Hello everyone,


I am confused about the meaning and function of -fpic and -fPIC option of gcc. I am more confused when after reading the manual of gcc (for example, what means PIC, GOT, constant addresses, etc.). Could anyone explain to me what are their functions in easy to understand words?

Here are the descriptions from gcc manual, which makes me confused. :-)

--------------------
-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, PowerPC and SPARC.
Chapter 3: GCC Command Options 193
Position-independent code requires special support, and therefore works only
on certain machines.
--------------------


thanks in advance,
George

Hi -

The basic issue is:
1. If you've got a shared library

2. How can you share static data in the library among multiple processes, each of which
will map that data to completely different addresses?

"Position Independent Code" ("PIC") is one common solution.

Here are a couple of links that might help explain the details a bit better:
http://docs.hp.com/en/B2355-90655/ch07s03.html
http://www.iecc.com/linker/linker10.html
http://en.wikipedia.org/wiki/Shared_library
http://en.wikipedia.org/wiki/Position_Independent_Code

'Hope that helps .. PSM

Thank you paulsm4,


I have read some of the materials. Seems that -fpic generates PIC and -fPIC does not generate PIC at all, is my understanding correct?

But if -fPIC does not generate PIC in shared library, how could other processes use the shared library? Since the address can not be decided until the execution of a process (so, non-PIC code, for example, specific address based on virtual memory, is not suitable).


regards,
George