* FAQ: updated FAQ with config file issues.
[vlc.git] / src / misc / threads.c
1 /*****************************************************************************
2  * threads.c : threads implementation for the VideoLAN client
3  *****************************************************************************
4  * Copyright (C) 1999, 2000, 2001, 2002 VideoLAN
5  * $Id: threads.c,v 1.19 2002/10/03 17:01:58 gbazin Exp $
6  *
7  * Authors: Jean-Marc Dressler <polux@via.ecp.fr>
8  *          Samuel Hocevar <sam@zoy.org>
9  *          Gildas Bazin <gbazin@netcourrier.com>
10  *
11  * This program is free software; you can redistribute it and/or modify
12  * it under the terms of the GNU General Public License as published by
13  * the Free Software Foundation; either version 2 of the License, or
14  * (at your option) any later version.
15  * 
16  * This program is distributed in the hope that it will be useful,
17  * but WITHOUT ANY WARRANTY; without even the implied warranty of
18  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
19  * GNU General Public License for more details.
20  *
21  * You should have received a copy of the GNU General Public License
22  * along with this program; if not, write to the Free Software
23  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111, USA.
24  *****************************************************************************/
25
26 #include <vlc/vlc.h>
27
28 #define VLC_THREADS_UNINITIALIZED  0
29 #define VLC_THREADS_PENDING        1
30 #define VLC_THREADS_ERROR          2
31 #define VLC_THREADS_READY          3
32
33 /*****************************************************************************
34  * Global mutex for lazy initialization of the threads system
35  *****************************************************************************/
36 static volatile int i_initializations = 0;
37
38 #if defined( PTH_INIT_IN_PTH_H )
39 #elif defined( ST_INIT_IN_ST_H )
40 #elif defined( WIN32 )
41 #elif defined( PTHREAD_COND_T_IN_PTHREAD_H )
42     static pthread_mutex_t once_mutex = PTHREAD_MUTEX_INITIALIZER;
43 #elif defined( HAVE_CTHREADS_H )
44 #elif defined( HAVE_KERNEL_SCHEDULER_H )
45 #endif
46
47 /*****************************************************************************
48  * vlc_threads_init: initialize threads system
49  *****************************************************************************
50  * This function requires lazy initialization of a global lock in order to
51  * keep the library really thread-safe. Some architectures don't support this
52  * and thus do not guarantee the complete reentrancy.
53  *****************************************************************************/
54 int __vlc_threads_init( vlc_object_t *p_this )
55 {
56     static volatile int i_status = VLC_THREADS_UNINITIALIZED;
57
58     libvlc_t *p_libvlc = (libvlc_t *)p_this;
59     int i_ret = VLC_SUCCESS;
60
61     /* If we have lazy mutex initialization, use it. Otherwise, we just
62      * hope nothing wrong happens. */
63 #if defined( PTH_INIT_IN_PTH_H )
64 #elif defined( ST_INIT_IN_ST_H )
65 #elif defined( WIN32 )
66     HINSTANCE hInstLib;
67 #elif defined( PTHREAD_COND_T_IN_PTHREAD_H )
68     pthread_mutex_lock( &once_mutex );
69 #elif defined( HAVE_CTHREADS_H )
70 #elif defined( HAVE_KERNEL_SCHEDULER_H )
71 #endif
72
73     if( i_status == VLC_THREADS_UNINITIALIZED )
74     {
75         i_status = VLC_THREADS_PENDING;
76
77         /* We should be safe now. Do all the initialization stuff we want. */
78         vlc_object_create( p_libvlc, VLC_OBJECT_ROOT );
79         p_libvlc->b_ready = VLC_FALSE;
80
81 #if defined( PTH_INIT_IN_PTH_H )
82         i_ret = pth_init();
83
84 #elif defined( ST_INIT_IN_ST_H )
85         i_ret = st_init();
86
87 #elif defined( WIN32 )
88         /* Dynamically get the address of SignalObjectAndWait */
89         if( GetVersion() < 0x80000000 )
90         {
91             /* We are running on NT/2K/XP, we can use SignalObjectAndWait */
92             hInstLib = LoadLibrary( "kernel32" );
93             if( hInstLib )
94             {
95                 p_libvlc->SignalObjectAndWait =
96                     (SIGNALOBJECTANDWAIT)GetProcAddress( hInstLib,
97                                                      "SignalObjectAndWait" );
98             }
99         }
100         else
101         {
102             p_libvlc->SignalObjectAndWait = NULL;
103         }
104
105         p_libvlc->b_fast_mutex = 0;
