027 命令行变元

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/**********************027 命令行变元******************************** * 使用，命令行变元写一个小程序cuntdown。 程序对来自命令行的值连续减 * 量，达到零时产生蜂鸣。 * C语言精彩编程百例 第27*///Countdown program#include<stdio.h>#include<stdlib.h>#include<ctype.h>#include<string.h>int main(int argc,char* argv[]){    int disp,count;    if(argc<2)    {        printf("You must enter the length of the count\n");        printf("on the command line.Try again\n");        exit(1);    }    if (argc==3 && !strcmp(argv[1],"display"))        disp = 1;    else        disp=0;    for(count = atoi(argv[2]);count;--count)        if(disp==1)            printf("%d\n",count);    putchar('\a');    printf("Down");    return 0;}

对应的汇编

    .file   "027.c"    .def    ___main;    .scl    2;  .type   32; .endef    .text    .align 32LC0:    .ascii "You must enter the length of the count\12\0"    .align 32LC1:    .ascii "on the command line.Try again\12\0"LC2:    .ascii "display\0"LC3:    .ascii "%d\12\0"LC4:    .ascii "Down\0"    .align 2.globl _main    .def    _main;  .scl    2;  .type   32; .endef_main:    pushl   %ebp    movl    %esp, %ebp    subl    $24, %esp    andl    $-16, %esp    movl    $0, %eax    movl    %eax, -12(%ebp)    movl    -12(%ebp), %eax    call    __alloca    call    ___main    cmpl    $1, 8(%ebp)    # if argc<2    jg  L74    subl    $12, %esp  # printf    pushl   $LC0    call    _printf    addl    $16, %esp    subl    $12, %esp  # printf    pushl   $LC1    call    _printf    addl    $16, %esp    subl    $12, %esp  # exit(1)    pushl   $1    call    _exitL74:    cmpl    $3, 8(%ebp)    # if argc==3    jne L75    subl    $8, %esp   # 准备调用_strcmp    pushl   $LC2       # "display"    movl    12(%ebp), %eax  # eax=argv     addl    $4, %eax   # eax=argv+4    pushl   (%eax)      # argv[1]    call    _strcmp    addl    $16, %esp  # 调整堆栈    testl   %eax, %eax  # if strcmp    jne L75    movl    $1, -4(%ebp)   # disp =1    jmp L76L75:    movl    $0, -4(%ebp)   # disp =0L76:    subl    $12, %esp  # 准备调用 atoi    movl    12(%ebp), %eax  # eax=argv    addl    $8, %eax   # argv+8    pushl   (%eax)      # argv[2]    call    _atoi    addl    $16, %esp  # 平衡堆栈    movl    %eax, -8(%ebp)  # count = atoi(argv[2])L77:    cmpl    $0, -8(%ebp)   # if count    jne L80    jmp L78L80:    cmpl    $1, -4(%ebp)    # if disp=1    jne L79    subl    $8, %esp   # printf    pushl   -8(%ebp)    pushl   $LC3    call    _printf    addl    $16, %espL79:    leal    -8(%ebp), %eax  # &count    decl    (%eax)      # count--    jmp L77L78:    subl    $12, %esp  # putchar    pushl   $7    call    _putchar    addl    $16, %esp    subl    $12, %esp    pushl   $LC4    call    _printf     # printf    addl    $16, %esp    movl    $0, %eax    leave    ret    .def    _putchar;   .scl    2;  .type   32; .endef    .def    _atoi;  .scl    2;  .type   32; .endef    .def    _strcmp;    .scl    2;  .type   32; .endef    .def    _exit;  .scl    2;  .type   32; .endef    .def    _printf;    .scl    2;  .type   32; .endef

命令行变元和函数的参数传递方式比较类似，32位编译器下，都是通过 8(%ebp)，12(%ebp)传递的。
char* argv[]，和 char ** argv 在这里应该是一样的，目前指向地址的指针还没开始研究，后面看见较深入的例子后再深入理解。

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