pass string from java to c++ and using popen with NDK cased errors?

Question

i am using NDK to join java with C++ this my code :

MainActivity.java

public class MainActivity extends Activity {

/*Don't forget to load the library!!*/
static {
    System.loadLibrary("NDK1");
}

public native String exec(String cmd);

Android.mk

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
 # Here we give our module name and source file(s)
LOCAL_MODULE    := NDK1
LOCAL_SRC_FILES := NDK1.cpp
include $(BUILD_SHARED_LIBRARY)

Application.mk

APP_ABI := armeabi armeabi-v7a
APP_STL := gnustl_static

NDK1.cpp

#include <string>
#include <iostream>
#include <stdio.h>
 #include <jni.h>

   std::string exec(char* cmd) {
  FILE* pipe = popen(cmd, "r");
  if (!pipe) return "ERROR";
 char buffer[128];
 std::string result = "";
 while(!feof(pipe)) {
     if(fgets(buffer, 128, pipe) != NULL)
     result += buffer;
 } 
 pclose(pipe);
 return result;
}

  jstring Java_com_example_ndk1_MainActivity_exec(JNIEnv* env, jobject javaThis , jstring   cmd) {
char * res;

    res = env->GetStringUTFChars(cmd ) ;

std::string result = exec(res);
  return env->NewStringUTF(result);
 }

but have this error !!!

    $ /cygdrive/c/android-ndk-r4/ndk-build
   make: Warning: File `/cygdrive/c/android-ndk-r4/build/core/import-locals.mk' has    modification time 140680622 s in the future
 Compile++ thumb  : NDK1 <= NDK1.cpp
 jni/NDK1.cpp: In function '_jstring* Java_com_example_ndk1_MainActivity_exec(JNIEnv*,   jobject, jstring)':
 jni/NDK1.cpp:22:39: error: no matching function for call to   '_JNIEnv::GetStringUTFChars(_jstring*&)'
 jni/NDK1.cpp:22:39: note: candidate is:
 C:/android-ndk-r4/platforms/android-14/arch-arm/usr/include/jni.h:860:17: note: char   const* _JNIEnv::GetStringUTFChars(jstring, jboolean*)
 C:/android-ndk-r4/platforms/android-14/arch-arm/usr/include/jni.h:860:17: note:     candidate expects 2 arguments, 1 provided
 jni/NDK1.cpp:25:36: error: no matching function for call to  '_JNIEnv::NewStringUTF(std::string&)'
 jni/NDK1.cpp:25:36: note: candidate is:
 C:/android-ndk-r4/platforms/android-14/arch-arm/usr/include/jni.h:854:13: note: _jstring*  _JNIEnv::NewStringUTF(char const*)
 C:/android-ndk-r4/platforms/android-14/arch-arm/usr/include/jni.h:854:13: note:   no known  conversion for argument 1 from 'std::string {aka std::basic_string<char>}' to 'char const*'
 /cygdrive/c/android-ndk-r4/build/core/build-binary.mk:269: recipe for target  `obj/local/armeabi/objs/NDK1/NDK1.o' failed
 make: *** [obj/local/armeabi/objs/NDK1/NDK1.o] Error 1

Answer 1

after many tries to catch the problem this is the solution without causing any error :

jstring Java_com_example_ndk1_MainActivity_exec(JNIEnv* env, jobject javaThis , jstring cmd) {
const char * res;

    jboolean isCopy;
    res = env->GetStringUTFChars(cmd, &isCopy);

    if (isCopy == JNI_TRUE) {
        (env)->ReleaseStringUTFChars(cmd, res);
    }

std::string result = exec(res);
return (env)->NewStringUTF((const char* )result.c_str());
 }

Answer 2

[edited to make the answer more generic - there are several errors in the code]

For every error of the type no matching function for call to, look up the definition of the function in the JNI reference

Make sure that you are:

• Passing all required parameters
• The parameters are of the required type

For example, the call to GetStringUTFChars is missing parameters and the call to NewStringUTF doesn't take a std::string as parameter. It wants a const char *.

For the NewStringUTF, try:

return (*env)->NewStringUTF(env, (const char*)result.c_str());

This JNI tutorial is also nice: http://www.steveolyo.com/JNI/JNI.html