汇编返回int到C函数段 [英] Assembly Return int to C function segfaults

问题描述

我正在完成一个汇编程序，该程序用给定的替换字符替换字符串中的字符.汇编代码调用C函数，而汇编程序本身是从我的.c文件中的main调用的.但是，当尝试完成并从汇编程序向C返回最终的int值时，我遇到了段错误.我的.asm文件如下:

I am finishing up an assembly program that replaces characters in a string with a given replacement character. The assembly code calls C functions and the assembly program itself is called from main in my .c file. However, when trying to finish and return a final int value FROM the assembly program TO C, I get segfaults. My .asm file is as follows:

; File: strrepl.asm
; Implements a C function with the prototype:
;
;   int strrepl(char *str, int c, int (* isinsubset) (int c) ) ;
;
; 
; Result: chars in string are replaced with the replacement character and string is returned.

    SECTION .text
    global  strrepl


_strrepl:   nop
strrepl:
    push    ebp         ; set up stack frame
    mov ebp, esp

    push    esi         ; save registers
    push    ebx
    xor eax, eax
    mov ecx, [ebp + 8]      ;load string (char array) into ecx
    jecxz   end         ;jump if [ecx] is zero
    mov al, [ebp + 12]      ;move the replacement character into esi
    mov edx, [ebp + 16]     ;move function pointer into edx

firstLoop:

    xor eax, eax

    mov edi, [ecx]
    cmp edi, 0
    jz  end

    mov edi, ecx        ; save array


    movzx   eax, byte [ecx]     ;load single byte into eax  
    push    eax         ; parameter for (*isinsubset)
    mov edx, [ebp + 16]         
    call    edx         ; execute (*isinsubset)


    mov ecx, edi        ; restore array
    cmp eax, 0
    jne secondLoop  
    add esp, 4          ; "pop off" the parameter
    mov ebx, eax        ; store return value
    add ecx, 1
    jmp firstLoop

secondLoop:
    mov eax, [ebp+12]
    mov [ecx], al
    mov edx, [ebp+16]
    add esp, 4
    mov ebx, eax
    add ecx, 1
    jmp     firstLoop

end:
    pop ebx         ; restore registers
    pop esi
    mov esp, ebp        ; take down stack frame
    pop ebp
    mov eax, 9
    push    eax         ;test
    ret

我的c文件是:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

//display *((char *) $edi)
// These functions will be implemented in assembly:
//

int strrepl(char *str, int c, int (* isinsubset) (int c) ) ;


int isvowel (int c) {

   if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') 
      return 1 ;

   if (c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U') 
      return 1 ;

   return 0 ;
}

int main(){
    char *str1;
    int r;


    str1 = strdup("ABC 123 779 Hello World") ;
    r = strrepl(str1, '#', &isdigit) ;
    printf("str1 = \"%s\"\n", str1) ;
    printf("%d chararcters were replaced\n", r) ;
    free(str1) ;
    return 0;
}

在我的汇编代码中，您可以最终看到

In my assembly code, you can see in end

mov eax, 9
push    eax 

我只是试图将值9返回到值"r"，该值是C文件中的int值.这只是一个测试，看是否可以在c文件中将int返回给r.最终，我将返回的字符数返回给r.但是，我需要弄清楚为什么上面的以下代码是段错误的.有任何想法吗?

I am simply trying to return the value 9 to the value "r" which is an int in the C file. This is just a test to see if I can return an int back to r in the c file. Eventually I will be returning the number of characters that were replaced back to r. However, I need to figure out why the following code above is segfaulting. Any ideas?

推荐答案

mov     eax, 9
push    eax          ; NOT a good idea
ret

那是一个大错误.它会根据堆栈中最低的内容返回，而您只是将肯定不是有效的返回地址的内容压入堆栈中.

That is a big mistake. It's going to return based on the lowest thing on the stack and you've just pushed something on to the stack that's almost certainly not a valid return address.

大多数函数只需将其放入eax中就可以返回代码(这当然取决于调用约定，但这是很常见的)，通常无需将其压入堆栈，当然还有很多不足之处这样做.

Most functions return a code by simply placing it into eax (this depends on calling convention of course but that's a pretty common one), there's generally no need to push it on to the stack, and certainly plenty of downside to doing so.

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