Saturday, April 2, 2011

address of a java object (and virtual/physical memory)

XR,

(another blog post) We once discussed how to find the address of a java object. The address has to be hidden from application programs since the garbage collector often need to relocate the object through the generational heap. Therefore any reference variable we use in java will let us read/write the "pointee" object but won't reveal address.

However, the address is visible to the garbage collector and some of the C code integrating with java via JNI or other means. It has to be visible because C uses pointers. A pointer holds a memory address. If a C function uses a pointer, then the C function can print out the address.

By the way, all along we are talking about virtual memory addresses, which could be anything from 0 to 0xFFFFFFFF ie 32-bit integer, even on a 128MB RAM laptop.

The virtual memory module in the kernel translates between virtual memory address and physical RAM address.

Q: Is it every possible for a C program to see the physical RAM address of an object? Here are my tentative answers so please correct me --

A: yes for the C program implementing the virtual memory module itself. This module runs in probably the lowest layer in the kernel. Virtual memory module probably gets loaded first so that a 32MB RAM laptop can load a 50M operation system. Virtual memory continues to be extremely relevant since no machine has enough RAM to fill up a 64 bit address space.

A: no for any other C program running on top of virtual memory module.

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