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      1 /*
      2     SDL - Simple DirectMedia Layer
      3     Copyright (C) 1997-2012 Sam Lantinga
      4 
      5     This library is free software; you can redistribute it and/or
      6     modify it under the terms of the GNU Lesser General Public
      7     License as published by the Free Software Foundation; either
      8     version 2.1 of the License, or (at your option) any later version.
      9 
     10     This library is distributed in the hope that it will be useful,
     11     but WITHOUT ANY WARRANTY; without even the implied warranty of
     12     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     13     Lesser General Public License for more details.
     14 
     15     You should have received a copy of the GNU Lesser General Public
     16     License along with this library; if not, write to the Free Software
     17     Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
     18 
     19     Sam Lantinga
     20     slouken (at) libsdl.org
     21 */
     22 #include "SDL_config.h"
     23 
     24 /* An implementation of mutexes using semaphores */
     25 
     26 #include "SDL_thread.h"
     27 #include "SDL_systhread_c.h"
     28 
     29 
     30 struct SDL_mutex {
     31 	int recursive;
     32 	Uint32 owner;
     33 	SDL_sem *sem;
     34 };
     35 
     36 /* Create a mutex */
     37 SDL_mutex *SDL_CreateMutex(void)
     38 {
     39 	SDL_mutex *mutex;
     40 
     41 	/* Allocate mutex memory */
     42 	mutex = (SDL_mutex *)SDL_malloc(sizeof(*mutex));
     43 	if ( mutex ) {
     44 		/* Create the mutex semaphore, with initial value 1 */
     45 		mutex->sem = SDL_CreateSemaphore(1);
     46 		mutex->recursive = 0;
     47 		mutex->owner = 0;
     48 		if ( ! mutex->sem ) {
     49 			SDL_free(mutex);
     50 			mutex = NULL;
     51 		}
     52 	} else {
     53 		SDL_OutOfMemory();
     54 	}
     55 	return mutex;
     56 }
     57 
     58 /* Free the mutex */
     59 void SDL_DestroyMutex(SDL_mutex *mutex)
     60 {
     61 	if ( mutex ) {
     62 		if ( mutex->sem ) {
     63 			SDL_DestroySemaphore(mutex->sem);
     64 		}
     65 		SDL_free(mutex);
     66 	}
     67 }
     68 
     69 /* Lock the semaphore */
     70 int SDL_mutexP(SDL_mutex *mutex)
     71 {
     72 #if SDL_THREADS_DISABLED
     73 	return 0;
     74 #else
     75 	Uint32 this_thread;
     76 
     77 	if ( mutex == NULL ) {
     78 		SDL_SetError("Passed a NULL mutex");
     79 		return -1;
     80 	}
     81 
     82 	this_thread = SDL_ThreadID();
     83 	if ( mutex->owner == this_thread ) {
     84 		++mutex->recursive;
     85 	} else {
     86 		/* The order of operations is important.
     87 		   We set the locking thread id after we obtain the lock
     88 		   so unlocks from other threads will fail.
     89 		*/
     90 		SDL_SemWait(mutex->sem);
     91 		mutex->owner = this_thread;
     92 		mutex->recursive = 0;
     93 	}
     94 
     95 	return 0;
     96 #endif /* SDL_THREADS_DISABLED */
     97 }
     98 
     99 /* Unlock the mutex */
    100 int SDL_mutexV(SDL_mutex *mutex)
    101 {
    102 #if SDL_THREADS_DISABLED
    103 	return 0;
    104 #else
    105 	if ( mutex == NULL ) {
    106 		SDL_SetError("Passed a NULL mutex");
    107 		return -1;
    108 	}
    109 
    110 	/* If we don't own the mutex, we can't unlock it */
    111 	if ( SDL_ThreadID() != mutex->owner ) {
    112 		SDL_SetError("mutex not owned by this thread");
    113 		return -1;
    114 	}
    115 
    116 	if ( mutex->recursive ) {
    117 		--mutex->recursive;
    118 	} else {
    119 		/* The order of operations is important.
    120 		   First reset the owner so another thread doesn't lock
    121 		   the mutex and set the ownership before we reset it,
    122 		   then release the lock semaphore.
    123 		 */
    124 		mutex->owner = 0;
    125 		SDL_SemPost(mutex->sem);
    126 	}
    127 	return 0;
    128 #endif /* SDL_THREADS_DISABLED */
    129 }
    130