Calling printf in x86_64 using GNU assembler

Question

I\'ve written a program using AT&T syntax for use with GNU assembler:

            .data
format:   .ascii \"%d\\n\"  

            .text
            .glob         


        

Answer 1

You can look at assembly code generated from an equivalent c file.
Running gcc -o - -S -fno-asynchronous-unwind-tables test.c with test.c

#include 
int main() {
   return printf("%d\n", 1);
}

This output the assembly code:

        .file   "test.c"
        .section        .rodata
.LC0:
        .string "%d\n"
        .text
        .globl  main
        .type   main, @function
main:
        pushq   %rbp
        movq    %rsp, %rbp
        movl    $1, %esi
        movl    $.LC0, %edi
        movl    $0, %eax
        call    printf
        popq    %rbp
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 6.1.1 20160602"
        .section        .note.GNU-stack,"",@progbits

This give you a sample of an assembly code calling printf that you can then modify.

---

Comparing with your code, you should modify 2 things:

• %rdi should point to the format, you should not unreferenced %rbx, this could be done with mov $format, %rdi
• printf has a variable number of arguments, then you should add mov $0, %eax

Applying these modifications will give something like :

    .data
format: .ascii "%d\n"  
.text
    .global main  
main:
  mov  $format, %rdi
  mov  $1, %rsi
  mov  $0, %eax
  call printf
  ret

And then running it print :

1