What are libtool's .la file for?

It is a textual file that includes a description of the library.

It allows libtool to create platform-independent names.

For example, libfoo goes to:

Under Linux:

/lib/libfoo.so       # Symlink to shared object
/lib/libfoo.so.1     # Symlink to shared object
/lib/libfoo.so.1.0.1 # Shared object
/lib/libfoo.a        # Static library
/lib/libfoo.la       # 'libtool' library

Under Cygwin:

/lib/libfoo.dll.a    # Import library
/lib/libfoo.a        # Static library
/lib/libfoo.la       # libtool library
/bin/cygfoo_1.dll    # DLL

Under Windows MinGW:

/lib/libfoo.dll.a    # Import library
/lib/libfoo.a        # Static library
/lib/libfoo.la       # 'libtool' library
/bin/foo_1.dll       # DLL

So libfoo.la is the only file that is preserved between platforms by libtool allowing to understand what happens with:

• Library dependencies
• Actual file names
• Library version and revision

Without depending on a specific platform implementation of libraries.

According to http://blog.flameeyes.eu/2008/04/14/what-about-those-la-files, they're needed to handle dependencies. But using pkg-config may be a better option:

In a perfect world, every static library needing dependencies would have its own .pc file for pkg-config, and every package trying to statically link to that library would be using pkg-config --static to get the libraries to link to.

I found very good explanation about .la files here http://openbooks.sourceforge.net/books/wga/dealing-with-libraries.html

Summary (The way I understood): Because libtool deals with static and dynamic libraries internally (through --diable-shared or --disable-static) it creates a wrapper on the library files it builds. They are treated as binary library files with in libtool supported environment.