problem with c++ function called from assembly

hello
I'm new of this forum use visual c++ 2012 under windows 10.
I've this error when compiling
error LNK2019: _Dimensione_Variabile1@4
defined
<extern "C" int _stdcall Dimensione_Variabile1(char *dato);>
the routine is
<
#include "stdafx.h"
int Dimensione_Variabile1(char *dato)
{ return sizeof(dato); }>
the assembly definescion is
<extern Dimensione_Variabile1@4:Proc
Dimensione_Variabile1 Equ Dimensione_Variabile1@4>
where is my error?
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