Re: [Help-gsl] gsl_vector_alloc question.

[Sang Oh]

Actually I wanting to pass more than one gsl_vector (or gsl_matrix) to 
functions to get allocated.
I am doing this now.  John is right in that it's more a c question than 
gsl problem.  =)

void ruler(double xa, double xb, double dx, gsl_vector **vc)
{
*vc = gsl_vector_alloc(len);
}

main()
{
gsl_vector *v;
ruler(xa,xb,dx,&v);
}

regards,

Sang


John Lamb wrote:

> Sang Oh wrote:
>
> > I'm trying to allocate memory in a function.
> >
> > void ruler(double xa, double xb, double dx, gsl_vector *vc) {
> >  vc = gsl_vector_alloc(len);
> > }
>
>
> Looks like a common programming error, nothing to do with gsl. You 
> cannot globally change the _value_ of a parameter. You can only change 
> the value of the struct it points to. The gsl_vector_alloc call just 
> changes a local copy of vc and then throws it away when the function 
> returns, leaving a vector on the heap that you can't point to.
>
> Try rewriting as
>
> gsl_vector *ruler(double xa, double xb, double dx){
>   ...
>   vc = gsl_vector_alloc(len);
>   ...
>   return vc;
> }