Android棒棒糖从本机代码c ++写入sdcard

Question

I have android application which uses a lot of c++ native code. Application needs to work with files located on sdcard(read, create, append). But Kitkat+ denied writing to sdcard for 3rd party applications. Android 5 introduced new API which allows that again.

How to use the new SD card access API presented for Android 5.0 (Lollipop)?

All examples and documentation what I found are mostly for Java side. Examples for native code don't exist or are very unclear. So I want ask few questions.

The link above contains valuable example how to get DocumentFile which can return ParcelFileDescriptor. From this object I am able to receive native file descriptor - ParcelFileDescriptor.getFd(). It's integer which I am sending to c++ code through jni.

In c++ I am opening file with fdopen(fd).

My questions are :

1. Is fdopen function correct way how to open the file with new api? Or the DocumentFile already opens the file and I should only use fd in further operations.
2. It's enough to close the file descriptor inside native code with fclose? or should I close it on java side with ParcelFileDescriptor.detachFd(). Or both.

Thank you

EDIT : I getFD and detachFD works. But I never found answer how to correctly replace ftruncate, which needs write access too, and I did not found ftruncate version which takes file descriptor like a input

Answer 1

1) yes, use file descriptors and fdopen 2)

1. Open the ParcelFileDescriptor .
2. getFd().
3. Pass the Fd to native code.
4. Close the ParcelFileDescriptor . (this is closing your java reference)

Fd is just an int representing a linux id for a file. In native:

1. Fdopen
2. Do stuff
3. Fclose (note this is closing your native file pointer)

The two closes are doing different things.

Note: You still need the SAF permission for the file or a higher root.

Answer 2

Try Below Links:

Android - writing/saving files from native code only: Android - writing/saving files from native code only

Android NDK Write File: Android NDK Write File

File Operations in Android NDK: File Operations in Android NDK