106         p_libvlc->i_win9x_cv = 0;
107
108 #elif defined( PTHREAD_COND_T_IN_PTHREAD_H )
109 #elif defined( HAVE_CTHREADS_H )
110 #elif defined( HAVE_KERNEL_SCHEDULER_H )
111 #endif
112
113         vlc_mutex_init( p_libvlc, &p_libvlc->global_lock );
114
115         if( i_ret )
116         {
117             i_status = VLC_THREADS_ERROR;
118         }
119         else
120         {
121             i_initializations++;
122             i_status = VLC_THREADS_READY;
123         }
124     }
125     else
126     {
127         /* Just increment the initialization count */
128         i_initializations++;
129     }
130
131     /* If we have lazy mutex initialization support, unlock the mutex;
132      * otherwize, do a naive wait loop. */
133 #if defined( PTH_INIT_IN_PTH_H )
134     while( i_status == VLC_THREADS_PENDING ) msleep( THREAD_SLEEP );
135 #elif defined( ST_INIT_IN_ST_H )
136     while( i_status == VLC_THREADS_PENDING ) msleep( THREAD_SLEEP );
137 #elif defined( WIN32 )
138     while( i_status == VLC_THREADS_PENDING ) msleep( THREAD_SLEEP );
139 #elif defined( PTHREAD_COND_T_IN_PTHREAD_H )
140     pthread_mutex_unlock( &once_mutex );
141 #elif defined( HAVE_CTHREADS_H )
142     while( i_status == VLC_THREADS_PENDING ) msleep( THREAD_SLEEP );
143 #elif defined( HAVE_KERNEL_SCHEDULER_H )
144     while( i_status == VLC_THREADS_PENDING ) msleep( THREAD_SLEEP );
145 #endif
146
147     if( i_status != VLC_THREADS_READY )
148     {
149         return VLC_ETHREAD;
150     }
151
152     return i_ret;
153 }
154
155 /*****************************************************************************
156  * vlc_threads_end: stop threads system
157  *****************************************************************************
158  * FIXME: This function is far from being threadsafe. We should undo exactly
159  * what we did above in vlc_threads_init.
160  *****************************************************************************/
161 int __vlc_threads_end( vlc_object_t *p_this )
162 {
163 #if defined( PTH_INIT_IN_PTH_H )
164     i_initializations--;
165     if( i_initializations == 0 )
166     {
167         return pth_kill();
168     }
169
170 #elif defined( ST_INIT_IN_ST_H )
171     i_initializations--;
172
173 #elif defined( WIN32 )
174     i_initializations--;
175
176 #elif defined( PTHREAD_COND_T_IN_PTHREAD_H )
177     pthread_mutex_lock( &once_mutex );
178     i_initializations--;
179     pthread_mutex_unlock( &once_mutex );
180
181 #elif defined( HAVE_CTHREADS_H )
182     i_initializations--;
183
184 #elif defined( HAVE_KERNEL_SCHEDULER_H )
185     i_initializations--;
186
187 #endif
188     return VLC_SUCCESS;
189 }
190
191 /*****************************************************************************
192  * Prototype for GPROF wrapper
193  *****************************************************************************/
194 #ifdef GPROF
195 /* Wrapper function for profiling */
196 static void *      vlc_thread_wrapper ( void *p_wrapper );
197
198 #   ifdef WIN32
199
200 #       define ITIMER_REAL 1
201 #       define ITIMER_PROF 2
202
203 struct itimerval
204 {
205     struct timeval it_value;
206     struct timeval it_interval;
207 };  
208
209 int setitimer(int kind, const struct itimerval* itnew, struct itimerval* itold);
210 #   endif /* WIN32 */
211
212 typedef struct wrapper_t
213 {
214     /* Data lock access */
215     vlc_mutex_t lock;
216     vlc_cond_t  wait;
217     
218     /* Data used to spawn the real thread */
219     vlc_thread_func_t func;
220     void *p_data;
221     
222     /* Profiling timer passed to the thread */
223     struct itimerval itimer;
224     
225 } wrapper_t;
226
227 #endif /* GPROF */
228
229 /*****************************************************************************
230  * vlc_mutex_init: initialize a mutex
231  *****************************************************************************/
232 int __vlc_mutex_init( vlc_object_t *p_this, vlc_mutex_t *p_mutex )
233 {
234     p_mutex->p_this = p_this;
235
236 #if defined( PTH_INIT_IN_PTH_H )
237     return pth_mutex_init( &p_mutex->mutex );
238
239 #elif defined( ST_INIT_IN_ST_H )
240     p_mutex->mutex = st_mutex_new();
241     return ( p_mutex->mutex == NULL ) ? errno : 0;
242
243 #elif defined( WIN32 )
244     /* We use mutexes on WinNT/2K/XP because we can use the SignalObjectAndWait
245      * function and have a 100% correct vlc_cond_wait() implementation.
