What are .a and .so files?

488
votes

I'm currently trying to port a C application to AIX and am getting confused. What are .a and .so files and how are they used when building/running an application?

5
cunixcompilationshared-libraries.a
one is library archive and other is shared object – Shiplu Mokaddim
Possible duplicate of What's the difference between .so, .la and .a library files?. – Peter Mortensen

5 Answers

548
votes

Archive libraries (.a) are statically linked i.e when you compile your program with -c option in gcc. So, if there's any change in library, you need to compile and build your code again.

The advantage of .so (shared object) over .a library is that they are linked during the runtime i.e. after creation of your .o file -o option in gcc. So, if there's any change in .so file, you don't need to recompile your main program. But make sure that your main program is linked to the new .so file with ln command.

This will help you to build the .so files. http://www.yolinux.com/TUTORIALS/LibraryArchives-StaticAndDynamic.html

Hope this helps.

244
votes

.a are static libraries. If you use code stored inside them, it's taken from them and embedded into your own binary. In Visual Studio, these would be .lib files.

.so are dynamic libraries. If you use code stored inside them, it's not taken and embedded into your own binary. Instead it's just referenced, so the binary will depend on them and the code from the so file is added/loaded at runtime. In Visual Studio/Windows these would be .dll files (with small .lib files containing linking information).

13
votes

.a files are usually libraries which get statically linked (or more accurately archives), and
.so are dynamically linked libraries.

To do a port you will need the source code that was compiled to make them, or equivalent files on your AIX machine.

11
votes

They are used in the linking stage. .a files are statically linked, and .so files are sort-of linked, so that the library is needed whenever you run the exe.

You can find where they are stored by looking at any of the lib directories... /usr/lib and /lib have most of them, and there is also the LIBRARY_PATH environment variable.

2
votes

Wikipedia is a decent source for this info.

To learn about static library files like .a read Static libarary

To learn about shared library files like .so read Library_(computing)#Shared_libraries On this page, there is also useful info in the File naming section.