无法将参数从 C 传递给汇编代码 [英] Can't pass parameter from C to Assembly code

问题描述

据我了解，当在 C 中的函数调用中传递参数时，被调用者可以在 [ebp+8] 处找到第一个参数.
通过 eax 返回一个值对我有用，从堆栈中读取正确的参数值不起作用.

From what I understand, when a parameter is passed in a function call in C, the callee can find the first parameter at [ebp+8].
Returning a value through eax works for me, reading the right parameter value from the stack doesn't.

现在我只是想写一个汇编函数，它可以从 C 调用并返回相同的值，它正在被传递.

Right now I'm just trying to write an assembly function, that can be called from C and returns the same value, that it is being passed.

当我运行以下程序时，它会将 number: 1 打印到控制台，无论将什么值传递给 myFunc.我做错了什么?

When I run the following program, it prints number: 1 to the console, no matter what value is passed into myFunc. What am I doing wrong?

assembly.s

section .text
    global _myFunc

    _myFunc:
        mov eax, [ebp+8]
        ret

ma​​in.c

#include <stdio.h>

extern unsigned int myFunc(unsigned int somedata);

int main() {
    unsigned int i = myFunc(6);
    printf("number: %i\n",i);
    return 0;
}

我用的是 Mac，用 nasm 来汇编代码，用 gcc 来编译 C.

I'm using a Mac, nasm to assemble the code and gcc for C compilation.

Makefile

macho32:
    nasm -f macho32 assembly.s
    gcc -m32 -o macho32 assembly.o main.c

推荐答案

您通过从 [EBP+offset] 中读取来引用堆栈上的参数 - EBP 是否已设置为实际指向堆栈?如果不是，您可能必须先执行此操作，通常通过以下方式完成:

you refer to argument on stack, by reading from [EBP+offset] - has EBP been set up to actually point at stack? If no, you may have to do that first, conventionally done by:

push  ebp    
mov   ebp,esp

然后才将 EBP 指向其堆叠的先前内容、堆叠的返回地址下方以及传递的参数下方.

Only then points EBP to its stacked previous contents, below stacked return address, and below passed arguments.

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