What are the main differences between binding to a shared object or to an ordinary object? Also how is this possible to share some variables between some programs and knowing that our variables are never changed by another program?
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What are the main differences between binding to a shared object or to an ordinary object? Also how is this possible to share some variables between some programs and knowing that our variables are never changed by another program?
Variables are not shared between programs, ever. (Although specially-allocated shared memory can be shared, this is an “object” and not a “variable” in C terminology.) Where you’re confused is that the on-disk backing is what’s shared between processes, and this is the same whether it’s the main program (static or dynamic linked) or a shared library file. The operating system’s virtual memory implementation takes care of using the same page of physical memory for multiple processes when the contents are unchanged from what’s on-disk, and making physical duplicates of pages at runtime if they’re written to. All of this is transparent to your application, which sees a linear 32- or 64-bit address space consisting of nothing but its own code and data.
In practice, the dynamic linking system makes a number of storage optimizations which isolate the data which will be changed per-process to a few pages, allowing the vast majority of pages to be shared between processes that use the same executable file or same libraries.