Question

If I have the following C struct:

struct group_of_pointers {
    double *p1, *p2, *p3;
} *pointers;

Now, I want to allocate n spaces (each with sizeof double) to pointers->p1, pointers->p2, pointers->p3. How do I do this? Do I have to allocate any space to the pointer to the structure ‘pointers’ itself?

Thanks.

ETA: Related question:
The reason I need this struct is because I want to return 3 variable length arrays from a function. Should I just do

void foo(const double* const input, double *output1, double *output2, double *output3) 

or should I do

struct group_of_pointers *foo(const double* const input)

The former looks a bit confusing with input and output all bundled together. But is it just the way C is?

Answer 1

Yes, since your pointers variable is itself a pointer, you need to give it some memory to point to.

struct group_of_pointers {
    double *p1, *p2, *p3;
} *pointers;

pointers = malloc(sizeof(*pointers));

pointers->p1 = malloc(n*sizeof(*pointers->p1));
pointers->p2 = malloc(m*sizeof(*pointers->p2));
pointers->p3 = malloc(o*sizeof(*pointers->p3));