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

Question

I have been involved in some debate with respect to libraries in Linux, and would like to confirm some things.

It is to my understanding (please correct me if I am wrong and I will edit my post later), that there are two ways of using libraries when building an application:

1. Static libraries (.a files): At link time, a copy of the entire library is put into the final application so that the functions within the library are always available to the calling application
2. Shared objects (.so files): At link time, the object is just verified against its API via the corresponding header (.h) file. The library isn't actually used until runtime, where it is needed.

The obvious advantage of static libraries is that they allow the entire application to be self-contained, while the benefit of dynamic libraries is that the ".so" file can be replaced (ie: in case it needs to be updated due to a security bug) without requiring the base application to be recompiled.

I have heard some people make a distinction between shared objects and dynamic linked libraries (DLL's), even though they are both ".so" files. Is there any distinction between shared objects and DLLs when it comes to C/C++ development on Linux or any other POSIX compliant OS (ie: MINIX, UNIX, QNX, etc)? I am told that one key difference (so far) is that shared objects are just used at runtime, while DLL's must be opened first using the dlopen() call within the application.

Finally, I have also heard some developers mention "shared archives", which, to my understanding, are also static libraries themselves, but are never used by an application directly. Instead, other static libraries will link against the "shared archives" to pull some (but not all) functions/resources from the shared archive into the static library being built.

Thank you all in advance for your assistance.

Update

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In the context in which these terms were provided to me, it was effectively erroneous terms used by a team of Windows developers that had to learn Linux. I tried to correct them, but the (incorrect) language norms stuck.

1. Shared Object: A library that is automatically linked into a program when the program starts, and exists as a standalone file. The library is included in the linking list at compile time (ie: LDOPTS+=-lmylib for a library file named mylib.so). The library must be present at compile time, and when the application starts.
2. Static Library: A library that is merged into the actual program itself at build time for a single (larger) application containing the application code and the library code that is automatically linked into a program when the program is built, and the final binary containing both the main program and the library itself exists as a single standalone binary file. The library is included in the linking list at compile time (ie: LDOPTS+=-lmylib for a library file named mylib.a). The library must be present at compile time.
3. DLL: Essentially the same as a shared object, but rather than being included in the linking list at compile time, the library is loaded via dlopen()/dlsym() commands so that the library does not need to be present at build time for the program to compile. Also, the library does not need to be present (necessarily) at application startup or compile time, as it is only needed at the moment the dlopen/dlsym calls are made.
4. Shared Archive: Essentially the same as a static library, but is compiled with the "export-shared" and "-fPIC" flags. The library is included in the linking list at compile time (ie: LDOPTS+=-lmylibS for a library file named mylibS.a). The distinction between the two is that this additional flag is required if a shared object or DLL wants to statically link the shared archive into its own code AND be able to make the functions in the shared object available to other programs, rather than just using them internal to the DLL. This is useful in the case when someone provides you with a static library, and you wish to repackage it as an SO. The library must be present at compile time.

Additional Update

The distinction between "DLL" and "shared library" was just a (lazy, inaccurate) colloquialism in the company I worked in at the time (Windows developers being forced to shift to Linux development, and the term stuck), adhering to the descriptions noted above.

Additionally, the trailing "S" literal after the library name, in the case of "shared archives" was just a convention used at that company, and not in the industry in general.

Answer 1

I've always thought that DLLs and shared objects are just different terms for the same thing - Windows calls them DLLs, while on UNIX systems they're shared objects, with the general term - dynamically linked library - covering both (even the function to open a .so on UNIX is called dlopen() after 'dynamic library').

They are indeed only linked at application startup, however your notion of verification against the header file is incorrect. The header file defines prototypes which are required in order to compile the code which uses the library, but at link time the linker looks inside the library itself to make sure the functions it needs are actually there. The linker has to find the function bodies somewhere at link time or it'll raise an error. It ALSO does that at runtime, because as you rightly point out the library itself might have changed since the program was compiled. This is why ABI stability is so important in platform libraries, as the ABI changing is what breaks existing programs compiled against older versions.

