printf float in nasm assembly 64-bit

Question

I want to print a float value with printf

global main
extern printf

section .data
   string: db `%f\n`, 0

section .bss
   rs: resq 1

[...]

   movq xmm0, [rs]
   mov rdi, string
   mov rax, 0
   call printf

rs contains the floating value 1.6

(gdb) x/fg &rs
0x600ad8 <rs>:  1.6000000000000001

but the program prints

[username@localhost folder]$ ./programname
0.000000

who can I get the program to print 1.6? what am I doing wrong?

Answer 1

what am I doing wrong?

First: make sure you are using the right calling convention (stack, registers, left to right, right to left, etc.). If your program indeed prints a floating point number, although it is not the one you required, then at least the format string is being passed correctly (or you are having a lot of luck and printf found the address of the format string at the right place even if you didn't put its address there).

Second: the number you are trying to print... is it a float or a double? rs is defined to hold a quadword value (64 bits), but floats are 32 bits. So, if the first point has been checked and it's ok, I suggest you to use "%lf" as format, instead of "%f".

BTW: why do you put RAX = 0? What does it mean regarding the call to printf?

UPDATE: This may help you. A disassembly of a silly program (f.c):

#include <stdio.h>

main()
{
  float x;

  x = 1.6;
  printf ("%f\n", x);
}

$ gcc -c -S f.c

$ less f.s

        .file   "f.c"
        .section        .rodata
.LC1:
        .string "%f\n"
        .text
.globl main
        .type   main, @function
main:
.LFB0:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        movq    %rsp, %rbp
        .cfi_offset 6, -16
        .cfi_def_cfa_register 6
        subq    $16, %rsp
        movl    $0x3fcccccd, %eax
        movl    %eax, -4(%rbp)
        movss   -4(%rbp), %xmm0
        cvtps2pd        %xmm0, %xmm0
        movl    $.LC1, %eax
        movq    %rax, %rdi
        movl    $1, %eax
        call    printf
        leave

Answer 2

The first problem is your code setting AL (via RAX) to 0 whereas it must be 1 because you pass a floating point argument (see here for details). Basically AL should contain the number of variable arguments passed in xmmN registers.

Fixing that without other changes would result in a segfault inside printf, caused by stack misalignment. The program crashes at a movaps instruction (which expects the memory operand to be aligned on 16-byte boundary):

=> 0x7ffff7a65f84 <__printf+36>:    movaps %xmm0,0x50(%rsp)
   0x7ffff7a65f89 <__printf+41>:    movaps %xmm1,0x60(%rsp)
   0x7ffff7a65f8e <__printf+46>:    movaps %xmm2,0x70(%rsp)
   0x7ffff7a65f93 <__printf+51>:    movaps %xmm3,0x80(%rsp)
   0x7ffff7a65f9b <__printf+59>:    movaps %xmm4,0x90(%rsp)
   0x7ffff7a65fa3 <__printf+67>:    movaps %xmm5,0xa0(%rsp)
   0x7ffff7a65fab <__printf+75>:    movaps %xmm6,0xb0(%rsp)
   0x7ffff7a65fb3 <__printf+83>:    movaps %xmm7,0xc0(%rsp)

When entering main (or any other function), the stack is aligned RSP%16 == 8, but you need RSP%16 == 0 before a call. If you fix this the program works fine. Below is my test program (notice the sub rsp, 8 in the beginning):

global main
default rel     ; use a more efficient [rel foo] addressing mode by default
extern printf

section .data
    string db `%f\n`, 0
    rs dq 1.6

section .text

main:
    sub  rsp, 8                ; align the stack

    movsd xmm0, qword [rs]     ; load a Scalar Double
    mov  rdi, string           ; 64-bit absolute address of format string
    mov  eax, 1                ; AL=1  number of FP args in XMM regs
    call printf

    add  rsp, 8
    xor  eax, eax           ; return 0
    ret                     ; or  call exit with EDI=0,  not call _exit or syscall
; don't use _exit() after printf, it won't flush stdio buffers.
; which will be non-empty if output stdout is redirected to a file.
;       mov eax, 60         ; __NR_exit from unistd_64.h
;       xor edi, edi
;       syscall

Possible pitfalls and related Q&As:

• Using printf in assembly leads to empty output when piping, but works on the terminal - Don't mix printf with raw _exit() system calls.
• How to load address of function or label into register - for more efficient ways to put the format-string address into RDI
• For maintaining 16-byte stack alignment, see: glibc scanf Segmentation faults when called from a function that doesn't align RSP
• Can't call C standard library function on 64-bit Linux from assembly (yasm) code - if your linker won't insert a PLT stub for call printf, e.g. in a PIE executable.

Answer 3

; file: pf.asm
; assemble with:  nasm -f elf64 pf.asm -o pf.o
; link with: ld -pie -z noexecstack -e _start -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o pf pf.o -lc

default rel             ; Use PC relative offsets (for PIE executable)

global _start           ; Execute at _start (don't use any provided startup code)

extern printf           ; printf is our only external function

section .rodata         ; The format string in printf should be read only
    string: db "Your number is %f", 0xa, 0

section .data           ; We could make this read only, but if we want to change it
    rs dq 1.6           ; through a scanf or something we can

section .text           ; Program code

_start:
;    Uncomment stack frame stuff if stack space needed, not needed for this example

;    push rbp           ; Setup our stack frame
;    mov rbp, rsp

;    sub rsp, 8         ; Create space on the stack

    movq xmm0, qword [rs]       ; Put float in xmm0
    lea rdi, [string]   ; Load address of string, lea is needed for PC relative
    mov eax, 1          ; Using floating point numbers, 0 extend into rax
    call printf wrt ..plt  ; PC relative function call

;    add rsp, 8         ; Remove allocated space on the stack

;    pop rbp            ; Undo our stack frame 

    mov eax, 60         ; Exit syscall
    xor edi, edi        ; No error, 0 extend into rdi
    syscall