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A shared library or shared object is a file that is intended to be shared by multiple programs. Symbols used by a program are loaded from shared libraries into memory at load time or runtime.
Simply put, A shared library/ Dynamic Library is a library that is loaded dynamically at runtime for each application that requires it. Dynamic Linking doesn't require the code to be copied, it is done by just placing name of the library in the binary file.
The advantages of shared libraries are: Less disk space is used because the shared library code is not included in the executable programs. Less memory is used because the shared library code is only loaded once. Load time may be reduced because the shared library code may already be in memory.
PIC stands for Position Independent Code.
To quote man gcc:
If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on AArch64, m68k, PowerPC and SPARC.
Use this when building shared objects (*.so) on those mentioned architectures.
The f is the gcc prefix for options that "control the interface conventions used
in code generation"
The PIC stands for "Position Independent Code", it is a specialization of the fpic for m68K and SPARC.
Edit: After reading page 11 of the document referenced by 0x6adb015, and the comment by coryan, I made a few changes:
This option only makes sense for shared libraries and you're telling the OS you're using a Global Offset Table, GOT. This means all your address references are relative to the GOT, and the code can be shared accross multiple processes.
Otherwise, without this option, the loader would have to modify all the offsets itself.
Needless to say, we almost always use -fpic/PIC.
man gcc says:
-fpic Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC and 32k on the m68k and RS/6000. The 386 has no such limit.) Position-independent code requires special support, and therefore works only on certain machines. For the 386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent. -fPIC If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on the m68k and the SPARC. Position-independent code requires special support, and therefore works only on certain machines.
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