246      * As this function is not available on Win9x, we can use the faster
247      * CriticalSections */
248     if( p_this->p_vlc->SignalObjectAndWait && !p_this->p_vlc->b_fast_mutex )
249     {
250         /* We are running on NT/2K/XP, we can use SignalObjectAndWait */
251         p_mutex->mutex = CreateMutex( 0, FALSE, 0 );
252         return ( p_mutex->mutex != NULL ? 0 : 1 );
253     }
254     else
255     {
256         p_mutex->mutex = NULL;
257         InitializeCriticalSection( &p_mutex->csection );
258         return 0;
259     }
260
261 #elif defined( PTHREAD_COND_T_IN_PTHREAD_H )
262 #   if defined(DEBUG) && defined(SYS_LINUX)
263     {
264         /* Create error-checking mutex to detect problems more easily. */
265         pthread_mutexattr_t attr;
266         int                 i_result;
267
268         pthread_mutexattr_init( &attr );
269         pthread_mutexattr_setkind_np( &attr, PTHREAD_MUTEX_ERRORCHECK_NP );
270         i_result = pthread_mutex_init( &p_mutex->mutex, &attr );
271         pthread_mutexattr_destroy( &attr );
272         return( i_result );
273     }
274 #   endif
275     return pthread_mutex_init( &p_mutex->mutex, NULL );
276
277 #elif defined( HAVE_CTHREADS_H )
278     mutex_init( p_mutex );
279     return 0;
280
281 #elif defined( HAVE_KERNEL_SCHEDULER_H )
282     /* check the arguments and whether it's already been initialized */
283     if( p_mutex == NULL )
284     {
285         return B_BAD_VALUE;
286     }
287
288     if( p_mutex->init == 9999 )
289     {
290         return EALREADY;
291     }
292
293     p_mutex->lock = create_sem( 1, "BeMutex" );
294     if( p_mutex->lock < B_NO_ERROR )
295     {
296         return( -1 );
297     }
298
299     p_mutex->init = 9999;
300     return B_OK;
301
302 #endif
303 }
304
305 /*****************************************************************************
306  * vlc_mutex_destroy: destroy a mutex, inner version
307  *****************************************************************************/
308 int __vlc_mutex_destroy( char * psz_file, int i_line, vlc_mutex_t *p_mutex )
309 {
310     int i_result;
311     /* In case of error : */
312     int i_thread = -1;
313     const char * psz_error = "";
314
315 #if defined( PTH_INIT_IN_PTH_H )
316     return 0;
317
318 #elif defined( ST_INIT_IN_ST_H )
319     i_result = st_mutex_destroy( p_mutex->mutex );
320
321 #elif defined( WIN32 )
322     if( p_mutex->mutex )
323     {
324         CloseHandle( p_mutex->mutex );
325     }
326     else
327     {
328         DeleteCriticalSection( &p_mutex->csection );
329     }
330     return 0;
331
332 #elif defined( PTHREAD_COND_T_IN_PTHREAD_H )    
333     i_result = pthread_mutex_destroy( &p_mutex->mutex );
334     if ( i_result )
335     {
336         i_thread = (int)pthread_self();
337         psz_error = strerror(i_result);
338     }
339
340 #elif defined( HAVE_CTHREADS_H )
341     return 0;
342
343 #elif defined( HAVE_KERNEL_SCHEDULER_H )
344     if( p_mutex->init == 9999 )
345     {
346         delete_sem( p_mutex->lock );
347     }
348
349     p_mutex->init = 0;
350     return B_OK;
351 #endif    
352
353     if( i_result )
354     {
355         msg_Err( p_mutex->p_this,
356                  "thread %d: mutex_destroy failed at %s:%d (%d:%s)",
357                  i_thread, psz_file, i_line, i_result, psz_error );
358     }
359     return i_result;
360 }
361
362 /*****************************************************************************
363  * vlc_cond_init: initialize a condition
364  *****************************************************************************/
365 int __vlc_cond_init( vlc_object_t *p_this, vlc_cond_t *p_condvar )
366 {
367     p_condvar->p_this = p_this;
368
369 #if defined( PTH_INIT_IN_PTH_H )
370     return pth_cond_init( &p_condvar->cond );
371
372 #elif defined( ST_INIT_IN_ST_H )
373     p_condvar->cond = st_cond_new();
374     return ( p_condvar->cond == NULL ) ? errno : 0;
375
376 #elif defined( WIN32 )
377     /* Initialize counter */
378     p_condvar->i_waiting_threads = 0;
379
380     /* Misc init */
