Is global variable in a shared library / dll, shared across process

Question

I am developing a shared library(.so) and dll. I have a global variable which is updated in multiple threads. So I have mutex lock for synchronization.

I am not clear whether global data in shared library is shared across process. If it is then I need to use semaphores for synchronization. As I understand global variables are part of data segment so I wanted to understand how dll manages the global data across processes. Any information w.r.t. dll format and segment will be helpful.

Thanks.

Answer 1

By default, no, global variables are not shared across processes.

However, you can use a data segment (data_seg) in order to share global variables across processes. You can find more information on MSDN in the article titled "How do I share data in my DLL with an application or with other DLLs?"

Answer 2

Absolutely NO. Each process has its own virtual memory space and do not see memory of other processes. Two processes can even store different value at the same address, say 1000000 - because theirs virtual addresses "1000000" are mapped to different physical memory cells (for example to "2000000" for first process and to "3000000" for second. Shared dll does not change anything in this.