Static libraries are just bundles of object files straight out of the compiler, just like the ones that you are building yourself as part of your project's compilation, so they get pulled in and fed to the linker in exactly the same way, and unused bits are dropped in exactly the same way.

Answer 2

A static library(.a) is a library that can be linked directly into the final executable produced by the linker,it is contained in it and there is no need to have the library into the system where the executable will be deployed.

A shared library(.so) is a library that is linked but not embedded in the final executable, so will be loaded when the executable is launched and need to be present in the system where the executable is deployed.

A dynamic link library on windows(.dll) is like a shared library(.so) on linux but there are some differences between the two implementations that are related to the OS (Windows vs Linux) :

A DLL can define two kinds of functions: exported and internal. The exported functions are intended to be called by other modules, as well as from within the DLL where they are defined. Internal functions are typically intended to be called only from within the DLL where they are defined.

An SO library on Linux doesn't need special export statement to indicate exportable symbols, since all symbols are available to an interrogating process.

Answer 3

I can elaborate on the details of DLLs in Windows to help clarify those mysteries to my friends here in *NIX-land...

A DLL is like a Shared Object file. Both are images, ready to load into memory by the program loader of the respective OS. The images are accompanied by various bits of metadata to help linkers and loaders make the necessary associations and use the library of code.

Windows DLLs have an export table. The exports can be by name, or by table position (numeric). The latter method is considered "old school" and is much more fragile -- rebuilding the DLL and changing the position of a function in the table will end in disaster, whereas there is no real issue if linking of entry points is by name. So, forget that as an issue, but just be aware it's there if you work with "dinosaur" code such as 3rd-party vendor libs.

Windows DLLs are built by compiling and linking, just as you would for an EXE (executable application), but the DLL is meant to not stand alone, just like an SO is meant to be used by an application, either via dynamic loading, or by link-time binding (the reference to the SO is embedded in the application binary's metadata, and the OS program loader will auto-load the referenced SO's). DLLs can reference other DLLs, just as SOs can reference other SOs.

In Windows, DLLs will make available only specific entry points. These are called "exports". The developer can either use a special compiler keyword to make a symbol an externally-visible (to other linkers and the dynamic loader), or the exports can be listed in a module-definition file which is used at link time when the DLL itself is being created. The modern practice is to decorate the function definition with the keyword to export the symbol name. It is also possible to create header files with keywords which will declare that symbol as one to be imported from a DLL outside the current compilation unit. Look up the keywords __declspec(dllexport) and __declspec(dllimport) for more information.

One of the interesting features of DLLs is that they can declare a standard "upon load/unload" handler function. Whenever the DLL is loaded or unloaded, the DLL can perform some initialization or cleanup, as the case may be. This maps nicely into having a DLL as an object-oriented resource manager, such as a device driver or shared object interface.

When a developer wants to use an already-built DLL, she must either reference an "export library" (*.LIB) created by the DLL developer when she created the DLL, or she must explicitly load the DLL at run time and request the entry point address by name via the LoadLibrary() and GetProcAddress() mechanisms. Most of the time, linking against a LIB file (which simply contains the linker metadata for the DLL's exported entry points) is the way DLLs get used. Dynamic loading is reserved typically for implementing "polymorphism" or "runtime configurability" in program behaviors (accessing add-ons or later-defined functionality, aka "plugins").

The Windows way of doing things can cause some confusion at times; the system uses the .LIB extension to refer to both normal static libraries (archives, like POSIX *.a files) and to the "export stub" libraries needed to bind an application to a DLL at link time. So, one should always look to see if a *.LIB file has a same-named *.DLL file; if not, chances are good that *.LIB file is a static library archive, and not export binding metadata for a DLL.

Answer 4

You are correct in that static files are copied to the application at link-time, and that shared files are just verified at link time and loaded at runtime.

The dlopen call is not only for shared objects, if the application wishes to do so at runtime on its behalf, otherwise the shared objects are loaded automatically when the application starts. DLLS and .so are the same thing. the dlopen exists to add even more fine-grained dynamic loading abilities for processes. You dont have to use dlopen yourself to open/use the DLLs, that happens too at application startup.