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Shared memory and C++ Objects (cont)

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# 1  
Old 01-28-2002
Shared memory and C++ Objects (cont)

I asked the next question in a former thread :
Can I safely share objects that have virtual functions (i.e. have
virtual function table pointers) between two processes ?
Where will the pointers point to in each process ?
The objects are supposed to reside in shared memory

I received a partial answer from Wizard (thanx, btw), but
was not satisfied : I am using AIX, and want to know if
I can solve the pointer problem via a OS mechanism, or,
given that this is not possible, a language interface.
What is that "shared libraries"
gizmo Wizard referred to ?
# 2  
Old 01-30-2002
Sorry about the original post. I thought you had somehow gotten the pointer to the virtual function table and wanted to pass it to another process so the other process could dereference it. I've seen this done when creating dynamic code. Apparently my brain doesn't work all the time.

Anyway, when you say you want to "share Objects" between processes, do you mean you want to reuse your classes in different binaries? If that's the case, then compiling the code into a binary creates a virtual function table with an accompanying pointer to the table in the executable text area of the binary. Each binary gets a private copy of the virtual function table, so references to virtual functions are resolved. In this case, you don't necessarily need a shared library.

If you are familiar with an archive library, then you know you place code that you reuse often in the archive library to make it easier to use at link time. The disadvantage of a standard archive library is that if you change the code or data in the source and rebuild the archive library, you have to rebuild all the code that uses the library. You also have larger binaries than you need, because each binary has a private copy of the executable text from the archive library. If it's a small archive library, this isn't a big deal. However, if you've ended up with a large library, you can waste a good deal of memory each time you run your binary.

The advantages you get using a shared library are that you can change the code in the shared library and you don't have to recompile all of your binaries. You also have only one copy of the executable text in memory and all processes share the text. Each process gets a private copy of any data contained in the shared library. In this case, each process will still get a local copy of the virtual function table.

The internals of the AIX XCOFF image and how the linking is actually done is fairly complex, but the process of building a shared library is fairly simple. If you have the AIX hypertext docs loaded, search on shared libraries and there should be some examples.

Last edited by wizard; 01-30-2002 at 01:09 PM..
# 3  
Old 01-31-2002
Shared in Runtime !

What I want to acheive is to actually share (in run time)
in a shared memory section, a series of objects that
may contain a virtual function pointer.
Is that possible ?
# 4  
Old 01-31-2002
As far as I can understand the objects that contain pointers to "virtual" functions suit exactly the definition of a shared library (maybe u have seen these libraries with the .so.0 extension in other OSes). This is the AIX way. Else u have to reinvent all the appropriate mechanism to do the job. Look at this :

Shared libraries and shared objects (normally called Dynamically Loaded Libraries, or DLLs in Windows terminology) are terms used to refer to object code components that are handled in a special way.
Shared libraries are used in two stages when creating an executable. At link time, the link editor (the ld command) searches the specified library to resolve all undefined symbols that are referenced in the main application code. If a shared library contains the referenced symbols, the loader section
of the header of the created executable contains a reference to the shared library . Unlike using the static library, the object files containing the referenced symbols are not incorporated into the executable. At run time, the system loader (the kernel
component that starts new processes) reads the header information of the executable and attempts to locate and load any referenced shared libraries.
Assuming all the referenced shared libraries are found, the executable can be started. This process is known as dynamic linking.

The header of a shared library is composed only of
Header information
Program code
Program data
Shared object information

And an example

For example, to make the shared object from the source files
source1.c and source2.c, use:
cc -g -c source1.c
cc -g -c source2.c
ld -G -o source1.o source2.o

U can seek some redbook from IBM for more info
# 5  
Old 02-01-2002
Data More questions

It seems I just cant make myself understood....Smilie
I'll try one more time...

Here is what I want to do :
Process A creats a shared memory object
The given pointer is treated as pointing to Object
Obj1 of class
Class1 defines a virual functions F1().
Therefore, Class1 has a
virtual function table pointer
pointing to an area containing the table.
ProcessB then maps to
casting the resulting pointer to Class1.
Thus Obj1 is shared.
The problem I forsee is sharing the
virtual function table pointer,
which, in ProcessB's context,
point to nothing meaningfull.

How can this problem be avoided, if at all ?
# 6  
Old 02-01-2002
Ok. Apparently I understood you correctly in the first post. First of all, when you create an object containing a virtual function, you do not create a virtual function table. At compile time, a pointer to the virtual function is created and placed into a module wide virtual function table which is a table of pointers to virtual functions defined within the compiled object. If you can get the value of the pointer pointing to the virtual function table, it will only have meaning in the executable that contains the instantiation of Class1, in this case object Obj1.

Secondly, if you create a shared memory segment and place a pointer to the virtual function, the virtual function table, or a pointer to anything inside the address space of ProcessA, you are NOT "sharing the object". You are passing a virtual memory address with meaning in ProcessA, to ProcessB where it has no meaning. When ProcessB dereferences the pointer, you will more than likely get a SIGSEGV.

Shared memory is used for passing DATA objects, not executable text (i.e. a virtual function) or a pointer.

Obviously, I don't know why ProcessB needs to access the Obj1 object defined in ProcessA. If the Class1 template (with virtual functions and all) is declared inside both source modules (or more likely in a header file and included in the source), then you could have separate instantiations of the template in both processes. If you need to share data values between the instantiated objects, then you could pass the data values using shared memory, but you cannot pass a pointer to the executable text of the virtual function. You don't need to as you would have the same executable text in each process.

Now, that said, you should look at the load() system call in AIX. I've used this to dynamically create modules and load them into the process address space of a running process. Realize that this is a non-portable system call. Since I don't know what your ultimate goal is here, this is the only possible option I can come up with.
# 7  
Old 02-02-2002

To all those who helped calrify matters.
It has been most helpful.


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