Why is scanf returning 0.000000 when it is supplied with a double?

Question

I have the following assembly code (written for NASM on Linux):

; This code has been generated by the 7Basic
; compiler <http://launchpad.net/7basic&gt;

extern printf
extern scanf

      SECTION .data
printf_f: db "%f",10,0
scanf_f: db "%f",0

      SECTION .bss
v_0 resb 8

      SECTION .text

global main
  main:
push ebp
mov ebp,esp
push v_0         ; load the address of the variable
push scanf_f     ; push the format string
call scanf       ; call scanf()
add esp,8
push dword [v_0+4]  ; load the upper-half of the double
push dword [v_0]    ; load the bottom-half
push printf_f     ; push the format string
call printf       ; call printf
add esp,12
mov esp,ebp
pop ebp
mov eax,0
ret

When I assemble and run the program, I get a prompt as expected. However, no matter what number I enter, the output is always 0.000000.

What am I doing wrong?

Answer 1

You are trying to scan a float with the '%f' token but providing a double. Pass in a float variable to scanf and then convert to a double or pass in '%lf' as the format string to scanf.