Question

Suppose I have the following struct and function returning a pointer:

typedef struct {
  int num;
  void *nums;
  int size;
} Mystruct;

Mystruct *mystruct(int num, int size)
{ 
   //Is the following correct? Is there a more efficient way?
   Mystruct mystruct;
   mystruct.num = num;
   mystruct.size = size;
   mystruct.nums = malloc(num*sizeof(size));
   Mystruct *my;
   *my = mystruct;
   return my;
}

I want to define any Mystruct pointer using the above function. Should I declare a Mystruct variable, define the properties of Mystruct, assign a pointer to it, and return the pointer or define the properties of a mystruct property through a pointer immediately?

Answer 1

Should I declare a Mystruct variable,
define the properties of Mystruct,
assign a pointer to it, and return the
pointer

Definitely not, because the variable defined in the function (in “auto” storage class) will disappear as the function exits, and you’ll return a dangling pointer.

You could accept a pointer to a Mystruct (caller’s responsibility to allocate that) and fill it in; or, you can use malloc to create a new one (caller’s responsibility to free it when it’s done). The second option at least lets you keep the function signature you seem to be keen on:

Mystruct *mystruct(int num, int size)
{
   Mystruct *p = malloc(sizeof(MyStruct));
   ....
   return p;
}

but it’s often an inferior one — since the caller has to have responsibilities anyway, may as well go with the first option and potentially gain performance (if the caller can use an auto-class instance because it knows the scope of use is bounded).