Marcus Clyne wrote: > 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? You're asking a lot of questions here! Code in a shared library is PIC, so its pages are not writable and all fixups go via the GOT or the PLT. Hint: type "GOT PLT" into Google and all will become clear. Andrew.
