pthread_once emulation...

pthread_once.c

// best version
// would like to do CreateEvent in static initializer; but that's not a constant expression.

#define PTHREAD_ONCE_INIT   { 0, 0  }

struct pthread_once {
	HANDLE event;
	volatile LONG inited;
};
typedef struct pthread_once pthread_once_t;

static inline int
pthread_once(pthread_once_t *once, void (*cb) (void) ) {
	if( !InterlockedExchange( &once->inited, 1 ) ) {
		once->event = CreateEvent( NULL, TRUE, FALSE, NULL );
		cb();
		SetEvent( once->event );
	}
	while( !once->event ) Sleep( 0 );
	DWORD rc = WaitForSingleObject( once->event, INFINITE );
	if( rc == WAIT_OBJECT_0 )
		return 0;
	return 1;
}

//-------------------------------------------------------------------
// interlocked-exchange/spin-wait version
// modifies pthread_once and initializer
struct pthread_once {
	volatile LONG inited;
	volatile LONG initDone;
};
typedef struct pthread_once pthread_once_t;

static inline BOOL CALLBACK
_pthread_once_win32_cb(PINIT_ONCE once, PVOID param, PVOID *context)
{
	void (*cb) (void) = param;
	cb();
	return TRUE;
}

static inline void wait( int *on ) {
	while( !on[0] ) Sleep(0);
}

#define pthread_once(o,cb) (                            \
	(( !InterlockedExchange( &((o)[0]).inited, 1 ) )?  \
		(cb(),                                  \
		((o)[0]).initDone = 1):0),                  \
	wait( &((o)[0]).initDone ),0                      \
)


//------------------------------------------------------
// simple one-time-runner macro; static is not allowed in expressions :(

#define pthread_once(o,cb) (                            \
	static int inited,                              \
	static volatile int initDone,                   \
	( !InterlockedExchange( &inited, 1 ) )?         \
		(cb(),                                  \
		initDone = 1):0                         \
	wait( &initDone ),0                             \
)

//-----------------------------------------------------------
// original method - non XP compatible.

struct pthread_once {
	INIT_ONCE once;
};
typedef struct pthread_once pthread_once_t;

static inline BOOL CALLBACK
_pthread_once_win32_cb(PINIT_ONCE once, PVOID param, PVOID *context)
{
	void (*cb) (void) = param;
	cb();
	return TRUE;
}

static inline int
pthread_once(pthread_once_t *once, void (*cb) (void))
{
	BOOL rc = InitOnceExecuteOnce(&once->once, _pthread_once_win32_cb, cb, NULL);
	if (rc == 0)
		return -1;
	else
		return 0;
}

Thanks for sharing this gist. Is the cast at line 78 allowed? I mean, can't data pointers and function pointers have different sizes in some architectures?

Well; as a consolation, one would expect code pointers to be less than or equal to data pointers (?)
A compiler with an issue like that would generate a warning probably on that line... casting data pointer to function pointer; but this is windows, so I don't really expect asymmetry?

Got it. I'm not very familiar with Windows, so I wasn't sure if that could even be an issue.