C functions to create dynamic array of structs

Question

can someone help with this piece of code? I leaved out check of allocations to keep it brief.

typedef struct {
  int x;
  int y;
} MYSTRUCT;

void init(MYSTRUCT **p_point);
void plusOne(MYSTRUCT **p_point, int *p_size);

int main()
{
  MYSTRUCT *point;
  int size = 1;

  init(&point);

  plusOne(&point, &size);
  plusOne(&point, &size);

  point[1]->x = 47;  //  this was the problem
  point[1].x = 47;   //  this is solution    

  return 0;
}

void init(MYSTRUCT **p_point)
{
  *p_point = (MYSTRUCT *) malloc( sizeof(MYSTRUCT) );
}

void plusOne(MYSTRUCT **p_point, int *p_size) 
{
  (*p_size)++;

  *p_point = realloc(*p_point, *p_size * sizeof(MYSTRUCT) ); // also calling to the function is fixed
}

I don't understand why index notation doesn't work after calling to functions.

Sergey Kalinichenko · Accepted Answer · 2014-05-21 15:22:15Z

2

This is because you are not multiplying the p_size by sizeof(MYSTRUCT) in the call of realloc, and not assigning the results back to p_point:

*p_point = realloc(*p_point, *p_size * sizeof(MYSTRUCT));

Notes:

You do not need to cast the result of malloc or realloc in C.
For consistency, consider passing &size to init, and set it to 1 there.

answered May 21, 2014 at 15:22

Sergey Kalinichenko

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3 Comments

the_Imp Over a year ago

Thank you for incredibly fast answer. I've fixed the realloc() as you said but still can't use the index notation. I need it because in next step I want to make function which fills x and y of the array.

Sergey Kalinichenko Over a year ago

@the_Imp You need to use ., not -> there, because point[1] is MYSTRUCT itself, not a pointer to MYSTRUCT.

the_Imp Over a year ago

Thank you. It works. I try to make other functions and hope it'll works.

twalberg · Accepted Answer · 2014-05-21 15:53:21Z

You have some type confusion going on... Here:

MYSTRUCT *point;

you declare point to be a pointer to a MYSTRUCT structure (or an array of them).

The syntax point[i] is equivalent to *(point + i) - in other words, it already dereferences the pointer after the addition of the appropriate offset, yielding a MYSTRUCT object, not a pointer to one.

The syntax p->x is equivalent to (*p).x. In other words, it also expects p to be a pointer, which it dereferences, and then yields the requested field from the structure.

However, since point[i] is no longer a pointer to a MYSTRUCT, using -> on it is wrong. What you are looking for is point[i].x. You could alternatively use (point + i) -> x, but that's considerably less readable...

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