undefined reference to `android::Mutex::lock()'

轉自http://blog.csdn.net/keensword007/article/details/5720636

 

用NDK編譯時出現這么個錯誤undefined reference to `android::Mutex::lock()'

起初以為沒有鏈接必要的so，結果加上了  -lutils 也不行。所以google 了一下，發現有人遇到過此問題，如下：

 

察看了一下ndk中的STABLE-APIS.TXT文檔，上面有這樣一句：

 

Note that the Android C library includes support for pthread (<pthread.h>),

so "LOCAL_LIBS := -lpthread" is not needed. The same is true for real-time

extensions (-lrt on typical Linux distributions).

也就是說使用多線程，並不需要增加-lpthread編譯選項，看來解決問題還需要從源代碼入手。察看frameworks/base/include/utils/threads.h文件，關於Mutex有如下代碼片斷：

 

#if defined(HAVE_PTHREADS)

 

inline Mutex::Mutex() {

    pthread_mutex_init(&mMutex, NULL);

}

inline Mutex::Mutex(const char* name) {

    pthread_mutex_init(&mMutex, NULL);

}

inline Mutex::Mutex(int type, const char* name) {

    if (type == SHARED) {

        pthread_mutexattr_t attr;

        pthread_mutexattr_init(&attr);

        pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);

        pthread_mutex_init(&mMutex, &attr);

        pthread_mutexattr_destroy(&attr);

    } else {

        pthread_mutex_init(&mMutex, NULL);

    }

}

inline Mutex::~Mutex() {

    pthread_mutex_destroy(&mMutex);

}

inline status_t Mutex::lock() {

    return -pthread_mutex_lock(&mMutex);

}

inline void Mutex::unlock() {

    pthread_mutex_unlock(&mMutex);

}

inline status_t Mutex::tryLock() {

    return -pthread_mutex_trylock(&mMutex);

}

 

#endif // HAVE_PTHREADS

也就是說如果定義了HAVE_PTHREADS宏，就有Mutex的相關方法實現，如果沒有定義這個宏，Mutex的方法又是如何實現的，下面看看frameworks/base/libs/utils/Threads.cpp中的代碼：

#if defined(HAVE_PTHREADS)

// implemented as inlines in threads.h

#elif defined(HAVE_WIN32_THREADS)

Mutex::Mutex()

{

    HANDLE hMutex;

    assert(sizeof(hMutex) == sizeof(mState));

    hMutex = CreateMutex(NULL, FALSE, NULL);

    mState = (void*) hMutex;

}

Mutex::Mutex(const char* name)

{

    // XXX: name not used for now

    HANDLE hMutex;

    assert(sizeof(hMutex) == sizeof(mState));

    hMutex = CreateMutex(NULL, FALSE, NULL);

    mState = (void*) hMutex;

}

Mutex::Mutex(int type, const char* name)

{

    // XXX: type and name not used for now

    HANDLE hMutex;

    assert(sizeof(hMutex) == sizeof(mState));

    hMutex = CreateMutex(NULL, FALSE, NULL);

    mState = (void*) hMutex;

}

Mutex::~Mutex()

{

    CloseHandle((HANDLE) mState);

}

status_t Mutex::lock()

{

    DWORD dwWaitResult;

    dwWaitResult = WaitForSingleObject((HANDLE) mState, INFINITE);

    return dwWaitResult != WAIT_OBJECT_0 ? -1 : NO_ERROR;

}

void Mutex::unlock()

{

    if (!ReleaseMutex((HANDLE) mState))

        LOG(LOG_WARN, "thread", "WARNING: bad result from unlocking mutex/n");

}

status_t Mutex::tryLock()

{

    DWORD dwWaitResult;

    dwWaitResult = WaitForSingleObject((HANDLE) mState, 0);

    if (dwWaitResult != WAIT_OBJECT_0 && dwWaitResult != WAIT_TIMEOUT)

        LOG(LOG_WARN, "thread", "WARNING: bad result from try-locking mutex/n");

    return (dwWaitResult == WAIT_OBJECT_0) ? 0 : -1;

}

#else

#error "Somebody forgot to implement threads for this platform."

#endif

也就是說，要么定義HAVE_PTHREADS宏，要么定義HAVE_WIN32_THREADS宏，否則，Mutex類的方法就沒有實現代碼。對於andorid來說，需要定義HAVE_PTHREADS宏。從上面的代碼中還可以看出，android的線程實現實際上還是使用了pthread庫，只是在上面進行了一個封裝。在Android.mk中增加下面一條語句，問題解決：

LOCAL_CXXFLAGS := -DHAVE_PTHREADS