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From: Alexander Terekhov (terekhov_at_[hidden])
Date: 2002-10-18 14:04:30
David Abrahams wrote:
>
> * On condition.html
>
> a. What is the effective difference between notify_one() and
> notify_all()? In the absence of any information about the
> underlying system's scheduling mechanism, it seems as though there
> is none: either way, one blocked thread will get to execute with
> the mutex locked. If there is a difference between the two in
> practice, the documentation should give some indication of it.
A) http://www.opengroup.org/onlinepubs/007904975/functions/pthread_cond_signal.html
(don't miss "RATIONALE" bits)
B) http://www.linux.org.uk/~ajh/ols2002_proceedings.pdf.gz
(Acrobat: "479 of 631", read on "futexes")
C) consider the following .cpp that might help you [as usual,
modulo bugs and any thing(s) that I've myself missed and/
or misunderstood] understand how it is all meant to work.
//=======================================================
// DESIGN-futex-CV.cpp DRAFT <mailto:terekhov_at_[hidden]>
//=======================================================
#include <cerrno>
#include <cassert>
#include <climits>
#include <pthread.h>
//*** ES aside for a moment...
//*** pthread mutex wrapper
class mutex {
//*** unimplemented since it's non-copyable/non-copy-constructible
mutex( const mutex& );
mutex& operator=( const mutex& );
pthread_mutex_t m_mutex;
public:
//*** RAII scoped locking guard
class guard {
//*** unimplemented since it's non-copyable/non-copy-constructible
guard( const guard& );
guard& operator=( const guard& );
mutex& m_mutex;
public:
guard( mutex& mtx ) : m_mutex(mtx) {
m_mutex.lock();
}
~guard() {
m_mutex.unlock();
}
mutex& mutex_ref() { return m_mutex; }
pthread_mutex_t& c_mutex_ref() { return m_mutex.c_mutex(); }
}; //*** class mutex::guard
//*** RAII scoped "unlocking"/release guard
class release_guard {
//*** unimplemented since it's non-copyable/non-copy-constructible
release_guard( const release_guard& );
release_guard& operator=( const release_guard& );
mutex& m_mutex;
public:
release_guard( mutex::guard& guard ) : m_mutex(guard.mutex_ref()) {
m_mutex.unlock();
}
~release_guard() {
m_mutex.lock();
}
mutex& mutex_ref() { return m_mutex; }
pthread_mutex_t& c_mutex_ref() { return m_mutex.c_mutex(); }
}; //*** class mutex::release_guard
mutex() {
int status = pthread_mutex_init( &m_mutex,0 );
assert( !status );
}
~mutex() {
int status = pthread_mutex_destroy( &m_mutex );
assert( !status );
}
void lock() {
int status = pthread_mutex_lock( &m_mutex );
assert( !status );
}
void unlock() {
int status = pthread_mutex_unlock( &m_mutex );
assert( !status );
}
pthread_mutex_t& c_mutex() { return m_mutex; }
}; //*** class mutex
//*** pthread condvar wrapper
class condvar {
//*** unimplemented since it's non-copyable/non-copy-constructible
condvar( const condvar& );
condvar& operator=( const condvar& );
pthread_cond_t m_condvar;
public:
condvar() {
int status = pthread_cond_init( &m_condvar,0 );
assert( !status );
}
~condvar() {
int status = pthread_cond_destroy( &m_condvar );
assert( !status );
}
void wait( mutex::guard& guard ) {
int status = pthread_cond_wait( &m_condvar,&guard.c_mutex_ref() );
assert( !status );
}
bool timedwait( mutex::guard& guard, const timespec& abstime ) {
int status = pthread_cond_timedwait(
&m_condvar,&guard.c_mutex_ref(),&abstime );
assert( !status || ETIMEDOUT == status );
return ETIMEDOUT == status;
}
void broadcast() {
int status = pthread_cond_broadcast( &m_condvar );
assert( !status );
}
}; //*** class condvar
//*** futex emulator
class futex {
//*** unimplemented since it's non-copyable/non-copy-constructible
futex( const futex& );
futex& operator=( const futex& );
//*** RAII helper [used inside {timed}wait() only]
struct waiter {
waiter( waiter*& queue ) : next(queue), pqueue(&queue) {
queue = this;
}
~waiter() {
if ( !is_reset() ) { // timedout/canceled
while ( *pqueue && this != *pqueue )
pqueue = &(*pqueue)->next;
assert( this == *pqueue );
*pqueue = next;
}
}
waiter* reset() {
waiter* w = next;
next = this;
return w;
}
bool is_reset() {
return this == next;
}
waiter* next;
waiter** pqueue;
}* m_queue;
int m_value;
mutex mutable m_mutex;
condvar m_condvar;
public:
