Write a program to subtract two big number( greater than 100 digit)

#include<stdio.h>
#include<string.h>

int compare_number(char dec1[], char dec2[])
{
int length1, length2, result;

//If one of the number is negative, process them here.
if (dec1[0] != '-' && dec2[0] == '-') return(1);
if (dec1[0] == '-' && dec2[0] != '-') return(-1);

//If the numbers are not having equal sign, they will not come here.
length1 = strlen(dec1);
length2 = strlen(dec2);

if (length1 == length2) result = strcmp(dec1, dec2);
   else result = length1 - length2;

   //If both the number are negative.
   if (dec1[0] == '-') return(-result);
   else return(result);
}

void character_subtraction(char num1[], char num2[], char result[])
{
int  i, j, k;
char *swap;

   //If 'num1' < 'num2', then swap them and put '-' at the first of the 'result'.
if (compare_number(num1, num2) < 0)
{
swap = num1; num1 = num2; num2 = swap;
      sprintf(result, "-%s", num1);
}

else strcpy(result, num1);

for(i = strlen(result) - 1, j = strlen(num2) - 1; j >= 0; i--, j--)
{
      if (result[i] >= num2[j])
      {
         result[i] = result[i] - num2[j] + '0';
      continue;
      }

result[i] = (result[i] + 10) - num2[j] + '0';
for(k = i - 1; k > 0 && result[k] == '0'; k--) result[k] = '9';
result[k] -= 1;
}

   //Ignoring all the leading zeros.
   for(i = (result[0] == '-') ? 1 : 0; result[i] == '0'; i++);

   //If the result is 0, then previous loop will end in 'NULL'.
   if (result[i] == NULL) --i;

   //Launch all the digits other than leading zeros.
   for(j = (result[0] == '-') ? 1 : 0; result[i] != NULL; j++, i++) result[j] = result[i];

   result[j] = NULL;

return;
}

int main()
{
char num1[100] , num2[100] ,res[100];
while(scanf(" %s %s" , num1 , num2)==2)
{
character_subtraction(num1 , num2 , res);
printf("%s\n" , res);
}
return 0;
}