The objective of the exercise was to accept two operands with any length (not restricted to any datatype size limitation) and perform a sum operation with both.

sum.c

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

int digit_to_int(char d)
{
 	char str[2];
 	str[0] = d;
 	str[1] = '\0';
 	return (int) strtol(str, NULL, 10);
}

typedef struct _digit 
{
	int value;
	struct _digit *next;
} digit;

typedef struct 
{
	digit *first;
	digit *last;
} operand;

operand* newOperand()
{
	operand *o = malloc(sizeof(operand));
	o->first = NULL;
	return o;
}

void insertDigitToOperand(operand *o, int c)
{
	digit *d = malloc(sizeof(digit));
	d->value = c;
	d->next = NULL;
	if(o->first == NULL)
		o->first = d;
	else
		o->last->next = d;
	o->last = d;
}

operand* getOperand()
{
	printf("Enter the value for the operand: ");
	operand *o = newOperand();
	char c;
	while((c=getchar())!='\n')
	{
		if(c < '0' || c > '9')
		{
			printf("\n\nThe input was not a valid number.\n");
			return NULL;
		}
		insertDigitToOperand(o, digit_to_int(c));
	}
	return o;
}

void moveToNextOperandDigit(operand* o)
{
	if(o->first == NULL)
		return;
	digit *d = o->first;
	o->first = d->next;
	if(o->first == NULL)
		o->last = NULL;
	free(d);
}

void clearOperand(operand* o)
{
	while(o->first != NULL)
		moveToNextOperandDigit(o);
	free(o);
}

typedef struct 
{
	digit *top;
	int carry;
} sum;

sum* newSum()
{
	sum *s = malloc(sizeof(sum));
	s->top = NULL;
	s->carry = 0;
	return s;
}

void insertDigitToSum(sum *s, int i)
{
	digit *d = malloc(sizeof(digit));
	d->value = i;
	d->next = s->top;
	s->top = d;
}

sum* getSum(operand *o1, operand *o2)
{
	sum *s = newSum();
	if(o1->first == NULL && o2->first == NULL)
	{
		insertDigitToSum(s, 0);
		return s;
	}
	while(o1->first != NULL || o2->first != NULL)
	{
		int digitSum = 0;
		if(o1->first == NULL)
			digitSum = o2->first->value;
		else if(o2->first == NULL)
			digitSum = o1->first->value;
		else
			digitSum = o1->first->value + o2->first->value;
		digitSum += s->carry;

		int digitValue = digitSum % 10;
		int carryValue = digitSum / 10;

		insertDigitToSum(s, digitValue);
		s->carry = carryValue;

		moveToNextOperandDigit(o1);
		moveToNextOperandDigit(o2);
	}
	if(s->carry > 0)
	{
		insertDigitToSum(s, s->carry);
		s->carry = 0;
	}
	return s;
}

void moveToNextSumDigit(sum* s)
{
	if(s->top == NULL)
		return;
	digit *d = s->top;
	s->top = d->next;
	free(d);
}

void clearSum(sum* s)
{
	while(s->top != NULL)
		moveToNextSumDigit(s);
	free(s);
}

void showSum(sum* s)
{
	while(s->top != NULL)
	{
		printf("%d", s->top->value);
    moveToNextSumDigit(s);
	}
}

int main()
{
	operand *operand1 = getOperand();
	operand *operand2 = getOperand();
	sum *s = getSum(operand1, operand2);
	showSum(s);
	clearOperand(operand1);
	clearOperand(operand2);
	clearSum(s);
}

There have to be a better way for doing it.