How is `va_list` implemented in Assembler level on ARMv7 Android?

Question

I need to extract all arguments from CallStaticObjectMethodV JNI call on ARMv7 Android at Assembler level.

Can anyone advice how is va_list implemented on low level in ARMv7 Android?

Answer 1

From Procedure Call Standard for the ARM® Architecture (ARM IHI 0042E):

Typedef: va_list

Base type:

struct __va_list {
void *__ap;
}

Notes:

A va_list may address any object in a parameter list. Consequently, the first object addressed may only have word alignment (all objects are at least word aligned), but any double-word aligned object will appear at the correct double-word alignment in memory. In C++, __va_list is in namespace std.

So basically it's just a pointer to the word-aligned arguments in memory.