汇编：printf不打印新行

Question

I have the following code that prints the number of parameters passed to ./main . Notice the fmt in the rodata section. I've included the new line \\n , just like in C , but instead of printing the new line, it prints:

Number of parameters: 1 \\n

My code is:

;main.asm
GLOBAL main
EXTERN printf

section .rodata:
fmt db "Number of parameters: %d \n", 0 

section .text:

main:

    push ebp
    mov ebp, esp    ;stackframe

    push dword[ebp+8]       ;prepara los parametros para printf
    push fmt
    call printf
    add esp, 2*4

    mov eax, 0      ;return value

    leave           ;desarmado del stack frame
    ret

I know that including a 10 before the 0 and after the "Number..." in fmt will print it, but I want printf to do it. I assemble the code with NASM and then link it via GCC to create my executable.

Answer 1

When you use quotes or double quotes around a string in NASM , it doesn't accept C style escape sequences. On Linux you can encode \\n as ASCII 10 like this:

fmt db "Number of parameters: %d", 10, 0 

There is an alternative. NASM supports backquotes (backticks) which will allow NASM to process the characters between them as C style escape sequences. This should work as well:

fmt db `Number of parameters: %d \n`, 0

Please note: Those are not single quotes, but backticks . This is described in the NASM documentation :

3.4.2 Character Strings

A character string consists of up to eight characters enclosed in either single quotes ('...'), double quotes ("...") or backquotes ( ... ). Single or double quotes are equivalent to NASM (except of course that surrounding the constant with single quotes allows double quotes to appear within it and vice versa); the contents of those are represented verbatim. Strings enclosed in backquotes support C-style -escapes for special characters .

Answer 2

The assembler is not C. The C compiler understands \\n as an escape code for ASCII 10. The assembler does not and treats it as two characters. Add the 10 as you describe.