在带有64位Linux的Assembly for x86_64中编写putchar吗? [英] Writing a putchar in Assembly for x86_64 with 64 bit Linux?

问题描述

我正在尝试使用写入 syscall 来重现打印单个字符的 putchar 函数行为.我的代码如下，

I am trying to use the write syscall in order to reproduce the putchar function behavior which prints a single character. My code is as follows,

asm_putchar:
  push    rbp
  mov     rbp, rsp

  mov     r8, rdi

call:
  mov     rax, 1
  mov     rdi, 1
  mov     rsi, r8
  mov     rdx, 1
  syscall

return:
  mov     rsp, rbp
  pop     rbp
  ret

推荐答案

从 man 2 write 中，您可以看到 write 的签名是

From man 2 write, you can see the signature of write is,

ssize_t write(int fd, const void *buf, size_t count);

它需要一个指向内存中缓冲区的指针( const void * buf ).您无法按值传递 char ，因此必须将其存储到内存并传递指针.

It takes a pointer (const void *buf) to a buffer in memory. You can't pass it a char by value, so you have to store it to memory and pass a pointer.

(除非您只有一个要打印的字符，否则不要一次打印一个字符，这确实效率低下.在内存中构造一个缓冲区并进行打印.例如，此x86-64 Linux NASM函数:

(Don't print one char at a time unless you only have one to print, that's really inefficient. Construct a buffer in memory and print that. e.g. this x86-64 Linux NASM function: How do I print an integer in Assembly Level Programming without printf from the c library?)

NASA版本的 GCC:内联汇编中的putchar(char):

; x86-64 System V calling convention: input = byte in DIL
; clobbers: RDI, RSI, RDX,  RCX, R11 (last 2 by syscall itself)
; returns:  RAX = write return value: 1 for success, -1..-4095 for error
writechar:
    mov    byte [rsp-4], dil      ; store the char from RDI

    mov     edi, 1                ; EDI = fd=1 = stdout
    lea     rsi, [rsp-4]          ; RSI = buf
    mov     edx, edi              ; RDX = len = 1
    syscall                    ; rax = write(1, buf, 1)
    ret

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