linux - Passing argument from C to Assembly -
i'm trying make program in c uses function assembly. below can see code:
sum_c.c
#include <stdio.h> extern int _assemblysum(int x, int y); int main(int argc, char *argv[]){ int total; total = _assemblysum(4, 2); printf("%d\n", total); return 0; } assembly_sum.asm
section .data section .text global _assemblysum _assemblysum: push rbp mov rbp, rsp mov rax, [rbp+16] mov rbx, [rbp+24] add rax, rbx pop rbp ret compile
nasm -f elf64 assembly_sum.asm -o assembly_sum.o gcc c_sum.c assembly_sum.o -o sum ./sum when run program random numbers -1214984584 or 2046906200. know need use registers rdi, rsi, rdx , rcx because 64bit gnu/linux compiler uses them (passing arguments c 64bit linux assembly). how can that?
you may have confused calling convention being used.
linux uses 64-bit system v calling convention. under convention, registers preferred on stack passing of integer type parameters. registers integer passing used in following order:
- %rdi
- %rsi
- %rdx
- %rcx
- %r8
- %r9
if additional integer parameters used, passed on stack.
you can find detailed information in system v abi specification.
