File Operations in Android NDK

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?


Other answers are correct. You can open a file through the NDK using FILE and fopen, but don't forget to place a permission for it.

In the Android manifest place:

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/> 

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
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * 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/
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);

    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

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.

I can also verify that fopen() works correctly, but not if you're trying to access a file in the application's resources or assets folder. I recommend, to avoid having to reinvent the wheel, that you stick any assets you want shipped with your app in the assets folder, where they'll be packaged up for distribution.

In the assets folder case you need to do one of two things, depending on whether the file was compressed by the packager. Both use the AssetManager methods, and you can get the AssetManager from the context/app. File names are always relative to the assets folder, btw: If your have a file "foo.png" directly in the assets folder, you'd open "foo.png," not something like "assets/foo.png".

  1. If the file wasn't compressed (i.e., it's one of the extensions that doesn't get compressed, like .png), you can get a file descriptor from AssetManager.openFd() and pass it to C++. Then you can use fdopen(dup(fd),"r"); to open the file as a FILE*. Note you must fseek() to the offset, and keep track of the length of the file yourself. You're really getting a file handle to the entire assets package, and your file of interest is only a small part.

  2. If your file is compressed, you need to use the Java streaming reader: gives you an InputStream you can use the read the file in. This is a PITA because you can't query (AFAIK) the file size; I run a preprocessing step on my assets folder that generates a list of all the files with their respective sizes so I can know, e.g., how big of a buffer to allocate.

If your file is a resource, you may need to go through the Resource class to access it, though it appears resources are also packed into the same assets package. Resource has an openRawResource() call to get the InputStream and an openRawResourceFd() call to get the file descriptor, as above, though.

Good luck.

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