Union of sets using linked lists #2

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>

typedef struct node *position;

typedef struct node
{
	int num;
	position next;
}_NODE;

void ReadFromFile(position);
void Print(position);
void Union(position un, position p, position q);
int main(int argc, char *argv[])
{

	_NODE L1,L2,PRS,UN;

	L1.next=NULL;
	L2.next=NULL;
	PRS.next = NULL;

	ReadFromFile(&L1);
	Print(L1.next);

	ReadFromFile(&L2);
	Print(L2.next);

	printf("\n");

	Union(&UN, L1.next, L2.next);
	Print(UN.next);

	return 0;
}
void ReadFromFile(position p)
{
	FILE *input;
	char file[50];
	position tmp,z;

	printf("\nFile name:");
	scanf(" %s", &file);

	input = fopen(file, "r");

	if(input == NULL)
	{
		printf("File not opened.");
	}
	else
	{
		while(!feof(input))
		{
			z=(position) malloc(sizeof(_NODE));
			if(z==NULL)
				printf("Greska u alokaciji memorije.");
			else
			{
				fscanf(input," %d", &z->num);

				tmp=p;

				while(tmp->next != NULL && tmp->next->num < z->num)
					tmp = tmp->next;

				z->next = tmp->next;
				tmp->next = z;
			}

		}
		fclose(input);
	}

}
void Print(position p)
{
	while (p != NULL)
	{
		printf("%d\t",p->num);
		p=p->next;
	}
}


void Union(position un, position p, position q)
{
	position z,tmp;

	while (p != NULL && q != NULL)
	{

		z = (position)malloc(sizeof(_NODE));
		if (z == NULL)
		{
			printf("Memory allocation error!");
			break;
		}
		else
		{
			if (p->num < q->num)
			{
				z->num = p->num;
				z->next = un->next;
				un->next = z;

				p = p->next;
			}
			else if (p->num > q->num)
			{
				z->num = q->num;
				z->next = un->next;
				un->next = z;

				q = q->next;
			}
			else
			{
				z->num = p->num;

				z->next = un->next;
				un->next = z;

				p = p->next;
				q = q->next;
			}
			if (z->num == un->num)
			{
				un->next = z->next;
				free(z);
			}
		}
	}
	if (p == NULL)
	{
		tmp = q;
		while (tmp != NULL)
		{
			z = (position)malloc(sizeof(_NODE));
			if (z == NULL)
			{
				printf("Memory allocation error!.");
				break;
			}
			else
			{
				z->num = tmp->num;
				z->next = un->next;
				un->next = z;
				tmp = tmp->next;
			}
		}
	}
	if (q == NULL)
	{
		tmp = p;
		while (tmp != NULL)
		{
			z = (position)malloc(sizeof(_NODE));
			if (z == NULL)
			{
				printf("Memory allocation error!");
				break;
			}
			else
			{
				z->num = tmp->num;
				z->next = un->next;
				un->next = z;
			}
			tmp = tmp->next;
		}
	}
}