从NASM函数返回一个char数组

Question

I'm trying to do the following for the sake of practice in NASM:

1)Read a string from command-line in C

2)Pass that string to a NASM function which takes the string as its first parameter

3)Return that exact string from NASM function

prefix.asm:

;nasm -f elf32 prefix.asm -o prefix.o

segment .bss
pre resb 256

segment .text
global prefix

    prefix:
        push ebp          ;save the old base pointer value
        mov  ebp,esp      ;base pointer <- stack pointer

        mov eax,[ebp+8]   ;function argument

        add esp, 4
        pop ebp
        ret

prefix c:

//nasm -f elf32 prefix.asm -o prefix.o
//gcc prefix.c prefix.o -o prefix -m32
#include <stdio.h>
#include <string.h>

char* prefix(char *str);

int main(void)
{
    char str[256];
    char* pre;
    int a;

    printf("Enter string: ");
    scanf("%s" , str) ;
    pre = prefix(str);
    printf("Prefix array: %s\n", pre);
    return 0;
}

After I run(it compiles w/o any problem) and supply my string to the program I get a Segmentation fault (core dumped) error.

Answer 1

First try to write a C program to implement char* prefix(char *str) , disassemble it and understand it.

Problem 1: the add esp, 4 should be deleted. A function should preserve the stack pointer. Ie the esp should be the same before the first instruction and before the return instruction. Your assembly code increases esp by 4.

Problem 2: Don't name your .asm and .c to be the same. Use different names.