futex() : m_queue(0), m_value(0), m_mutex(), m_condvar() {}
~futex() { assert( !m_queue ); }
operator int() const {
mutex::guard guard( m_mutex );
return m_value;
}
futex& operator=( int value ) {
mutex::guard guard( m_mutex );
m_value = value;
return *this;
}
futex& operator++() {
mutex::guard guard( m_mutex );
++m_value;
return *this;
}
futex& operator--() {
mutex::guard guard( m_mutex );
--m_value;
return *this;
}
void wait( int value ) {
mutex::guard guard( m_mutex );
if ( value == m_value ) {
waiter this_waiter( m_queue );
do { m_condvar.wait( guard ); }
while ( !this_waiter.is_reset() );
}
}
bool timedwait( int value, const timespec& abstime ) {
mutex::guard guard( m_mutex );
if ( value == m_value ) {
waiter this_waiter( m_queue );
do { if ( m_condvar.timedwait( guard,abstime ) ) return true; }
while ( !this_waiter.is_reset() );
}
return false;
}
void wake( unsigned int wakeups ) {
mutex::guard guard( m_mutex );
while ( m_queue && wakeups-- )
m_queue = m_queue->reset();
m_condvar.broadcast();
}
}; //*** class futex
//*** RAII helper
class cancel_off_guard {
//*** unimplemented since it's non-copyable/non-copy-constructible
cancel_off_guard( const cancel_off_guard& );
cancel_off_guard& operator=( const cancel_off_guard& );
int m_old_cancel_state;
public:
cancel_off_guard() {
int status = pthread_setcancelstate( PTHREAD_CANCEL_DISABLE,
&m_old_cancel_state );
assert( !status );
}
~cancel_off_guard() {
int status = pthread_setcancelstate( m_old_cancel_state,
&m_old_cancel_state );
assert( !status );
}
}; //*** class cancel_off_guard
//*** futex-based condvar implementation
class futex_condvar {
//*** unimplemented since it's non-copyable/non-copy-constructible
futex_condvar( const futex_condvar& );
futex_condvar& operator=( const futex_condvar& );
mutex m_mutex;
futex m_futex;
unsigned int m_wakeups;
unsigned int m_waiters[2];
//*** Less stress: MIN = INT_MIN and MAX = INT_MAX
enum CONSTS { MIN = -2, MAX = 1, ALL = UINT_MAX };
//*** End-Of-Cycle value
int EOC() const { return (m_futex < 0) ? -1 : MAX; }
//*** RAII helper [used inside {timed}wait() only]
struct waiter {
waiter( futex_condvar* fcv, mutex& mtx ) :
m_fcv(fcv), m_mtx(mtx), m_ftx(fcv->enter_wait( mtx )) { }
~waiter() { m_fcv->leave_wait( m_ftx ); m_mtx.lock(); }
int ftx() const { return m_ftx; }
futex_condvar* m_fcv;
mutex& m_mtx;
int m_ftx;
}; //*** struct futex_condvar::waiter
friend struct waiter; // calls enter_wait()/leave_wait()
int enter_wait( mutex& mtx ) {
mutex::guard guard( m_mutex );
mtx.unlock();
++m_waiters[ EOC() == m_futex ];
return m_futex;
}
void leave_wait( int ftx ) {
mutex::guard guard( m_mutex );
if ( ftx != m_futex ) {
assert( m_waiters[0] );
--m_waiters[0];
if ( 0 != m_wakeups &&
0 == --m_wakeups &&
EOC() == m_futex ) {
m_futex = ( m_futex < 0 ) ? 0 : MIN;
m_waiters[0] = m_wakeups = m_waiters[1];
m_waiters[1] = 0;
m_futex.wake( ALL );
}
}
else
--m_waiters[ EOC() == ftx ];
}
public:
futex_condvar() : m_mutex(), m_futex(), m_wakeups(0) {
m_waiters[0] = m_waiters[1] = 0;
}
~futex_condvar() {
mutex::guard guard( m_mutex );
assert( m_waiters[0] == m_wakeups );
while ( m_waiters[0] ) {
int ftx = m_futex = EOC();
mutex::release_guard release_guard( guard );
cancel_off_guard no_cancel;
m_futex.wait( ftx );
}
}
void wait( mutex& mtx ) {
waiter this_waiter( this, mtx );
m_futex.wait( this_waiter.ftx() );
}
bool timedwait( mutex& mtx, const timespec& abstime ) {
waiter this_waiter( this, mtx );
return m_futex.timedwait( this_waiter.ftx(), abstime );
}
void signal() {
unsigned int wakeups;
{
mutex::guard guard( m_mutex );
if ( 0 != (wakeups = m_waiters[0] > m_wakeups) ) {
if ( EOC() == ++m_futex ) {
wakeups = ALL;
m_wakeups = m_waiters[0];
}
else
++m_wakeups;
}
}
if ( wakeups )
m_futex.wake( wakeups );
}
void broadcast() {
bool wakeups;
{
mutex::guard guard( m_mutex );
if ( false != (wakeups = m_waiters[0] > m_wakeups) ) {
++m_futex;
m_wakeups = m_waiters[0];
}
}
if ( wakeups )
m_futex.wake( ALL );
}
}; //*** class futex_condvar
//*** yet another "tennis" (but with futex_condvars this time)
#include <ctime>
#include <iostream>
using namespace std;
enum GAME_STATE {
[snip; see my other message that should contain this code]
regards,
alexander.
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