Android read text file from asset folder using C (ndk)

Question

I need to read text file from asset folder in android, by searching through internet I found that there is asset_manager api available from android 2.3 onwards. As I am targeting only tablet devices this is useful. But as I am not expert in C language I am not able to find any example on how to read/write files using file descriptor. I found many examples using FILE* (file pointers)

My goal is to decrypt a js file from asset folder which is encrypted using C (for securing the code), as js code is visible if end user decompiled my apk. Because asset folder is inside zip file is it possible to do?

score 35 · Accepted Answer · edited May 26 '12 at 20:50

35

Here is the code I used to read file from android assets folder using asset_manager ndk lib

    AAssetManager* mgr = AAssetManager_fromJava(env, assetManager);
    AAsset* asset = AAssetManager_open(mgr, (const char *) js, AASSET_MODE_UNKNOWN);
    if (NULL == asset) {
        __android_log_print(ANDROID_LOG_ERROR, NF_LOG_TAG, "_ASSET_NOT_FOUND_");
        return JNI_FALSE;
    }
    long size = AAsset_getLength(asset);
    char* buffer = (char*) malloc (sizeof(char)*size);
    AAsset_read (asset,buffer,size);
    __android_log_print(ANDROID_LOG_ERROR, NF_LOG_TAG, buffer);
    AAsset_close(asset);

Added following line to my Android.mk

# for native asset manager
LOCAL_LDLIBS    += -landroid

And don't forget the include in source file

#include <android/asset_manager.h>

edited May 26 '12 at 20:50

Mirsaes

43
2

answered May 03 '12 at 09:03

Sandeep Manne

6,030
5
39
55

1

Great answer! Works for me in conjunction with Assimp to load 3D models. First, I load a file into memory like described above, then I pass the buffer to aiImportFileFromMemory – j00hi Aug 22 '14 at 22:10
Can you help me figuring out where did you pass the file name ? – Nishant May 26 '15 at 14:12
@Nishant Could you figure out how to pass the file name? – re3el Nov 04 '15 at 05:56
@re3el Sorry, I worked on it months ago and do not have access to the source codes. – Nishant Nov 04 '15 at 09:49
1

#include for AAssetManager_fromJava – Rakib Ansary Dec 06 '15 at 17:52

score 2 · Answer 2 · answered May 02 '12 at 07:07

Practically FILE* and 'int' descriptors are equivalent and fread/fwrite/fopen/fclose are the counterparts of open/close/read/write functions (the functions are not equivalent however, the latter are non-blocking).

To get 'int' from 'FILE*' you can use

int fileno(FILE* f);

in header and to do the inverse you can use fdopen()

FILE *fdopen(int fd, const char *mode);

So either replace everything using the FILE* to int or just take one of the samples and insert this conversion code before the file reading.

OK. That has nothing to do with NDK actually, so if you post some C code here that you're having trouble with, the community will be able to help you better. Good luck with the coding :) — Viktor Latypov, May 02 '12 at 07:33

score 2 · Answer 3 · answered May 02 '12 at 07:12

It's pretty similar to regular fread/fseek functions. Here's read function declaraton:

ssize_t read(int fd, void *buf, size_t count);

It reads from fd file descriptor into buf buffer count bytes. If you think about fread, then instead of:

fread(buf, count, size, file);

you will call:

read(fd, buf, count*size);

And that's it. It is so simple.

Seeking is also similar. Just look up the function declaration and read the argument names/description. It will be obvious.

Android read text file from asset folder using C (ndk)

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