Error using memcpy : "Access violation reading location 0x0000000000000000" - c

Question

I am trying to write something similar to std::vector but in c to store a bunch of mathematical vectors.
Here is the line that is casing the error.
pVl->pData = memcpy(pNewData, pVl->pData, sizeof(pVl->pData));
My Intention: Copy data from pVl->pData to pNewData. Then assign the return value, which is the
pointer to start of the newly copied data memory and assign it to pVl->pData. I am not sure what I am doing wrong.
MRE:
#include <stdlib.h>
#include <string.h>
typedef enum R_Code { R_OK, R_WARNING, R_FAIL, R_FATAL } R_Code;
struct Vector2_s
{
float x;
float y;
} const Default_Vector2 = { 0.0f, 0.0f };
typedef struct Vector2_s Vector2;
struct Vector2List_s
{
//current capacity of the List
size_t capacity;
//current size of the list
size_t size;
//data buffer
Vector2* pData;
} const Default_Vector2List = { 0, 0, NULL };
typedef struct Vector2List_s Vector2List;
R_Code Vector2List_ReAllocateMem(Vector2List* pVl) {
if (pVl->capacity == 0) {
pVl->capacity++;
}
Vector2* pNewData = malloc(pVl->capacity * 2 * sizeof(Vector2));
if (pNewData == NULL) {
return R_FAIL;
}
pVl->capacity *= 2;
pVl->pData = memcpy(pNewData, pVl->pData, sizeof(pVl->pData));//EXPECTION THROWN IN THIS LINE
free(pNewData);
return R_OK;
}
R_Code Vector2List_PushBack(Vector2List* pVl, const Vector2 v) {
if (pVl->size == pVl->capacity) {
R_Code rcode = Vector2List_ReAllocateMem(pVl);
if (rcode == R_FAIL) {
return rcode;
}
}
pVl->pData[pVl->size] = v;
pVl->size++;
return R_OK;
}
int main() {
Vector2List vl = Default_Vector2List;
Vector2List_PushBack(&vl, Default_Vector2);
return 0;
}

Answer 1

Within the function Vector2List_ReAllocateMem you allocated dynamically memory
Vector2* pNewData = malloc(pVl->capacity * 2 * sizeof(Vector2));
then in this statement
pVl->pData = memcpy(pNewData, pVl->pData, sizeof(pVl->pData));
you are using the null pointer pVl->pData as a source of data that invokes undefined behavior.
Moreover you freed the allocated memory.
free(pNewData);
Also using this expression sizeof(pVl->pData) does not make a sense.
It seems what you need is the following
pVl->pData = pNewData;
Though if you are going to reallocate memory then instead of malloc you need to use realloc.
You need to rewrite the function entirely.