Linux NASM检测EOF [英] Linux NASM detect EOF

问题描述

我正在尝试学习Linux上的asm基础知识，但找不到很好的参考资料.NASM文档似乎假设您已经知道masm ......我没有在 cmp (在Intel指令参考之外)的文档中找到任何示例.

I'm trying to learn the basics asm on linux and I can't find a very good reference. The NASM docs seem to assume you already know masm... I found no examples in the documentation of the cmp (outside the Intel instruction reference).

我编写了一个程序，该程序从stdin读取一个字节并将其写入stdout.下面是我的修改，以尝试在stdin上检测EOF并在达到EOF时退出.问题是它永远不会退出.我只是继续打印从stdin读取的最后一个字符.问题是我检测到EOF( cmp ecx，EOF )和/或我跳到 _exit 标签( je _exit )

I'd written a program that reads a single byte from stdin and writes it to stdout. Below is my modification to try to detect EOF on stdin and exit when EOF is reached. The issue is it never exits. I just keeps printing the last char read from stdin. The issue is either in my EOF detection (cmp ecx, EOF) and/or my jump to the _exit label (je _exit) I think.

我在做什么错了?

%define EOF     -1

section .bss
        char:   resb    1

section .text
        global  _start

_exit:
        mov     eax,    1       ; exit
        mov     ebx,    0       ; exit status
        int     80h

_start:
        mov     eax,    3       ; sys_read
        mov     ebx,    0       ; stdin
        mov     ecx,    char    ; buffer
        cmp     ecx,    EOF     ; EOF?
        je      _exit
        mov     edx,    1       ; read byte count
        int     80h

        mov     eax,    4       ; sys_write
        mov     ebx,    1       ; stdout
        mov     ecx,    char    ; buffer
        mov     edx,    1       ; write byte count
        int     80h

        jmp     _start

出于理智，我确认此C的EOF为-1:

For the sake of sanity, I verified EOF is -1 with this C:

#include <stdio.h>
int main() { printf("%d\n", EOF); }

推荐答案

您正在将缓冲区的地址与EOF(-1)(而不是存储在缓冲区中的字符)进行比较.

You are comparing the address of the buffer to EOF (-1) instead of the character stored in the buffer.

话虽如此， read 系统调用在到达文件末尾时不会返回EOF的值，但是会返回零且不会在缓冲区中粘贴任何内容(请参见男人2阅读).要识别文件结尾，只需在调用 read 后检查 eax 的值:

Having said that, the read system call does not return the value of EOF when end of file is reached, but it returns zero and doesn't stick anything in the buffer (see man 2 read). To identify end of file, just check the value of eax after the call to read:

section .bss
    buf:   resb    1

section .text
    global  _start

_exit:
    mov     eax,    1       ; exit
    mov     ebx,    0       ; exit status
    int     80h

_start:
    mov     eax,    3       ; sys_read
    mov     ebx,    0       ; stdin
    mov     ecx,    buf    ; buffer
    mov     edx,    1       ; read byte count
    int     80h

    cmp     eax, 0
    je      _exit

    mov     eax,    4       ; sys_write
    mov     ebx,    1       ; stdout
    mov     ecx,    buf    ; buffer
    mov     edx,    1       ; write byte count
    int     80h

    jmp     _start

如果您确实想将字符正确地与某个值进行比较，请使用:

If you did want to properly compare the character to some value, use:

cmp byte [buf], VALUE

此外，我将 char 重命名为 buf . char 是基本的C数据类型，并且是变量名的错误选择.

Also, I renamed char to buf. char is a basic C data type and a bad choice for a variable name.

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