NASM-printf不打印第一个字符

Question

I want to write a program which returns the string you write. But it always prints it without the first character. Also, it doesn't return anything past the first space.

Example:

IN: test
OUT: est

Code:

extern printf, scanf, exit
global main

section .text
main:
    push rbp
                            ;input of string
    mov rdi, format
    mov rsi, string
    mov rax, 0
    call scanf

                            ;output of string
    mov rdi, format_out
    mov rsi, qword [string]
    mov rax, 0
    call printf
_exit:                        ;end of program
    pop rbp
    call exit

section .data
  format     db "%s", 0
  string     db 0
  format_out db "You wrote: %s", 0

I noticed that if I change string db 0 to string db ", it shows an error, but the program works correctly, printing the entire sentence to the first space. Unfortunately I don't have any clue what to do with that info. Thanks for the answer.

Answer 1

printf("%s", ...) takes a pointer arg, but you're passing it the first 8 bytes of the contents of your string. Use mov rsi, string to get a mov-immediate instead of mov rsi, [string] (which is a load).

(Or to save 2 code bytes, mov esi, string since static addresses are guaranteed to fit in 32 bits in the default code model.)

---

As pointed out in comments, your static buffer is too small. Since your format string has no size limit, you need an infinite-size buffer to avoid any risk of a long input line overflowing it.

But let's pretend that a 1024B buffer is safe enough. Or better, use %1023s so it is guaranteed to be enough. We don't want 1024B of zeros in our executable; that would be silly. So we should instead reserve 1024B in the BSS:

section .rodata
  format:     db "%1023s", 0         # actually set a size limit, leaving room for the \0
  format_out: db "You wrote: %s", 0

section .bss
  string:     resb 1024              # RESERVE 1024 bytes.  Only works in .bss

There's no reason to omit the : after label names on data, and good reason not to .