give structure offset attribute to assembler

Question

how can I send the offset of a C struct to en assembly code ? For example

In my C code I have

typedef struct
{
  unsigned int a;
  unsigned int b;
} CMyStruct;

I send to an ASM function a pointer of a CMyStruct structure Let suppose that my pointer is into R0

To access to a and b attribute I need to do that.

ldr      r1, [r0, #0] // read a
ldr      r2, [r0, #4] // read b

Is there anyway to not specify #0 and #4 as contant value ? Something like

ldr      r1, [r0, CMyStruct.a] // read a
ldr      r2, [r0, CMyStruct.b] // read b

Thank's Etienne

Answer 1

There is a way, actually. The solution is contains a little bit of magic, but it's works. It's just works.

in c file:

#define DEFINE(sym, val) asm volatile("\n-> " #sym " %0 " #val "\n" : : "i" (val))
#define OFFSETOF(s, m) \
    DEFINE(offsetof_##s##_##m, offsetof(s, m));

#define SIZEOF(s) \
    DEFINE(sizeof_##s, sizeof(s));

void foo()
{
    OFFSETOF(KERNEL, error);
    OFFSETOF(KERNEL, pool);
    SIZEOF(KERNEL);
}

in Makefile:

asm_defines.h: asm_defines.c
    $(GCC) $(FLAGS_CC) -S $< -o - | awk '($$1 == "->") { print "#define " $$2 " " $$3 }' > $(BUILD_DIR)/$@

Finally you will got asm_defines.h, which you can include in your .S file.

#define offsetof_KERNEL_error #16
#define offsetof_KERNEL_pool #4
#define sizeof_KERNEL #120

Answer 2

How about something like this:

#include <stddef.h>     /* offsetof */

struct CMyStruct 
{
  unsigned int a;
  unsigned int b;
};

int main()
{
   asm("ldr r1, [r0, %[OffsetOfA]] \n\t"
       "ldr r2, [r0, %[OffsetOfB]] \n\t" 
   : /* no outputs */
   : [OffsetOfA] "i" (offsetof(struct CMyStruct, a)), [OffsetOfB] "i" (offsetof(struct CMyStruct, b)));
}

Obviously that's not quite right, since you don't want to hard code r0/r1/r2, but it should point you in the right direction.

Answer 3

You could use GCC extended inline assembly code and use the offsetof macro.