIpthread_cond_broadcast\fP(), and \fIpthread_cond_destroy\fP() is
undefined.
.LP
Attempting to initialize an already initialized condition variable
results in undefined behavior.
.LP
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
\fIpthread_cond_init\fP() with parameter \fIattr\fP specified as NULL,
except that no error checks are performed.
.SH RETURN VALUE
.LP
If successful, the \fIpthread_cond_destroy\fP() and \fIpthread_cond_init\fP()
functions shall return zero; otherwise, an error
number shall be returned to indicate the error.
.LP
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
\fIcond\fP.
.SH ERRORS
.LP
The \fIpthread_cond_destroy\fP() function may fail if:
.TP 7
.B EBUSY
The implementation has detected an attempt to destroy the object referenced
by \fIcond\fP while it is referenced (for example,
while being used in a \fIpthread_cond_wait\fP() or \fIpthread_cond_timedwait\fP())
by another thread.
.TP 7
.B EINVAL
The value specified by \fIcond\fP is invalid.
.sp
.LP
The \fIpthread_cond_init\fP() function shall fail if:
.TP 7
.B EAGAIN
The system lacked the necessary resources (other than memory) to initialize
another condition variable.
.TP 7
.B ENOMEM
Insufficient memory exists to initialize the condition variable.
.sp
.LP
The \fIpthread_cond_init\fP() function may fail if:
.TP 7
.B EBUSY
The implementation has detected an attempt to reinitialize the object
referenced by \fIcond\fP, a previously initialized, but
not yet destroyed, condition variable.
.TP 7
.B EINVAL
The value specified by \fIattr\fP is invalid.
.sp
.LP
These functions shall not return an error code of [EINTR].
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
A condition variable can be destroyed immediately after all the threads
that are blocked on it are awakened. For example,
consider the following code:
.sp
.RS
.nf

\fBstruct list {
    pthread_mutex_t lm;
    ...
}
.sp

struct elt {
    key k;
    int busy;
    pthread_cond_t notbusy;
    ...
}
.sp

/* Find a list element and reserve it. */
struct elt *
list_find(struct list *lp, key k)
{
    struct elt *ep;
.sp

    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);
}
.sp

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);
}
\fP
.fi
.RE
.LP
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.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
See \fIpthread_mutex_init\fP() ; a similar rationale applies to condition
variables.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIpthread_cond_broadcast\fP() , \fIpthread_cond_signal\fP() , \fIpthread_cond_timedwait\fP()
, the Base Definitions volume of IEEE\ Std\ 1003.1-2001,
\fI<pthread.h>\fP
.SH 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 .