프로그램 사용/gcc2016.06.22 12:25

-f 니까 플래그고

PIC(Position Independent Code)

[링크 : http://devanix.tistory.com/198]


-shared 하면 자동으로 -fpic나 -fPIC가 설정되는건 아닌듯?

아무튼. .x86에는 연관이 없지만, 다른 아키텍쳐에서는 GOT 크기 문제로 -fpic와 -fPIC가 다르게 적용된다고 한다.

-shared

Produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options that were used to generate code (-fpic, -fPIC, or model suboptions) when you specify this option.[1]


-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.


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


-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 .

Position-independent code requires special support, and therefore works only on certain machines.


When this flag is set, the macros "__pic__" and "__PIC__" are defined to 2.


-fpie

-fPIE

These options are similar to -fpic and -fPIC, but generated position independent code can be only linked into executables. Usually these options are used when -pie GCC option will be used during linking.

-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The macros have the value 1 for -fpie and 2 for -fPIE. 

[링크 : http://linux.die.net/man/1/gcc]




Pseudo-assembly:


PIC: This would work whether the code was at address 100 or 1000

100: COMPARE REG1, REG2

101: JUMP_IF_EQUAL CURRENT+10

...

111: NOP


Non-PIC: This will only work if the code is at address 100

100: COMPARE REG1, REG2

101: JUMP_IF_EQUAL 111

...

111: NOP


If your code is compiled with -fPIC, it's suitable for inclusion in a library - the library must be able to be relocated from its preferred location in memory to another address, there could be another already loaded library at the address your library prefers. 

[링크 : http://stackoverflow.com/questions/5311515/gcc-fpic-option?rq=1]

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