在NDK共享内存区域 [英] Shared memory region in NDK

问题描述

我想有一个共享的内存块（一个 ashmem 区域）被映射和从本地code访问。我也想被多个应用此块。我也希望它工作在SDK 7级（Android 2.1的）

I want to have a shared memory block (an ashmem region) that's mapped and accessed from native code. I also want this block to be used by several applications. I also want it to work on SDK level 7 (Android 2.1)

有两种途径。我可以创建本地code的ashmem区域;但随后的问题是 - 我怎么整数关口文件描述符到另一个进程？您可以通过元帅的FileDescriptor 对象包裹，但没有办法，构成一个围绕原始FD。还有 ParcelFileDescriptor ，支持各地建设和获取整数FD的，但相关的方法只支持SDK 12级甚至更高。

There are two routes. I can create an ashmem region in native code; but then the question is - how do I pass an integer file descriptor to another process? You can marshal FileDescriptor objects via a Parcel, but there's no way to construct one around a raw FD. There's also ParcelFileDescriptor which supports constructing around and retrieving integer FD's, but the relevant methods are only supported in SDK level 12 or even higher.

另外，我可以创建一个 MemoryFile 。有一个的fugly方式在地块周围通过。但我怎么检索它文件描述符，使原生code的东西的mmap（）？

Alternatively, I can create a MemoryFile. There's a fugly way to pass it around in Parcels. But how do I retrieve a file descriptor from it so that native code has something to mmap()?

推荐答案

在所有版本的Andr​​oid，因为1.5到4.1，的FileDescriptor 有一个 INT 称为数据成员描述。这是包私人早期版本的Andr​​oid，私人最近的例子。带着几分刻意的访问控制颠覆，您可以访问它 - 无论是通过反射，或通过JNI。每个人都可以绕过访问控制 - 如果反射，通过 Field.setAccessible（），如果JNI的 - 默认

On all versions of Android since 1.5 to 4.1, FileDescriptor has an int data member called descriptor. It's package-private on earlier versions of Android, private on recent ones. With a bit of deliberate access control subversion, you can access it - either via reflection, or via JNI. Each can bypass access control - in case of reflection, via Field.setAccessible(), in case of JNI - by default.

考虑到这一点，您可以构造的FileDescriptor 围绕本地FD就好了。构造一个空白的，然后设置描述。这就是比特和建设的时候，Android的code件事情。

With that in mind, you can construct a FileDescriptor around a native FD just fine. Construct a blank one, then set descriptor. That's what bits and pieces of Android code do when constructing those.

这肮脏的黑客是否会打破最终，谁也不知道。幸运的是，它不是一个核心部分，在我的情况下的功能 - 有一些优雅降级

Whether this dirty hack will break eventually, who knows. Fortunately, it's not a core piece of functionality in my case - there's some graceful degradation.

人们可以有条件地聘请支持 ParcelFileDescriptor 方法，如果平台允许，使用字段访问黑客作为后备。这样一来，这将是相对面向未来。

One may conditionally employ supported ParcelFileDescriptor methods, if the platform allows, using the field access hack as a fallback. This way, it'll be relatively future proof.

这篇关于在NDK共享内存区域的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！