Initializing pointers in a struct in a minimum of source lines - c

Question

I'm currently new to C programming, and appreciate for any tip.
Is there a shorter way to initialize struct pointers in C without removing the pointer tags?
typedef struct {
int x, y, z;
} Point3;
typedef struct {
Point3 *pos, *direction;
} Vector;
int main() {
Vector *p;
p = malloc(sizeof(Vector));
p->pos = malloc(sizeof(Point3));
p->direction = malloc(sizeof(Point3));
return 0;
}

Answer 1

Yes, there is a shorter way — one which is one malloc() call shorter.
Vector *p = malloc(sizeof(Vector));
if (p != 0)
{
p->pos = malloc(2 * sizeof(Point3));
if (p->pos != 0)
p->direction = &p->pos[1];
}
Allocate an array of 2 Point3 values. p->pos points to the first, and p->direction points to the second (or vice versa).
It is still 3 statements (plus error checking) and two calls to malloc(), though.
In practice, you could almost certainly get away with:
Vector *p = malloc(sizeof(Vector) + 2 * sizeof(Point3));
if (p != 0)
{
p->pos = (void *)((char *)p + sizeof(Vector));
p->direction = (void *)((char *)p + sizeof(Vector) + sizeof(Point3));
}
I am not sure that is sanctioned by the C standard, but I can't immediately think of a plausible platform configuration where it would actually fail to work correctly. It would fail if you found some bizarre platform where addresses were 16-bits each but int was 8 bytes and had to be 8-byte aligned, but that's hardly plausible.

Answer 2

To me, it makes far more sense to put the Point3 members directly in the Vector, instead of pointers. Fewer allocations, less memory fragmentation, fewer de-references, fewer cache-misses.
typedef struct {
int x, y, z;
} Point3;
typedef struct {
Point3 pos, direction;
} Vector;
int main(void) {
/* Local (stack) allocation of a Vector, initialized to all zeros */
Vector v = {};
/* Dynamic (heap) allocation of a Vector, initialized to all zeros */
Vector *p;
p = malloc(sizeof(Vector));
if (!p) {
return 1; // failure
}
*p = (Vector){};
return 0;
}

Answer 3

Unfortunately, there is no other way. You can simplify memory allocation with another function, like this
Vector* allocate_vector( ) {
Vector* v = (Vector*)malloc( sizeof(Vector) );
if( v == NULL ) {
/**/
}
v->pos = (Point3*)malloc( sizeof(Point3) );
if( v->pos == NULL ) {
/**/
}
v->direction = (Point3*)malloc( sizeof(Point3) );
if( v->direction == NULL ) {
/**/
}
return v;
}
And then use it, when you need new Vector.
Vector* v = allocate_vector( );