无效的值将传递给从共享库动态加载的函数

Question

im从Android 4.4/System/lib/libgui.so上的共享库中动态函数

#include <jni.h>
#include <string>
#include <android/log.h>
#include <sys/types.h>
#include <iostream>

#include <link.h>
#include <dlfcn.h>

#define LOGI(...) \
  ((void)__android_log_print(ANDROID_LOG_INFO, "screencast::", __VA_ARGS__))
#define LOGE(...) \
  ((void)__android_log_print(ANDROID_LOG_ERROR, "screencast::", __VA_ARGS__))

typedef void* (*getBuiltInDisplay_t)(int32_t);

extern "C" JNIEXPORT jstring JNICALL
Java_com_example_screencast_1lib_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject) {
    LOGI("Loading libgui...");

    void *handle = dlopen("/system/lib/libgui.so", RTLD_LAZY);

    if (!handle) {
        LOGE("Cannot load library: %s", dlerror());
        return env->NewStringUTF("error while loading system library");
    }

    LOGI("Loading symbol getBuiltInDisplay...");

    dlerror();
    getBuiltInDisplay_t getBuiltInDisplay = (getBuiltInDisplay_t) dlsym(handle, "_ZN7android21SurfaceComposerClient17getBuiltInDisplayEi");
    const char *dlsym_error = dlerror();
    if (dlsym_error) {
        LOGE("Cannot load symbol: %s", dlsym_error);
        dlclose(handle);
        return env->NewStringUTF("error while loading system function");
    }

    LOGI("Calling getBuiltInDisplay...");
    getBuiltInDisplay(0);

    LOGI("Closing library...");
    dlclose(handle);

    return env->NewStringUTF("success!!");
}

，并在日志中获得此

...
04-26 17:45:12.623 D/dalvikvm( 6659): VFY: replacing opcode 0x20 at 0x000c
04-26 17:45:12.633 I/screencast::( 6659): Loading libgui...
04-26 17:45:12.633 I/screencast::( 6659): Loading symbol getBuiltInDisplay...
04-26 17:45:12.633 I/screencast::( 6659): Calling getBuiltInDisplay...
04-26 17:45:12.633 E/SurfaceFlinger(  175): getDefaultDisplay: id=1076775671 is not a valid default display id
04-26 17:45:12.633 F/libc    ( 6659): Fatal signal 11 (SIGSEGV) at 0x00000000 (code=1), thread 6659 (.screencast_lib)
04-26 17:45:12.693 F/libc    ( 6739): Fatal signal 13 (SIGPIPE) at 0x00001a53 (code=0), thread 6739 (sh)
...

功能，我也发现了 getBuiltIndisplay静态方法的实现。

sp<IBinder> SurfaceFlinger::getBuiltInDisplay(int32_t id) {
    if (uint32_t(id) >= DisplayDevice::NUM_BUILTIN_DISPLAY_TYPES) {
        ALOGE("getDefaultDisplay: id=%d is not a valid default display id", id);
        return NULL;
    }
    return mBuiltinDisplays[id];
}

因此，正如我所理解的，出于某种原因，我传递给函数的0值显示为随机值（对于这种情况，1076775671，但几乎每次都是随机的）

Answer 1

我发现了。正如@michael所说，这不是静态类方法，需要通过指针。因此，我分配了一些记忆，并将指针作为第一个参数。

typedef void* (*getBuiltInDisplay_t)(void*, int32_t);

// ...
    LOGI("Calling getBuiltInDisplay...");
    void *surfaceComposerClientInstance = malloc(1024);
    getBuiltInDisplay(surfaceComposerClientInstance, 0);
    free(surfaceComposerClientInstance);
// ...

而且有效！