381     p_condvar->i_win9x_cv = p_this->p_vlc->i_win9x_cv;
382     p_condvar->SignalObjectAndWait = p_this->p_vlc->SignalObjectAndWait;
383
384     if( (p_condvar->SignalObjectAndWait && !p_this->p_vlc->b_fast_mutex)
385         || p_condvar->i_win9x_cv == 0 )
386     {
387         /* Create an auto-reset event. */
388         p_condvar->event = CreateEvent( NULL,   /* no security */
389                                         FALSE,  /* auto-reset event */
390                                         FALSE,  /* start non-signaled */
391                                         NULL ); /* unnamed */
392
393         p_condvar->semaphore = NULL;
394         return !p_condvar->event;
395     }
396     else
397     {
398         p_condvar->semaphore = CreateSemaphore( NULL,       /* no security */
399                                                 0,          /* initial count */
400                                                 0x7fffffff, /* max count */
401                                                 NULL );     /* unnamed */
402
403         if( p_this->p_vlc->i_win9x_cv == 1 )
404             /* Create a manual-reset event initially signaled. */
405             p_condvar->event = CreateEvent( NULL, TRUE, TRUE, NULL );
406         else
407             /* Create a auto-reset event. */
408             p_condvar->event = CreateEvent( NULL, FALSE, FALSE, NULL );
409
410         InitializeCriticalSection( &p_condvar->csection );
411
412         return !p_condvar->semaphore || !p_condvar->event;
413     }
414
415 #elif defined( PTHREAD_COND_T_IN_PTHREAD_H )
416     return pthread_cond_init( &p_condvar->cond, NULL );
417
418 #elif defined( HAVE_CTHREADS_H )
419     /* condition_init() */
420     spin_lock_init( &p_condvar->lock );
421     cthread_queue_init( &p_condvar->queue );
422     p_condvar->name = 0;
423     p_condvar->implications = 0;
424
425     return 0;
426
427 #elif defined( HAVE_KERNEL_SCHEDULER_H )
428     if( !p_condvar )
429     {
430         return B_BAD_VALUE;
431     }
432
433     if( p_condvar->init == 9999 )
434     {
435         return EALREADY;
436     }
437
438     p_condvar->thread = -1;
439     p_condvar->init = 9999;
440     return 0;
441 #endif
442 }
443
444 /*****************************************************************************
445  * vlc_cond_destroy: destroy a condition, inner version
446  *****************************************************************************/
447 int __vlc_cond_destroy( char * psz_file, int i_line, vlc_cond_t *p_condvar )
448 {
449     int i_result;
450     /* In case of error : */
451     int i_thread = -1;
452     const char * psz_error = "";
453
454 #if defined( PTH_INIT_IN_PTH_H )
455     return 0;
456
457 #elif defined( ST_INIT_IN_ST_H )
458     i_result = st_cond_destroy( p_condvar->cond );
459
460 #elif defined( WIN32 )
461     if( !p_condvar->semaphore )
462         i_result = !CloseHandle( p_condvar->event );
463     else
464         i_result = !CloseHandle( p_condvar->event )
465           || !CloseHandle( p_condvar->semaphore );
466
467 #elif defined( PTHREAD_COND_T_IN_PTHREAD_H )
468     i_result = pthread_cond_destroy( &p_condvar->cond );
469     if ( i_result )
470     {
471         i_thread = (int)pthread_self();
472         psz_error = strerror(i_result);
473     }
474
475 #elif defined( HAVE_CTHREADS_H )
476     return 0;
477
478 #elif defined( HAVE_KERNEL_SCHEDULER_H )
479     p_condvar->init = 0;
480     return 0;
481 #endif
482
483     if( i_result )
484     {
485         msg_Err( p_condvar->p_this,
486                  "thread %d: cond_destroy failed at %s:%d (%d:%s)",
487                  i_thread, psz_file, i_line, i_result, psz_error );
488     }
489     return i_result;
490 }
491
492 /*****************************************************************************
493  * vlc_thread_create: create a thread, inner version
494  *****************************************************************************
495  * Note that i_priority is only taken into account on platforms supporting
496  * userland real-time priority threads.
