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

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

用NDK编译时出现这么个错误undefined reference to `android::Mutex::lock()'

起初以为没有链接必要的so，结果加上了  -lutils 也不行。所以google 了一下，发现有人遇到过此问题，如下：

察看了一下ndk中的STABLE-APIS.TXT文档，上面有这样一句：

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).

也就是说使用多线程，并不需要增加-lpthread编译选项，看来解决问题还需要从源代码入手。察看frameworks/base/include/utils/threads.h文件，关于Mutex有如下代码片断：

#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

也就是说如果定义了HAVE_PTHREADS宏，就有Mutex的相关方法实现，如果没有定义这个宏，Mutex的方法又是如何实现的，下面看看frameworks/base/libs/utils/Threads.cpp中的代码：

#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

也就是说，要么定义HAVE_PTHREADS宏，要么定义HAVE_WIN32_THREADS宏，否则，Mutex类的方法就没有实现代码。对于andorid来说，需要定义HAVE_PTHREADS宏。从上面的代码中还可以看出，android的线程实现实际上还是使用了pthread库，只是在上面进行了一个封装。在Android.mk中增加下面一条语句，问题解决：

LOCAL_CXXFLAGS := -DHAVE_PTHREADS