Library extension .a/.so

[Andrew Hammond]

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Byron L. Sonne wrote:
>> What is the difference in libraries ending with extension .a and .so.
>> Can one convert  from somelib.a to somelib.so?
> 
> I'd like to know as well - I've tended to think that it's the difference
> between static and dynamic, but upon reflection I don't have a good
> reason for thinking that. Or if I did, I've forgotten it.

You are correct. Archive libraries end with the .a and can be created
and analyzed using ar ranlib and nm (the man pages for each are pretty
helpful). They are linked statically on compile to the binary you're
building. Statically linked libraries are great for debugging and, in a
few very special circumstances can yeild a performance boost.

Dynamically linked libraries are the libsomething.so.<versioning> that
you often see under /lib and /usr/lib with a whole bunch of symlinks
pointing to them. They're loaded dynamically, usually when a program
starts. There are a number of big wins that come from dynamically linked
libraries. The obvious ones are that you only need to have a single
image of the library binary in memory and that you can upgrade libraries
without re-compiling applications.

Creating solibs is done by gcc (unless you want to call the linker
directly, which is probably not a good idea)

gcc -shared -Wl,-soname,your_soname -o library_name file_list library_list

You could put a .a library in the library list to convert it to a solib
if you were so inclined. I don't know how to go in the other direction.

There's a good FAQ on this subject, which covers a lot of stuff I've
glossed over, at
http://www.faqs.org/docs/Linux-HOWTO/Program-Library-HOWTO.html

- --
Andrew Hammond    416-673-4138    ahammond-swQf4SbcV9C7WVzo/KQ3Mw at public.gmane.org
Database Administrator, Afilias Canada Corp.
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