Which uses more RAM at run time, dynamic linking or static linking?

Question

I know that dynamic linking are smaller on disk but do they use more RAM at run time. Why if so?

Answer 1

The answer is "it depends how you measure it", and also "it depends which platform you're running on".

Static linking uses less runtime RAM, because for dynamic linking the entire shared object needs to be loaded into memory (I'll be qualifying this statement in a second), whilst with static linking only those functions you actually need are loaded.

The above statement isn't 100% accurate. Only the shared object pages that actually contain code you use are loaded. This is still much less efficient than statically linking, which compresses those functions together.

On the other hand, dynamic linking uses much less runtime RAM, as all programs using the same shared object use the same in RAM copy of the code (I'll be qualifying this statement in a second).

The above is a true statement on Unix like systems. On Windows, it is not 100% accurate. On Windows (at least on 32bit Intel, I'm not sure about other platforms), DLLs are not compiled with position independent code. As such, each DLL carries the (virtual memory) load address it needs to be loaded at. If one executable links two DLLs that overlap, the loader will relocate one of the DLLs. This requires patching the actual code of the DLL, which means that this DLL now carries code that is specific to this program's use of it, and cannot be shared. Such collisions, however, should be rare, and are usually avoidable.

To illustrate with an example, statically linking glibc will probably cause you to consume more RAM at run time, as this library is, in all likelihood, already loaded in RAM before your program even starts. Statically linking some unique library only your program uses will save run time RAM. The in-between cases are in-between.