CODEWITHSUNDEEP
android-ndkshared-memory
Jurlie
Jurlie

Reputation: 1014

Android NDK shared memory: how to use ashmem_create_region?

I've found some guides on using shared memory in Ansroid OS. I've learned that shm_open is not exist in Android amymore due to memory leaks caused by forced killing processes by OS or user. ASHMEM functions are developed instead. But I cannot find in my NDK the declaration of ashmem_create_region() and other function. Where they are?

Upvotes: 1

Views: 6182

Answers (2)

Ian Ni-Lewis
Ian Ni-Lewis

Reputation: 2367

As with so many things in Android, the answer is to use JNI. The Java class java.nio.MappedByteBuffer wraps ashmem and provides read/write methods to access it.

Unfortunately, if you're using shared memory to boost performance, multiple round trips through JNI aren't an attractive proposition. Cedric Fung proposes using reflection to retrieve the ashmem handle by name, which will work but may break in future frameworks. (This does happen, BTW. All it takes is somebody deciding that "mFD" is too vague and "mFileDescriptor" would be a better name, or some such.) If you want to play with fire, I'd suggest retrieving the descriptor by type rather than by name, since the type is very unlikely to change.

Cedric also proposes implementing a Binder in C++, but this puts you back where you started because Binder is also not included in the NDK. Instead, you'd need to pass the handle via a binder service implemented in Java.

It's a lot of work for such a simple feature, I know. It's easier to just mmap a file and use that instead, which is too bad since a basic file mapping isn't nearly as mobile-friendly as ashmem. :-(

Upvotes: 3

ph0b
ph0b

Reputation: 14473

the header is inside system/core/include/cutils/ashmem.h of the aosp.

You must not use it for a regular application as ashmem functions aren't part of the NDK: https://groups.google.com/forum/#!topic/android-ndk/eS9QK8EY968

Upvotes: 1

Related Questions