Creating temporary files in Android with NDK

Question

I am currently working on a C-based, NDK-based, Android application. This application needs to create temporary files. On a regular Linux system, I would use tmpfile to ensure that these files are properly created in a temporary directory and cleaned-up at process end.

However, my investigations on various Android devices seem to indicate that

• tmpfile always fails;
• there is no /tmp directory;
• directory /data/local/tmp isn't present on all variants of Android;
• there is no TEMP environment variable set;
• mkstemp doesn't work any better than tmpfile.

Now, I'm sure that I could hack something together, but seeing that the SDK offers context.getCacheDir and File.createTempFile for Java applications, I hope that there is an equivalent at C-level.

Does anyone know of a good reliable and cross-Android method for creating a temporary file?

Answer 1

The best way we've found is to call Context.getCacheDir on startup, get its path with getAbsolutePath, then call a JNI function to store that path in a global. Any function that wants to create a temporary file simply appends a suitable temporary file name to that path.

If you really want to fetch it from JNI another alternative would be to pass in a Context to a JNI function and use a bunch of GetMethodID / CallObjectMethod stuff to call back into Java to getCacheDir, but the former approach is a lot simpler.

Unfortunately, there does not appear to be a more elegant solution at the moment.