On Thu, Jan 05, 2012 at 07:32:48PM -0800, Fredrick wrote:
Hi,
I am not able to understand the DEFINE macro used in arch/powerpc/kernel/asm-offsets.c
I suppose the DEFINE is present in include/linux/kbuild.h where it says #define DEFINE(sym, val) \ asm volatile("\n->" #sym " %0 " #val : : "i" (val))
What does the above mean?
This is just a trick to get the offsets of members into a generated header file asm-offsets.h. The inline assembly does NOT contain valid instructions, and in fact, asm-offsets.c is never actually assembled into a program. Instead, the build process generates the assembly language output asm-offsets.s, and processes it with a sed script to generate asm-offsets.h. For example (assume offsetof(struct thread_struct, regs) is 30): DEFINE(PT_REGS, offsetof(struct thread_struct, regs)); will generate within the assembly language output: ->PT_REGS $30 offsetof(struct thread_struct, regs) A sed script, executed on the assembly language output will generate a line in include/generated/asm-offsets.h: #define PT_REGS 30 /* offsetof(struct thread_struct, regs) */ Thats about it. You can find the exact sed script used, and the make magic involved in Kbuild (see cmd_offsets). -- joshc