497  *****************************************************************************/
498 int __vlc_thread_create( vlc_object_t *p_this, char * psz_file, int i_line,
499                          char *psz_name, void * ( *func ) ( void * ),
500                          int i_priority, vlc_bool_t b_wait )
501 {
502     int i_ret;
503
504     vlc_mutex_lock( &p_this->object_lock );
505
506 #ifdef GPROF
507     wrapper_t wrapper;
508
509     /* Initialize the wrapper structure */
510     wrapper.func = func;
511     wrapper.p_data = (void *)p_this;
512     getitimer( ITIMER_PROF, &wrapper.itimer );
513     vlc_mutex_init( p_this, &wrapper.lock );
514     vlc_cond_init( p_this, &wrapper.wait );
515     vlc_mutex_lock( &wrapper.lock );
516
517     /* Alter user-passed data so that we call the wrapper instead
518      * of the real function */
519     p_data = &wrapper;
520     func = vlc_thread_wrapper;
521 #endif
522
523 #if defined( PTH_INIT_IN_PTH_H )
524     p_this->thread_id = pth_spawn( PTH_ATTR_DEFAULT, func, (void *)p_this );
525     i_ret = 0;
526
527 #elif defined( ST_INIT_IN_ST_H )
528     p_this->thread_id = st_thread_create( func, (void *)p_this, 1, 0 );
529     i_ret = 0;
530     
531 #elif defined( WIN32 )
532     {
533         unsigned threadID;
534         /* When using the MSVCRT C library you have to use the _beginthreadex
535          * function instead of CreateThread, otherwise you'll end up with
536          * memory leaks and the signal functions not working */
537         p_this->thread_id =
538                 (HANDLE)_beginthreadex( NULL, 0, (PTHREAD_START) func, 
539                                         (void *)p_this, 0, &threadID );
540     }
541
542     if ( p_this->thread_id && i_priority )
543     {
544         if ( !SetThreadPriority(p_this->thread_id, i_priority) )
545         {
546             msg_Warn( p_this, "couldn't set a faster priority" );
547             i_priority = 0;
548         }
549     }
550
551     i_ret = ( p_this->thread_id ? 0 : 1 );
552
553 #elif defined( PTHREAD_COND_T_IN_PTHREAD_H )
554     i_ret = pthread_create( &p_this->thread_id, NULL, func, (void *)p_this );
555
556     if ( i_priority )
557     {
558         struct sched_param param;
559         memset( &param, 0, sizeof(struct sched_param) );
560         param.sched_priority = i_priority;
561         if ( pthread_setschedparam( p_this->thread_id, SCHED_RR, &param ) )
562         {
563             msg_Warn( p_this, "couldn't go to real-time priority" );
564             i_priority = 0;
565         }
566     }
567
568 #elif defined( HAVE_CTHREADS_H )
569     p_this->thread_id = cthread_fork( (cthread_fn_t)func, (any_t)p_this );
570     i_ret = 0;
571
572 #elif defined( HAVE_KERNEL_SCHEDULER_H )
573     p_this->thread_id = spawn_thread( (thread_func)func, psz_name,
574                                       B_NORMAL_PRIORITY, (void *)p_this );
575     i_ret = resume_thread( p_this->thread_id );
576
577 #endif
578
579 #ifdef GPROF
580     if( i_ret == 0 )
581     {
582         vlc_cond_wait( &wrapper.wait, &wrapper.lock );
583     }
584
585     vlc_mutex_unlock( &wrapper.lock );
586     vlc_mutex_destroy( &wrapper.lock );
587     vlc_cond_destroy( &wrapper.wait );
588 #endif
589
590     if( i_ret == 0 )
591     {
592         if( b_wait )
593         {
594             msg_Dbg( p_this, "waiting for thread completion" );
595             vlc_cond_wait( &p_this->object_wait, &p_this->object_lock );
