Sharing heap memory in a dll between two separate applications

2
votes

Sorry if this question has been answered before; however all of the questions that are similar seem to be related to global or static variables in a DLL and sharing of those.

Is it possible to have one instance of a dll shared between two separate applications?

I have two applications (appA, appB) and a DLL (theDLL).

I am seeing if it is possible for appA to call a function in theDLL which then creates a variable on the heap and stores that in theDLL. At a later time, I would like to have appB connect to theDLL and be able to access that variable created earlier. Again, sorry if this answer is the same as static and global variables in dlls.

Here is some psuedo code:

(theDLL)

class VariableHolder
{
 public:
     void StoreVariable(int x)
     {
        mInt = new int(x);
     }

     int GetVariable()
     {
        return mInt;
     }
 private:
     int mInt;
}

(appA)

int main()
{
    ...
    (assuming we got access to a VariableHolder singleton created in theDLL)
    theVarialbeHolder.StoreVariable(5);
    ... 
}

(appB)

int main()
{
    ...
    (assuming we got access to a VariableHolder singleton created in theDLL)
    if (theVarialbeHolder.GetVariable() == 5)
    {
        cout << "Hurray, how did you know?";
    }
    ... 
}
6
c++dllshared-memory
If you are using the DLL from two different processes, you will have to use some form of interprocess communication. You could use a pipe, a memory mapped file, TCP/IP, etc. Different processes have different address spaces and different heaps. - Michael Graczyk
In the VariableHolder::StoreVariable you allocate a pointer and assign it to mInt, however mInt is not a pointer. - Some programmer dude
@Joachim, yes. I typed this in without a compiler and that was a silly mistake. - Eric Friedman

6 Answers

3
votes

This exactly is not possible - as the address spaces for the two processes are different (because they're virtual, having been created by the kernel), so a valid pointer in one won't work within the other. However, you can use shared memory to transport raw scalar data (strings, integers) between processes - here's how.

3
votes

Yes, this is possible using shared memory. It doesn't need to use a shared DLL though.

Depending on the operating, the approaches are somewhat different:

  • On Windows, a shared file is used on mapped into memory (see Creating Named Shared Memory).

  • On Linux and Unix, there are direct functions to create shared memory areas, e.g. System V IPC. Just google for it.

3
votes

Shared libraries on almost any modern operating system are implemented by shared read-only executable and data pages, mapped simultaneously into the address space of any process that uses the given library. On Windows though (in contrast to most Unix system) this sharing can also be extended to read-write data segments in DLLs, so it is possible to have global variables in a DLL, shared among all images that have the DLL loaded. To achieve this, there is a two-step process involved. First you tell the compiler to put the shared variables in a new named data section:

#pragma data_seg (".myshared")
int iscalar = 0;
int iarray[10] = { 0 };
#pragma data_seg ()

It is important to have all those variables statically intialised otherwise they will end up into the .bss section instead. Then you have to tell the linker that you'd like to have the .myshared section with shared read-write attributes using the /SECTION:.myshared,RWS option.

This mechanism is much simpler than creating and binding to named shared memory objects but it only allows to share statically allocated global variables - you cannot use it to share data on the heap as the heap is private to the process. For anything more complex you should use shared memory mappings, i.e. as shown on the MSDN page, linked in the answer from H2CO3.

1
votes

This is not possible. The DLL can be shared in the 2 process but the data isn't. It's the code or program image (i.e. the logic or instructions) that is shared and not the data. Every Dll is mapped into the virtual address space of the process that loads it so the data either is on the data section of the process or on stack if it is local to the function. When a process is executing the address of the other process data is not visible.

You need to do some reading on virtual memory and how memory management unit(MMU) works. The OS, CPU, MMU works together to make it possible. The reliable way to do this is inter process communication. You can use shared memory where each process has a copy of data in form of virtual address but it is eventually mapped to same location into the real memory i.e the real address. The OS makes it possible.

0
votes

This as @H2CO3 pointed out, is not possible because of different address spaces.

However, from your problem, it looks like you need either a surrogate process around that DLL or a Service and then different processes can connect to that surrogate process/exe and use the shared memory.

0
votes

You must use shared memory (as was written above).

I recommend to use boost interprocess library. See documentation about shared memory - Shared memory between processes