difference between *.a and *.so libs

[MikeCyber]

Hi
what's the difference between *.so and *.a libs?
Thanks

[Bachstelze]

Very roughly :

*.a libs are called static, which means that when you compile a program that uses the library, the library is built into the program. This has the advantage that you do not need to distribute the library along with the program, but the downside is that if you want to use a newer version of the library (e.g. for a bug fix), you need to recompile the entire program. It also makes your program larger.

*.so libraries are called shared (so stands for shared object), and they are basically the opposite of static libraries. When you compile a program against a shared library, the library is accessed at run-time, meaning that the library file must be present on the system when you run the program.

[Bachstelze]

By the way, if you are wondering what the .a stands for, it's for archive, because a static library is basically an archive of code that you can use in your program (it is created with the ar tool).

[MikeCyber]

Thanks. For ex.:
--------
michael@michael-ubuntu:/usr/local/lib$ find . -name "*Bullet*"
./libOgreBulletDyn.a
./libOgreBulletDyn.so.0.0.0
./libOgreBulletDyn.la
./libBulletDynamics.a
./libOgreBulletCol.a
./libBulletSoftBody.so.0
./pkgconfig/OgreBullet.pc
./libOgreBulletCol.so.0.0.0
./libBulletWorldImporter.a
./libBulletDynamics.so.0
./libBulletDynamics.la
./libBulletCollision.la
./libBulletDynamics.so.0.0.0
./libBulletSoftBody.la
./libBulletCollision.so
./libOgreBulletDyn.so.0
./libOgreBulletCol.la
./libBulletSoftBody.so.0.0.0
./libOgreBulletCol.so.0
./libBulletMultiThreaded.a
./libBulletCollision.so.0
./libBulletCollision.a
./libBulletCollision.so.0.0.0
./libBulletSoftBody.a
./libOgreBulletDyn.so
./libBulletSoftBody.so
./libOgreBulletCol.so
./libBulletSoftBodySolvers_OpenCL_Mini.a
./libBulletDynamics.so
./libBulletFileLoader.a
michael@michael-ubuntu:/usr/local/lib$

shows even *.la. What are those? Now how to link to a static or what's the difference to link to a shared lib? I've only linked to a shared lib so far, but would link now, as they're there, to static libs.
Many thanks

[Barrucadu]

Unless you have some specific need to (precise version requirements, needing to patch the library, etc), generally you don't link against static libraries as it just increases the size of your binary.