Allocator failing with higher numbers - c

Question

I have school project to create memory allocator. I´ve been debugging it over 3 days now with no reasonable outcome. It is suddenly failing when higher number is ment to be allocated for example i have an array of 65000Bytes and I want to allocate 12000, it fails. Lower numbers up to 800 work great tho.
Let´s say we have static array consisting of 65000Bytes.
We call init, which creates linked list which has pointer to next list and size of list. We also are allowed to have one global variable for pointer to start of array I named mine "start".
Looks like this:
typedef struct mallocstr head;
char* start;
struct mallocstr
{
head* next; //For moving through list
int size; //If allocated then value is <0
};
And here is my allocator function:
void* memory_alloc(unsigned int size)
{
head* p = start;
char* endd= end();
while (p != NULL)
{
if (p->size > 0 && p->size >= size + sizeof(p) && size != 0)
{
int newsize = p->size - size - sizeof(p);
//If head is smaller than newsize create next block and allocate this one
if (sizeof(p) < newsize)
{
hlavicka* var;
p->size = (p->size - newsize) * (-1);
int point = p->size;
point*= -1;
p->next= p + sizeof(p) + point;
var= p;
p = p->next;
p->size= newsize;
p->next = NULL;
return (void*)var + 2*(sizeof(int));
}
//If head is greater or same as newsize
else if (sizeof(p) >= newsize)
{
p->size= p->size* (-1);
p->next = NULL;
return (void*)p + 2*(sizeof(int));
}
}
//If we are unable to alocate this block because size is too small or it is allocated we move to
//next one
else if (p->size< size + sizeof(p) && p != endd|| p->size< 0)
{
p = p->next;
}
//When we cannot allocate any block
else if (p->size < size + sizeof(p) && p == endd|| size == 0)
{
return NULL;
}
}
}
Function end is for getting pointer to the end of list. Here it is:
void* end()
{
head* p = start;
while (p != NULL)
{
if (p->next== NULL)
{
break;
}
else
p = p->next;
}
return (void*)p;
}