File Operations in Android NDK

Question

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?

Answer 1

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!