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Re: what does -fno-pic do

On 2016年07月11日 16:44, Mason wrote:
On 10/07/2016 16:15, Yubin Ruan wrote:

      I am reading some OS kernel codes, and I find that in the Makefile
used to build the kernel, there is a **-fno-pic** flag. I totally don't
understand what that mean. I try to find some description from the man
pages but can find no direct description about that flag (same for the
**-fpic** flags).

Really? A search for gcc fpic returns many relevant results.

Here, pic stands for "position-independent code".

The latest gcc documentation states:
( )


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, 28k on AArch64 and 32k on the
m68k and RS/6000. The x86 has no such limit.)

Position-independent code requires special support, and therefore
works only on certain machines. For the x86, GCC supports PIC for
System V but not for the Sun 386i. Code generated for the IBM RS/6000
is always position-independent.

When this flag is set, the macros __pic__ and __PIC__ are defined to 1.


I really search carefully in the man pages on my Ubuntu/Linux-14.04-LTS. I cannot find much description about that. Afterwards I find the online manual that you describe.

Actually, I wonder why the man pages on my Ubuntu/Linux is so different from that online.


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