File Operations in Android NDK

user_135135 0
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I am using the Android NDK to make an application primarily in C for performance reasons, but it appears that file operations such as fopen do not work correctly in Android. Whenever I try to use these functions, the application crashes.

How do I create/write to a file with the Android NDK?

android-ndk java-native-interface file-io c
5 Answers

File IO works fine on Android using JNI. Perhaps you are trying to open a file with a bad path and not checking the return code? I modified the hello-jni example to demonstrate that it is indeed possible to open file and write to it. I hope this helps.

/*
 * Copyright (C) 2009 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */
#include <string.h>
#include <jni.h>
#include <stdio.h>

/* This is a trivial JNI example where we use a native method
 * to return a new VM String. See the corresponding Java source
 * file located at:
 *
 *   apps/samples/hello-jni/project/src/com/example/HelloJni/HelloJni.java
 */
jstring
Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env,
                                              jobject thiz )
{
    FILE* file = fopen("/sdcard/hello.txt","w+");

    if (file != NULL)
    {
        fputs("HELLO WORLD!\n", file);
        fflush(file);
        fclose(file);
    }

    return (*env)->NewStringUTF(env, "Hello from JNI (with file io)!");
}

Here is the result after running it on my phone (with an SD card):

$ adb -d shell cat /sdcard/hello.txt
HELLO WORLD!
user_1895110

Make sure to use the Java getExternalStorageDirectory() call to get the real path to the sdcard since newer devices don't simply map it to "/sdcard". In that case trying to use a hardcoded location of "/sdcard" will fail.

user_4950170

I want to add my two cents to the answers here, on top of setting the correct permissions specified in the answer to this question, make sure to give your app permission to access the storage in the OS. The permissions menu could change from phone to phone, an easy way to get to it is by going to the Settings menu and then searching for "Permissions." This will give you a chance to give your app permission to access the storage (ie sdcard directory) from NDK code.

user_44889450
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