src/thread/generic/SDL_sysmutex.c

/*
    SDL - Simple DirectMedia Layer
    Copyright (C) 1997-2010 Sam Lantinga

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
    License as published by the Free Software Foundation; either
    version 2.1 of the License, or (at your option) any later version.

    This library is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

    Sam Lantinga
    slouken@libsdl.org
*/
#include "SDL_config.h"

/* An implementation of mutexes using semaphores */

#include "SDL_thread.h"
#include "SDL_systhread_c.h"


struct SDL_mutex
{
    int recursive;
    SDL_threadID owner;
    SDL_sem *sem;
};

/* Create a mutex */
SDL_mutex *
SDL_CreateMutex(void)
{
    SDL_mutex *mutex;

    /* Allocate mutex memory */
    mutex = (SDL_mutex *) SDL_malloc(sizeof(*mutex));
    if (mutex) {
        /* Create the mutex semaphore, with initial value 1 */
        mutex->sem = SDL_CreateSemaphore(1);
        mutex->recursive = 0;
        mutex->owner = 0;
        if (!mutex->sem) {
            SDL_free(mutex);
            mutex = NULL;
        }
    } else {
        SDL_OutOfMemory();
    }
    return mutex;
}

/* Free the mutex */
void
SDL_DestroyMutex(SDL_mutex * mutex)
{
    if (mutex) {
        if (mutex->sem) {
            SDL_DestroySemaphore(mutex->sem);
        }
        SDL_free(mutex);
    }
}

/* Lock the semaphore */
int
SDL_mutexP(SDL_mutex * mutex)
{
#if SDL_THREADS_DISABLED
    return 0;
#else
    SDL_threadID this_thread;

    if (mutex == NULL) {
        SDL_SetError("Passed a NULL mutex");
        return -1;
    }

    this_thread = SDL_ThreadID();
    if (mutex->owner == this_thread) {
        ++mutex->recursive;
    } else {
        /* The order of operations is important.
           We set the locking thread id after we obtain the lock
           so unlocks from other threads will fail.
         */
        SDL_SemWait(mutex->sem);
        mutex->owner = this_thread;
        mutex->recursive = 0;
    }

    return 0;
#endif /* SDL_THREADS_DISABLED */
}

/* Unlock the mutex */
int
SDL_mutexV(SDL_mutex * mutex)
{
#if SDL_THREADS_DISABLED
    return 0;
#else
    if (mutex == NULL) {
        SDL_SetError("Passed a NULL mutex");
        return -1;
    }

    /* If we don't own the mutex, we can't unlock it */
    if (SDL_ThreadID() != mutex->owner) {
        SDL_SetError("mutex not owned by this thread");
        return -1;
    }

    if (mutex->recursive) {
        --mutex->recursive;
    } else {
        /* The order of operations is important.
           First reset the owner so another thread doesn't lock
           the mutex and set the ownership before we reset it,
           then release the lock semaphore.
         */
        mutex->owner = 0;
        SDL_SemPost(mutex->sem);
    }
    return 0;
#endif /* SDL_THREADS_DISABLED */
}

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