Mike - EMAIL IGNORED wrote: > When I use a shared library, is it all in memory, > or is only the object code currently in use in > memory? I ask because I have some object code > that will be used only rarely. Look up "demand paging" for details of how it gets *into* memory (the short answer is "it depends: Linux won't necessarily load all of a large library, but might well load all of a small one"). See below for how it gets *out* of memory. > Additionally, when the exe using the function exits and > no other exe is using it, is the memory freed? That's not how modern operating systems work. The same physical memory will be regarded *both* as the copy of the library which is actually executed (by any program that needs it) *and* as a cache of what's on disk. If all the programs using the library terminate, then the memory is regarded simply as a cache, in case the library is needed again in the near future. This *is* a Good Thing, because at that point in time, the system has no better use for the memory. Completely free memory isn't earning its keep. The system *does* keep a small pool of really free memory (so new programs, new requests for memory, or new data loaded from disk has somewhere to go). When this pool gets small, the kernel will attempt to free more memory by finding a 4K memory page that hasn't been used recently, and either "swapping" it out to disk, or (in the case of a library) noting that there's a copy on disk anyway, and discarding the memory page. If it needs the data again, it will read it back in. The important thing to note is that normally, the kernel will look at (almost) all memory, whether it's just cache, whether programs are running out of it, or whether it's working data. This happens automatically, and without special privileges, a program can't stop it happening. Basically, the kernel worries about this on your behalf, so you don't have to. More reading: http://lwn.net/Articles/147879/ http://lwn.net/Articles/83588/ James. -- E-mail address: james | A: Because people don't normally read bottom to top. @westexe.demon.co.uk | Q: Why is top-posting such a bad thing? | A: Top-posting. | Q: What is the most annoying thing in e-mail?
