Program location in the memory and static/shared libraries

Question

When I run a program (in linux) does it all get loaded into the physical memory? If so, is using shared libraries, instead of static libraries, help in terms of caching? In general, when should I use shared libraries and when should I use static libraries? My codes are either written in C or in C++ if that matters.

Answer 1

This article hits covers some decent ground on what you want. This article goes much deeper about the advantages of shared libraries

SO also has covered this topic in depth

Difference between static and shared libraries?

When to use dynamic vs. static libraries

Almost all the above mentioned articles are shared library biased. Wikipedia tries to rescue static libraries :)

From wiki,

There are several advantages to statically linking libraries with an executable instead of dynamically linking them. The most significant is that the application can be certain that all its libraries are present and that they are the correct version. This avoids dependency problems. Usually, static linking will result in a significant performance improvement.

Static linking can also allow the application to be contained in a single executable file, simplifying distribution and installation.

With static linking, it is enough to include those parts of the library that are directly and indirectly referenced by the target executable (or target library).

With dynamic libraries, the entire library is loaded, as it is not known in advance which functions will be invoked by applications. Whether this advantage is significant in practice depends on the structure of the library.

Answer 2

Shared libraries are used mostly when you have functionality that could be used and "shared" across different programs. In that case, you will have a single point where all the programs will get their methods. However, this creates a dependency problem since now your compiled programs are dependent on that specific version of the library.

Static libraries are used mostly when you don't want to have dependency issues and don't want your program to care which X or Y libraries are installed on your target system.

So, which one to use?. for that you should answer the following questions:

1. Will your program be used on different platforms or Linux distributions? (e.g. Red Hat, Debian, SLES11-SP1)
2. Do you have replicated code that is being used by different binaries?
3. Do you envision that in the future other programs could benefit from your work?

I think this is a case by case decision, and it is not a one size fits all kind of answer.