Difference between shared objects (.so), static libraries (.a), and DLL"s (.so)?

Understanding the Difference Between Shared Objects (.so), Static Libraries (.a), and DLL's (.so)

Are you confused about the difference between shared objects, static libraries, and DLL's? 🤔 Well, you're not alone! Many developers find these terms confusing, but fear not! In this article, I will demystify these concepts, clear up any confusion, and provide easy solutions to common issues. So let's dive right in! 💪

Static Libraries (.a)

Let's start with static libraries. A static library is essentially a collection of object files that are merged into the actual program at build time. This means that when you compile your program, all the necessary code from the static library is included in the final binary. The static library and the main program exist as a single standalone binary file.

To use a static library in your program, you include it in the linking list at compile time. For example, to link against a static library named mylib.a, you would use the linker option -lmylib. One advantage of static libraries is that they make your application self-contained, as all the necessary code is included in the final binary. However, this also means that if any changes are made to the static library, you need to recompile your entire application to incorporate those changes. 😕

Shared Objects (.so)

Now let's move on to shared objects. Unlike static libraries, shared objects are only verified against their API (Application Programming Interface) at link time. The actual use of the shared object happens at runtime, when the functions within the library are needed. This runtime loading of shared objects offers the advantage of flexibility. For example, if a security bug is discovered in the shared object, you can simply replace the .so file without having to recompile your entire application. 🔄

To use a shared object in your program, you include it in the linking list at compile time using the -lmylib option. However, it's important to note that the shared object must be present both at compile time and when the application starts. 🔒

DLL's (.so)

You may have heard the term DLL (Dynamic Link Library) in the context of shared objects. While shared objects and DLL's are both represented by the .so file extension, there are some key differences. In C/C++ development on Linux or other POSIX compliant operating systems, DLL's are loaded into the program dynamically at runtime using the dlopen() and dlsym() commands. This means that the DLL does not need to be present at build time or necessarily at application startup. It is only required when the dlopen() and dlsym() calls are made. 📚

Shared Archives

You may have also come across the term "shared archives". In essence, shared archives are similar to static libraries, but they are compiled with specific flags such as export-shared and -fPIC. These flags allow the shared archive to be statically linked into a shared object or DLL and make the functions in the shared object available to other programs. This is different from the regular static library usage, where functions are only used internally within the library itself. Shared archives are useful when you receive a static library and want to repackage it as a shared object. 💼

It's worth noting that the naming convention of having a trailing S in the library name for shared archives, such as mylibS.a, might be specific to certain companies and is not a general industry practice. 📝

Recap and Final Thoughts

To summarize, static libraries, shared objects, and DLL's serve different purposes in software development:

• Static libraries (*.a) are merged into the program at build time and make the application self-contained. They must be present at compile time.

• Shared objects (*.so) are verified at link time and used at runtime. They allow for flexibility and can be replaced without recompiling the entire application. They must be present at compile time and when the application starts.

• DLL's (*.so) are loaded into the program dynamically at runtime and do not need to be present at build time or necessarily at application startup. They are only required when the dlopen() and dlsym() calls are made.

• Shared archives (*.a) are similar to static libraries but compiled with flags that allow them to be statically linked into shared objects or DLL's. They must be present at compile time.

Understanding these distinctions will help you make informed decisions when it comes to library usage in your projects. So go forth and use the right tools for the right job! 💪

And if you're still unsure or have more questions, feel free to leave a comment or join the discussion below. Let's learn together! 🌟