Question

My C++ compiler creates “dylib” files which contain dynamic libraries. Whats the difference between .dylib and .so files?

And what is the difference between files in Mach-O format and files in an ELF format? I have to build files for later use under iOS (static libraries only/Mach-O) and Android (ELF).

Thanx!

Answer 1

I found that:

One Mach-O feature that hits many people by surprise is the strict
distinction between shared libraries and dynamically loadable modules.
On ELF systems both are the same; any piece of shared code can be used
as a library and for dynamic loading. Use otool -hv some_file to see
the filetype of some_file.

Mach-O shared libraries have the file type MH_DYLIB and carry the
extension .dylib. They can be linked against with the usual static
linker flags, e.g. -lfoo for libfoo.dylib. However, they can not be
loaded as a module. (Side note: Shared libraries can be loaded
dynamically through an API. However, that API is different from the
API for bundles and the semantics make it useless for an dlopen()
emulation. Most notably, shared libraries can not be unloaded.) [This
is no longer true—you can use dlopen() with both dylibs and bundles.
However, dylibs still can’t be unloaded.]

Loadable modules are called “bundles” in Mach-O speak. They have the
file type MH_BUNDLE. Since no component involved cares about it, they
can carry any extension. The extension .bundle is recommended by
Apple, but most ported software uses .so for the sake of
compatibility. Bundles can be dynamically loaded and unloaded via dyld
APIs, and there is a wrapper that emulates dlopen() on top of that
API. [dlopen is now the preferred API.] It is not possible to link
against bundles as if they were shared libraries. However, it is
possible that a bundle is linked against real shared libraries; those
will be loaded automatically when the bundle is loaded.

To compile a normal shared library on OS X, you should use -dynamiclib
and the extension .dylib. -fPIC is the default.