[Scilab-Dev] What is the Fortran function maxvol ?

[Vincent Lejeune]

Thank for your answer, 
infact I don't use lapack routine, but I try to reproduce them partially in C 
:)
I could do it whithout using maxvol, and in fact it seems that you were right 
about the role of the routine (I didn't know until wenesday that FORTRAN has 
not always allowed dynamic allocation...)

By the way, what is the main difference between OS malloc() and SciLab MALLOC ?
I always use classical malloc and I didnt encoutered any issue by now, but 
perhaps I'm missing something...

Vincent

Le mercredi 3 juin 2009 09:14:29, bernard.hugueney at scilab.org a écrit :
> On Tue, 2 Jun 2009 16:27:34 +0200, Vincent Lejeune
>
> <vincent.lejeune at institutoptique.fr> wrote:
> > Hi,
> >
> > In SciLabs Fortran part of code, there is a "maxvol" function that come
> > several time, for instance in intdgesv3.f, intdgecon.f, sci_f_mucomp.f
>
> ...
>
> > The function is typically called this way :
> > LWORK=maxvol(10,'d')
> >
> > (when the function is acting on complex, 'd' is replaced by 'z')
> > I am no Fortran expert, but I'd like to know what this function does. I
> > dont find its definition in scilab's git repository (I used nepomuk...),
> > and I
> > didnt catch any results on any google search. I suspect this function has
> >
> > to deal with some parallel activities, as it is frequently summoned
> > besides Lapack call...
> >
> > Thanks for the help !
>
> If I am not mistaken, this function returns the max number of elements in
>  a matrix of type double ('d') or complex ('z') that can be allocated on
> the Scilab stack. It is called before Lapack functions that require
> a temporary workspace of unbounded size (the more the better).
>
> The Fortran routine can allocates all the size available on the stack
> because
> no further allocation will be neded before the temporary workspace is
> needed.
>
> When coding in C, you won't use the Scilab stack for temporary workspace
> because
>  you can use malloc fir dynamic memory allocation (well, MALLOC in fact,
> with #include "MALLOC.h").
> However, you cannot malloc all the heap memory available like Fortran could
>
> alloc all the scilab stack memory available ! So you have to choose the
> best size
> and you can know which size is best by calling the Lapack routine with
> a special query mode by passing a worksize of -1 (but still passing
> relevent values
>  for nb of rows/cols for exemple because Lapack will use them to compute
> the optimal size). Lapack routines will then return the computed optimal
> size at
> the begining of the "workspace" (so you cannont pass NULL, but should
> instead
> pass &a_variable_of_type_double ). As small an example is worth a thousand
> words :
>  " with :
> static int dsyevWorkSizes(int iCols, int* optWorkSize, int* minWorkSize) {
>  int info=0, query=-1;
>  double opt;
>  C2F(dsyev)("N", "U", &iCols, NULL, &iCols, NULL, &opt, &query, &info);
>  *optWorkSize= (int)opt;
>  *minWorkSize= Max(1, 3*iCols -1); /* this is documented in Lapack API
> specifications */
>  return info;
>  }
>
>  you could write :
>
> int optWs, minWs;
> if(!dsyevWorkSizes(cols, &optWs, &minWs) {
>  double* workspace= NULL;
>  int wsSize= 0;
>
>  workspace=MALLOC(optWs);
>  if(workspace) { wsSize= optWs; }
>  else {
>    workspace= MALLOC(minWs);
>    if(workspace){ wsSize= minWs; }
>  }
>  if(workspace) {
>  /* now you can call Lapack routine with dynamicaly allocated workspace of
> size wsSize */
>  }
> [...]
>
>
>  HTH. (feel free to ask again if I was not clear).
>
> Regards,
>
> Bernard

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