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Android lollipop write to SDCard from native C ++ code

I have an Android app that uses a lot of native C ++ code. The application must work with files located on the SD card (read, create, add). But Kitkat + refused to write sdcard for third party applications. Android 5 introduced a new API that allows this again.

How to use the new SD Card Access API introduced for Android 5.0 (Lollipop)?

All examples and documentation I have found are mainly for Java. Examples for native code do not exist or are very unclear. So I want to ask a few questions.

The link above provides a valuable example of how to get the DocumentFile that ParcelFileDescriptor can return. From this object, I can get my own file descriptor - ParcelFileDescriptor.getFd (). It is an integer that I am posting to C ++ code via jni.

In C ++, I open a file using fdopen (fd).

My questions:

  • Is the fdopen function working correctly, how to open a file with the new api? Or DocumentFile is already opening the file and I have to use only fd in further operations.
  • Is it enough to close the file descriptor inside native code using fclose? or should I close it on the Java side using ParcelFileDescriptor.detachFd (). Or both.

thank

EDIT: I am getting FD and detachFD is working. But I never found an answer on how to properly replace ftruncate, which also needs write access, and I couldn't find a version of ftruncate that accepts a file descriptor as input

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Try the links below:

Android - write / save files from native code only: Android - write / save files from native code only



Android NDK Recording File : Android NDK Recording File

File Operations in Android NDK: File Operations in Android NDK

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1) yes, use file descriptors and fdopen 2)

  • Open up ParcelFileDescriptor

    .
  • getFd ().
  • Pass Fd to native code.
  • Close ParcelFileDescriptor

    . (this closes your Java link)

Fd is simply int

representing the linux id for a file. In native:



  • Fdopen
  • Make stuff
  • Fclose (note that this closes your own file pointer)

The two closures do different things.

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

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