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I am using NDK and I need to read resource media file. So, as far as I understand in order to access to resources I need to use AAssetManager, and eventually I need to get std::basic_istream to work with it.

So, question is, how to get std::basic_istream from AAssetManager?

Sirop4ik
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  • There's no such support in the API as far as I can tell. If you want to read from an asset, `AAsset_read` is the function to use. It would be problematic for NDK libraries to return STL objects to applications, since there could be incompatibilities between the STL implementation used by the system and the one used by the application. – Michael Apr 13 '20 at 15:44
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    Ok, but maybe there some kind of possible way to get a stream, that I can get by using `AAssetManager`? Because as far as I see I can call `AAsset_read`, but I did not get stream... – Sirop4ik Apr 13 '20 at 15:48
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    There's a function returning a void pointer to _"a buffer holding the entire contents of the assset"_. Whether you can create an `istream` for that, I don't know. – Michael Apr 13 '20 at 15:53
  • Is the use of `mmap` will work? – daparic Apr 21 '20 at 21:50

2 Answers2

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The answer is actually very different whether you have a compressed asset (e.g. text) or uncompressed (by default, Jpeg images and mp3 are stored by the packer). For these uncompressed assets, you can get the file descriptor with AAsset_openFileDescriptor(), and then follow the ways of How to construct a c++ fstream from a POSIX file descriptor?. For compressed assets, you may look for a (potentially API-level-dependent) hack that will let you get the file descriptor (or file path) to the transient file that the OS opens for you when it unpacks your asset.

Alex Cohn
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You can adapt this example to work with the "buffer" mode of AAssetManager.

This will read the entire asset into memory, though. You could make an implementation that works more like std::fstream by reading chunks into a memory buffer, but that is significantly more complex.

Note: I only test-compiled the code below.

class asset_streambuf : public std::streambuf {
    public:
        asset_streambuf(AAsset * the_asset)
            : the_asset_(the_asset) {
                char * begin = (char *)AAsset_getBuffer(the_asset);
                char * end = begin + AAsset_getLength64(the_asset);
                setg(begin, begin, end);
            }
        ~asset_streambuf() {
            AAsset_close(the_asset_);
        }
    private:
        AAsset * the_asset_;
};

Usage:

AAsset * asset = AAssetManager_open(mgr, "some_asset.bin", AASSET_MODE_BUFFER);
asset_streambuf sb(asset);
std::istream is(&sb);

EDIT: Found a shorter way based on this answer.

Botje
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