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: ( https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html ) > -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, 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. Regards.
