Ian Lance Taylor wrote:
Marcus Clyne <maccaday@xxxxxxxxx> writes:
If a shared object is loaded using dlopen etc and the symbols in its
object aren't used (specifically they are defined elsewhere already
inside that process), is the data in the symbols automatically loaded
into memory or is it skipped (assuming there are no other programs
using those symbols on the system) and the memory available for other
purposes?
If you open a shared library with dlopen, then that library is loaded
into memory. This will use up virtual address space. If you don't use
the library for anything, then the pages will never be loaded into RAM.
When the object loaded initially with dlopen(), is there anything
that's transferred to RAM (e.g. the page where the symbol names are),
or are they only copied to the swap space directly? If that's the
case, are they then loaded into RAM when dlsym() is called?
Most of a shared library is read-only and shared. Those pages are
simply mapped into your virtual address space--no copying occurs. In a
typical shared library, a few pages must be modified by the dynamic
linker when the library is loaded: those pages will be copied into RAM
and modified, and if not referenced will eventually be swapped out.
Would pages that used symbols from other libraries be included in the
pages that are copied and modified? If that's the case, what about
pages that have calls to symbols in the standard c library. Are they
also copied/modified? What about global variables inside the shared
object? Presumably they are copied, no?
Thanks,
Marcus.
