Garbage in string output function

问题

I'm trying to write a printf replacement in asm and so far have this code:

; string is loaded into r8
print_string:
    push rax
    push rbx
    push rsi
    push rdx

    ; load string pointer into ecx
    mov rsi, r8

    ; loop over every char
    print_string_loop0:
        cmp sil, 0 ; stop when encounter null character
        je print_string_return
        mov rax, 1 ; syscall (sys_write)
        mov rdi, 1 ; file descriptor for write (stdout = 1)
        mov rdx, 1 ; bytes to write (1 character)
        syscall
        inc rsi
        jmp print_string_loop0:

    print_string_return:
        pop rdx
        pop rsi
        pop rbx
        pop rax

which works, but I always get some sort of garbage after the string that I print.

Heres the code that uses print_string

global _start

section .text

_start:
    mov r8, string
    call print_string

    mov rax, 60 ; syscall (sys_exit)
    mov rdi, 0  ; exit code
    syscall

.section data

string:
    db "Hell! Oh, World.", 10, 0 ; string, newline, null

print_string is defined in the same file.

So why is garbage being printed after my string? The garbage is the same every time and if I modify the assembly at all, different garbage is output.

回答1:

You've got in R8 and thus in RSI an address not a character. So change the break condition cmp sil, 0 to cmp byte [rsi], 0.

来源：https://stackoverflow.com/questions/28668947/garbage-in-string-output-function