如何在没有系统调用的情况下在x86-64程序集(NASM)中将字符串打印到终端？

这是一个很好的练习.您将使用syscall(stdout否则无法访问),但您可以执行"裸机"写入,而无需任何外部库提供输出例程(如调用printf).作为stdoutx86_64中基本的"裸机"写入的示例,我将一个示例放在一起,没有任何内部或系统函数调用:

section .data
    string1 db  0xa, "  Hello StackOverflow!!!", 0xa, 0xa, 0

section .text
    global _start

    _start:
        ; calculate the length of string
        mov     rdi, string1        ; string1 to destination index
        xor     rcx, rcx            ; zero rcx
        not     rcx                 ; set rcx = -1
        xor     al,al               ; zero the al register (initialize to NUL)
        cld                         ; clear the direction flag
        repnz   scasb               ; get the string length (dec rcx through NUL)
        not     rcx                 ; rev all bits of negative results in absolute value
        dec     rcx                 ; -1 to skip the null-terminator, rcx contains length
        mov     rdx, rcx            ; put length in rdx
        ; write string to stdout
        mov     rsi, string1        ; string1 to source index
        mov     rax, 1              ; set write to command
        mov     rdi,rax             ; set destination index to rax (stdout)
        syscall                     ; call kernel

        ; exit 
        xor     rdi,rdi             ; zero rdi (rdi hold return value)
        mov     rax, 0x3c           ; set syscall number to 60 (0x3c hex)
        syscall                     ; call kernel

; Compile/Link
;
; nasm -f elf64 -o hello-stack_64.o hello-stack_64.asm
; ld  -o hello-stack_64 hello-stack_64.o

输出:

$ ./hello-stack_64

  Hello StackOverflow!!!

对于一般用途,我将过程分为两部分(1)获取长度和(2)写入stdout.strprn函数下面会写入任何字符串stdout.它调用strsz获取长度,同时保留堆栈上的目标索引.这减少了编写字符串的任务,stdout并防止代码中的大量重复.

; szstr computes the lenght of a string.
; rdi - string address
; rdx - contains string length (returned)
section .text
        strsz:
                xor     rcx, rcx                ; zero rcx
                not     rcx                     ; set rcx = -1 (uses bitwise id: ~x = -x-1)
                xor     al,al                   ; zero the al register (initialize to NUL)
                cld                             ; clear the direction flag
                repnz scasb                     ; get the string length (dec rcx through NUL)
                not     rcx                     ; rev all bits of negative -> absolute value
                dec     rcx                     ; -1 to skip the null-term, rcx contains length
                mov     rdx, rcx                ; size returned in rdx, ready to call write
                ret

; strprn writes a string to the file descriptor.
; rdi - string address
; rdx - contains string length
section .text
        strprn:
                push    rdi                     ; push string address onto stack
                call    strsz                   ; call strsz to get length
                pop     rsi                     ; pop string to rsi (source index)
                mov     rax, 0x1                ; put write/stdout number in rax (both 1)
                mov     rdi, rax                ; set destination index to rax (stdout)
                syscall                         ; call kernel
                ret

为了进一步自动化通用输出到stdoutNASM宏,提供了一个方便的解决方案.示例strn(简称string_n).它需要两个参数,字符串的地址和要写入的字符数:

%macro  strn    2
        mov     rax, 1
        mov     rdi, 1
        mov     rsi, %1
        mov     rdx, %2
        syscall
%endmacro

用于缩进,换行或编写完整的字符串.你可以通过传递3个参数来进一步概括,包括目的地rdi.