Compiler warning generated when using runge_kutta.c

[yantosca]

This is a minor issue but I just thought I'd bring it up. When compiling KPP 3.1.1 with gcc 10.2.0, I get this warning:

C_rk_Integrator.c: In function ‘RK_Integrator’:
C_rk_Integrator.c:1045:34: warning: ‘ErrOld’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 1045 |        FacGus = FacSafe*(H/Hacc)*pow(Err*Err/ErrOld,(double)(-0.25));
      |                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C_rk_Integrator.c:1045:27: warning: ‘Hacc’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 1045 |        FacGus = FacSafe*(H/Hacc)*pow(Err*Err/ErrOld,(double)(-0.25));
      |                         ~~^~~~~~

Which I've traced to this section of code:

 /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
 /*~~~> Accept/reject step */
 /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
      // accept:
      if (Err < ONE) { /*~~~> STEP IS ACCEPTED */
      	FirstStep = 0;
      	ISTATUS[Nacc]++;
      	if (Gustafsson == 1) {
	   /*~~~> Predictive controller of Gustafsson */
	   if (ISTATUS[Nacc] > 1) {
	      FacGus = FacSafe*(H/Hacc)*pow(Err*Err/ErrOld,(KPP_REAL)(-0.25));
	      FacGus = MIN(FacMax,MAX(FacMin,FacGus));
	      Fac = MIN(Fac,FacGus);	
	      Hnew = Fac*H;
	   } /* end if */
	   Hacc = H;
	   ErrOld = MAX((KPP_REAL)1.0e-02,Err);
      	} /* end if */

The compiler is flagging that ErrOld may be uninitialized. It doesn't look like it's used until after the first timestep.

Would it be safe to set e.g. ErrOld = Err just after the FirstStep=0 line? That would remove the warning.

Also a similar situation is in the F90 code but we don't get a warning for that.

This isn't an essential fix but would be more of a cleanup so that we wouldn't generate warnings when building the mechanism.

[RolfSander]

The code seems to be written in a way that ErrOld is never
uninitialized: ISTATUS starts with zero. It is then incremented to 1,
meaning that if (ISTATUS[Nacc] > 1) is still false and ErrOld is not
used yet. Anyway, in the spirit of clean coding, I agree that ErrOld
should be initialized with a dummy value.

I don't think that after FirstStep=0 is the right place. To be on the
safe side, I'd put it at the very beginning of the function, i.e.,
before Tdirection = SIGN....

For consistency, I think that the f90 code should be adjusted as well.