Shared library files

Question

Hi,
Can you please tell me the differences between the following shared library files: .a, .so, and .sl
Till now, I have a program which is linking to a .a files of a specific software. Now in the newer version of the software, they removed the .a files and replaced with .so files. Now is the linking process same?
thanks in advance.

Answer 1

Hi,
.a files are static libraries and .so are shared object/libraries.
Shared libraries are the preferred way. Please see docs.sun.com -> linker and library guide. for example at following link: http://docs.sun.com/ab2/coll.45.13/LLM/@Ab2PageView/6367?Ab2Lang=C&Ab2Enc=iso-8859-1
I linking of the executable with the library should be same. But creation of library itself will be different.
Here is the excerpt from one of the FAQ:
<pre>Q. What is a shared library?
A. A shared library, or shared object file, or dynamic library, is a
file in the ELF file format (see elf(3E)) that contains data and usually
code that is needed by application binaries. It, along with any other
shared libraries an application binary depends on, is transparently
loaded into memory when the binary is executed, making itself available
to the binary.
Q. What are the advantages of shared libraries?
A. The major advantage of a shared library is that it may contain code that
more than one binary depends on. Thus, this code is only stored once,
in the shared library, and not in each individual binary. This saves
storage space on disk, especially when dealing with very common code
that nearly every program needs. But the benefits go farther than the
space savings: if the common code needs to be updated (for example, a
patch to fix a bug), this need only be done once, in the shared library.
(There are also more reasons, covered below).
Another big advantage is that programs which use the non shared versions
of the system supplied libraries like libc, are not supported and are
not ABI compliant. See "Why does my program, statically linked under a
different version of Solaris, dump core?" and "Why do I get unresolved
references when trying to link statically?"
Q. What are the disadvantages of shared libraries?
A. It takes time for a program to load all of the shared library files it
needs and to link them to the main program and to other shared libraries
(called dynamic linking). The program will take longer to start up.
Programs that depend on shared libraries will not work if any of the
libraries they depend upon are not available. For people who distribute
software, this often means developers must restrict themselves to using
only the shared libraries that are sure to be present on every system.
Unfortunately of course, nothing is for sure and according to Murphy's
law, there will always be someone who doesn't have whatever library,
or has a different version that the developers used and the different
version doesn't work properly. Arguably, it is cleaner (more elegant)
to have a program that is self contained and requires nothing but
itself.
Finally, the system must be able to locate the require libraries
wherever they may be, which is, again, error prone. (See "What
is LD_LIBRARY_PATH?")
</pre>
Thanks
Kalpesh