What do pointer values actually represent?

[hackerNovitiate]

Hi all,

When debugging or experimenting with low-level code, I sometimes examine pointer values like this:

printf("Pointer points to %p\n", (void*)myPtr);

printf("Pointer points to %p\n", (void*)myPtr);

To copy to clipboard, switch view to plain text mode 

Usually, I'm only interested in confirming that two pointers have the same values, confirming that two pointers have different values, or finding the "positional differences" between pointer values. Thus, I've never needed to understand what the absolute values of those pointers are.

However, I'm now curious as to what those values actually represent... If I get the message "Pointer points to 006CBAC0", does 006CBAC0 refer to a unique location in RAM? If two different, simultaneously-running processes attempt to dereference 006CBAC0, will they read the same physical location?

[squidge]

It's a location in virtual RAM. Every process has it's own virtual memory. No process can access another processes data without ask the OS first and knowing the other processes process ID.

The OS keeps and manages a virtual to physical mapping table. Some data (such as runtime libraries) may be loaded in memory once and shared amongst several processes. The processes themselves will have no knowledge of this (and the virtual memory locations may be different in each process)

All the above is hardware accelerated by the CPU (ie, the virtual to physical lookups are done in hardware).

[stampede]

If two different, simultaneously-running processes attempt to dereference 006CBAC0, will they read the same physical location?

As squidge said, each process is given it's own "virtual" memory - from the process point of view it looks like its the only process in the system, having all the available memory to itself.
So, 006CBAC0 in one process will reference different physical memory location in another. Even if it was the same location, process can't access the memory which it does not own (without ask the OS first), trying to do so causes segmentation fault.