终端自动输入来自程序的输入作为命令

Question

I am trying to teach myself how to write Assembly in X86 on NASM. I'm attempting to write a program that takes a single integer value and prints it back to standard output before exiting.

My code:

section .data
        prompt: db "Enter a number: ", 0
        str_length: equ $ - prompt

section .bss
        the_number: resw 1

section .text

global _start

_start:
        mov eax, 4      ; pass sys_write
        mov ebx, 1      ; pass stdout
        mov edx, str_length     ; pass number of bytes for prompt
        mov ecx, prompt     ; pass prompt string
        int 80h
        
        mov eax, 3      ; sys_read
        mov ebx, 0      ; stdin
        mov edx, 1      ; number of bytes
        mov ecx, [the_number]       ; pass input of the_number
        int 80h
        
        mov eax, 4
        mov ebx, 1
        mov edx, 1
        mov ecx, [the_number]
        int 80h
        
        mov eax, 1          ; exit
        mov ebx, 0          ; status 0
        int 80h

From there I do the assembling nasm -felf -o input.o input.asm and linking ld -m elf_i386 -o input input.o .

I run a test and input an integer and when I press enter, the program exits and Bash tries to execute the number input as a command. I even echo 'd the exit status and been returned with 0.

So this is an odd behavior.

Answer 1

The call to read fails and doesn't read any input. When your program exits, that input is still waiting to be read on the TTY (which was this program's stdin), at which point bash reads it.

You should check the return status of your system calls. If EAX is a negative number when the system call returns, it is the error code. For example, in this case, EAX contains -14, which is EFAULT (“Bad address”).

The reason read fails is that you are passing an invalid pointer as the buffer address. You need to load the address of the_number , not its value. Use mov ecx, the_number .