Can a single pointer be initialized in a function and if so how?

Question

I have been given this function prototype void create_smth(smth* s) and it is asked that this function creates, initializes and returns a smth variable. Is this even possible? To my knowledge I would need a double pointer to do such a thing. What I am doing in this function is

s=(smth*)malloc(sizeof(smth));

This is probably the mistake.

In my main I have tried

smth* smth1=NULL;
create_smth(smth1);

and

smth smth1;
create_smth(&smth1);

but my code keeps crashing (Segmentation fault; core dump). I am starting to wonder if it is an instructors mistake but feel stupid to ask him straight up but the excercise requires what I stated above. Thanks everyone.

Answer 1

Looks like instructor's mistake to me. You are correct that the void create_smth(smth* s) prototype cannot return a value, either in the traditional return sense (it is void ), or using a double pointer. The only two ways to do this:

smth* creat_smth()
{
  smth* mySmth = malloc(sizeof *mySmth);
  // check malloc return valid pointer, initialize fields with mySmth->
  return mySmth;
}

int main(void)
{
  smth* smth1 = creat_smth();
  ...
  return 0;
}

or

void creat_smth(smth** mySmth)
{
  *mySmth = malloc(sizeof **mySmth);
  // check malloc returned valid pointer, initialize fields with (*mySmth)->
}

int main(void)
{
  smit* smth1;
  creat_smth(&smth1);
  ...
  return 0;
}

Answer 2

It's worth asking your instructor for clarification. That said, it's at least somewhat common to have functions that follow the pattern int init_smth(smth *x) — but note the name: such functions are usually called init_• , not create_• (because it doesn't create the storage for this object, it just fills it).

This makes sense when it's expected that stack-allocating the struct is the right thing to do. Instead of using malloc , the user would pass a pointer to a locally allocated structure:

smth my_smth;
init_smth(&my_smth);

And inside init_smth some moderately complex initialisation would take place (but again without malloc : the memory for the object is already allocated).

However, even then it's more common for the function to return a status code indicating success (hence the return type int in my prototype above).