Regarding STMP-command-style lookup: A more portable way to do that is to define a macro to assemble the 4 bytes into unsigned int:

  #define CHAR4_TO_UINT(a,b,c,d)       \
    (                                  \
        ((unsigned int)(a))      |     \
      ( ((unsigned int)(b))<<8  )|     \
      ( ((unsigned int)(c))<<16 )|     \
      ( ((unsigned int)(d))<<24 )      \
    )

then

  unsigned int  ui_cmdword = CHAR4_TO_UINT(
      cmdword[0], cmdword[1], cmdword[2], cmdword[3]
  );
  if( ui_cmdword == CHAR4_TO_UINT('H','E','L','O') )
      handle_helo();
  else if( ui_cmdword == CHAR4_TO_UINT('E','H','L','O') )
      handle_ehlo();

А compiler will generate memory-fetching code once, optimize right side of comparisons into constants and make everything flow fast and safe. You can even use switch statement if you like.