596         }
597
598         p_this->b_thread = 1;
599
600         msg_Dbg( p_this, "thread %d (%s) created at priority %d (%s:%d)",
601                  p_this->thread_id, psz_name, i_priority,
602                  psz_file, i_line );
603
604         vlc_mutex_unlock( &p_this->object_lock );
605     }
606     else
607     {
608         msg_Err( p_this, "%s thread could not be created at %s:%d (%s)",
609                          psz_name, psz_file, i_line, strerror(i_ret) );
610         vlc_mutex_unlock( &p_this->object_lock );
611     }
612
613     return i_ret;
614 }
615
616 /*****************************************************************************
617  * vlc_thread_ready: tell the parent thread we were successfully spawned
618  *****************************************************************************/
619 void __vlc_thread_ready( vlc_object_t *p_this )
620 {
621     vlc_mutex_lock( &p_this->object_lock );
622     vlc_cond_signal( &p_this->object_wait );
623     vlc_mutex_unlock( &p_this->object_lock );
624 }
625
626 /*****************************************************************************
627  * vlc_thread_join: wait until a thread exits, inner version
628  *****************************************************************************/
629 void __vlc_thread_join( vlc_object_t *p_this, char * psz_file, int i_line )
630 {
631     int i_ret = 0;
632
633 #if defined( PTH_INIT_IN_PTH_H )
634     i_ret = pth_join( p_this->thread_id, NULL );
635
636 #elif defined( ST_INIT_IN_ST_H )
637     i_ret = st_thread_join( p_this->thread_id, NULL );
638     
639 #elif defined( WIN32 )
640     WaitForSingleObject( p_this->thread_id, INFINITE );
641
642 #elif defined( PTHREAD_COND_T_IN_PTHREAD_H )
643     i_ret = pthread_join( p_this->thread_id, NULL );
644
645 #elif defined( HAVE_CTHREADS_H )
646     cthread_join( p_this->thread_id );
647     i_ret = 1;
648
649 #elif defined( HAVE_KERNEL_SCHEDULER_H )
650     int32 exit_value;
651     wait_for_thread( p_this->thread_id, &exit_value );
652
653 #endif
654
655     if( i_ret )
656     {
657         msg_Err( p_this, "thread_join(%d) failed at %s:%d (%s)",
658                          p_this->thread_id, psz_file, i_line, strerror(i_ret) );
659     }
660     else
661     {
662         msg_Dbg( p_this, "thread %d joined (%s:%d)",
663                          p_this->thread_id, psz_file, i_line );
664     }
665
666     p_this->b_thread = 0;
667 }
668
669 /*****************************************************************************
670  * vlc_thread_wrapper: wrapper around thread functions used when profiling.
671  *****************************************************************************/
672 #ifdef GPROF
673 static void *vlc_thread_wrapper( void *p_wrapper )
674 {
675     /* Put user data in thread-local variables */
676     void *            p_data = ((wrapper_t*)p_wrapper)->p_data;
677     vlc_thread_func_t func   = ((wrapper_t*)p_wrapper)->func;
678
679     /* Set the profile timer value */
680     setitimer( ITIMER_PROF, &((wrapper_t*)p_wrapper)->itimer, NULL );
681
682     /* Tell the calling thread that we don't need its data anymore */
683     vlc_mutex_lock( &((wrapper_t*)p_wrapper)->lock );
684     vlc_cond_signal( &((wrapper_t*)p_wrapper)->wait );
685     vlc_mutex_unlock( &((wrapper_t*)p_wrapper)->lock );
686
687     /* Call the real function */
688     return func( p_data );
689 }
690 #endif