pthread_cond_init (3p) - Linux Manuals

pthread_cond_init: destroy and initialize condition

PROLOG

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

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.

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 .