This   function   is   not   defined   in   ANSI-C   and   is   not
   part   of   C++,   but   is   part   of   POSIX   C   and   is 
  supported   by   some   compilers. 

POSIX,   or   or   Portable   Operating   System   Interface   for  
 uniX,   specifies   some   routines   that   should   be   available  
 over   and   above   those   in   the   C   standard   library   
proper;   these   are   often   implemented   alongside   the   C   
standard   library   functionality,   with   varying   degrees   of 
  closeness. 

"Traditional   POSIX   names   for   C   functions   are   declared  
 deprecated   by   Microsoft.   In   most   cases,   prepending   an  
 underscore   character   gives   the   standard   equivalent   name. 
  _itoa()   is   the   standard   equivalent   name   for   itoa().   "  

 --Deprecated   CRT   Functions,   Visual   C++   2005   documentation     

"The   names   of   Microsoft-specific   functions   and   global   
variables   begin   with   a   single   underscore.   These   names  
 can   be   overridden   only   locally,   within   the   scope   of 
  your   code.   For   example,   when   you   include   Microsoft   run-
time   header   files,   you   can   still   locally   override   the 
  Microsoft-specific   function   named   _open   by   declaring   a  
 local   variable   of   the   same   name.   However,   you   cannot 
  use   this   name   for   your   own   global   function   or   
global   variable. "   

--ANSI   C   Compliance,   Visual   C++   2005   documentation