Question

我正在研究android-ndk，其中我尝试将字符串从我的C代码传递到我的java然后在活动中。我的代码如下，但我收到以下错误。请帮帮我

#include <com_testing_ndk_FibLib.h>

JNIEXPORT jstring JNICALL Java_Sample1_stringMethod(JNIEnv *env, jobject obj,
        jstring string) {

    const char *name = (*env)->GetStringUTFChars(string, NULL); //Java String to C Style string
    char msg[60] = "HelloWorld";
    jstring result;

    strcat(msg, name);
    (*env)->ReleaseStringUTFChars(string, name);
    puts(msg);
    result = (*env)->NewStringUTF(msg); // C style string to Java String
    return result;
}

错误日志：

jni/com_testing_ndk_FibLib.c: In function 'Java_Sample1_stringMethod':
jni/com_testing_ndk_FibLib.c:7:55: error: 'NULL' undeclared (first use in this function)
  const char *name = (*env)->GetStringUTFChars(string, NULL); //Java String to C Style string
                                                       ^
jni/com_testing_ndk_FibLib.c:7:55: note: each undeclared identifier is reported only once for each function it appears in
jni/com_testing_ndk_FibLib.c:7:21: error: too few arguments to function '(*env)->GetStringUTFChars'
  const char *name = (*env)->GetStringUTFChars(string, NULL); //Java String to C Style string
                     ^
jni/com_testing_ndk_FibLib.c:11:2: warning: incompatible implicit declaration of built-in function 'strcat'
  strcat(msg, name);
  ^
jni/com_testing_ndk_FibLib.c:12:2: warning: passing argument 2 of '(*env)->ReleaseStringUTFChars' discards 'const' qualifier from pointer target type
  (*env)->ReleaseStringUTFChars(string, name);
  ^
jni/com_testing_ndk_FibLib.c:12:2: note: expected 'jstring' but argument is of type 'const char *'
jni/com_testing_ndk_FibLib.c:12:2: error: too few arguments to function '(*env)->ReleaseStringUTFChars'
jni/com_testing_ndk_FibLib.c:14:11: warning: passing argument 1 of '(*env)->NewStringUTF' from incompatible pointer type
  result = (*env)->NewStringUTF(msg); // C style string to Java String
           ^
jni/com_testing_ndk_FibLib.c:14:11: note: expected 'const struct JNINativeInterface **' but argument is of type 'char *'
jni/com_testing_ndk_FibLib.c:14:11: error: too few arguments to function '(*env)->NewStringUTF'
make: *** [obj/local/arm64-v8a/objs/com_testing_ndk_FibLib/com_testing_ndk_FibLib.o] Error 1

我的Java代码：

public class FibLib {

    static {
        System.loadLibrary("com_testing_ndk_FibLib");
    }



     public static native String stringMethod(String text);

        public static void main(String[] args)
        {
           System.loadLibrary("Sample1");
           FibLib sample = new FibLib();

           String  text   = sample.stringMethod("world");

           System.out.println("stringMethod: " + text);    
       }
}

Answer 1

当您编译C代码时，所有JNI调用都缺少第一个应该是env的参数。

如果您的代码使用的是C ++，则可以调用env->NewStringUTF("xx")之类的调用。但是在C中，JNIEnv *不是一个对象，因此你必须将它作为第一个参数传递，如下所示：(*env)->NewStringUTF(env, "xx")

对于未定义的NULL，您可以通过包含定义它的标头来解决此问题：#include <stddef.h>

Answer 2

第二个参数似乎不是可选的：

jboolean iscopy;
const char *name = (*env)->GetStringUTFChars(string, &iscopy);

＆＃39; NULL＆＃39;使用android ndk未声明（在此功能中首次使用）

2 个答案: