从汇编程序获取命令行参数 [英] Getting command line parameters from an assembly program

问题描述

通读《专业汇编语言书》；似乎它为读取命令行参数提供了错误的代码.我进行了一些更正，现在它从段错误变为读取参数计数再到段错误.

Reading through the "Professional Assembly Language Book"; it seems that it provides an erroneous code for reading command-line arguments. I corrected it a bit and now it went from segfaulting to reading argument count then segfaulting.

这是完整的代码:

.data
    output1:
        .asciz "There are %d params:\n"
    output2:
        .asciz "%s\n"

.text
.globl main
main:
    movl 4(%esp), %ecx  /* Get argument count.  */
    pushl %ecx
    pushl $output1
    call printf
    addl $4, %esp       /* remove output1  */

    /* ECX was corrupted by the printf call,
       pop it off the stack so that we get it's original
       value.  */
    popl %ecx

    /* We don't want to corrupt the stack pointer
       as we move ebp to point to the next command-line
       argument.  */
    movl %esp, %ebp
    /* Remove argument count from EBP.  */
    addl $4, %ebp

pr_arg:
    pushl (%ebp)
    pushl $output2
    call printf
    addl $8, %esp       /* remove output2 and current argument.  */
    addl $4, %ebp       /* Jump to next argument.  */
    loop pr_arg

    /* Done.  */
    pushl $0
    call exit

书中的代码:

.section .data
output1:
    .asciz "There are %d parameters:\n"
output2:
    .asciz "%s\n"
.section .text
.globl _start
_start:
    movl (%esp), %ecx
    pushl %ecx
    pushl $output1
    call printf
    addl $4, %esp
    popl %ecx
    movl %esp, %ebp
    addl $4, %ebp
loop1:
    pushl %ecx
    pushl (%ebp)
    pushl $output2
    call printf
    addl $8, %esp
    popl %ecx
    addl $4, %ebp
    loop loop1
    pushl $0
    call exit

用GCC(gcc cmd.S)编译，也许是问题所在? __libc_start_main是否以某种方式修改了堆栈?不太确定...

Compiled it with GCC (gcc cmd.S), maybe that's the problem? __libc_start_main modifies the stack in some way? Not quite sure...

更糟糕的是，尝试对其进行调试以查看堆栈，但是GDB似乎抛出了许多与printf相关的东西(其中之一是printf.c: File not found或类似的东西).

Even worse, trying to debug it to look at the stack but GDB seems to throw a lot of printf-related stuff (one of them was printf.c: File not found or something similar).

推荐答案

在@Michael的帮助下，我能够找到问题所在.

With the help from @Michael, I was able to track down the problem.

按照@Michael的建议，将％ebp用作argv(尽管他使用了%eax).另一个问题是我需要将(％ebp)的值与0(空终止符)进行比较，然后在该点结束程序.

Using %ebp as argv as @Michael suggested (he used %eax though). Another problem was that I needed to compare the value of (%ebp) with 0 (the null terminator) and end the program at that point.

代码:

    movl 8(%esp), %ebp  /* Get argv.  */

pr_arg:
    cmpl $0, (%ebp)
    je endit

    pushl %ecx
    pushl (%ebp)
    pushl $output2
    call printf
    addl $8, %esp       /* remove output2 and current argument.  */
    addl $4, %ebp

    popl %ecx
    loop pr_arg

    ret

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