Arbitrary specific integer c program - c

Question

So, recently I've asked question about arbitrary specific integer calculator program. It scans two string and is supposed to calculate it's sum and difference, multiplication, division and modulus. So far I think I'm really close to finishing the sum difference and multiplication functions. Here is how my code look like for addition function:
void sum(char* a, char* b)
{
//initializing values
int len1,len2,i;
int carry=0;
char *r;
// counting the digits
len1=strlen(a);
len2=strlen(b);
// checking which array is longer
if (len1>=len2)
{
// if first array is longer we allocate enough memory for the result
r=(char*)malloc((len1+1)*sizeof(char));
// starting from the LSB
for (i=0; i<=len2; ++i)
{ // mod 10 to extract only the last digit of the sum
// that becomes our new digit
*(r+len1-i)=(*(a+len1-i) + *(b+len2-i) + carry)%10;
carry=(*(a+len1-i)+*(b+len2-i))/10;
}
// copying the rest of the digits
for (i=len2+1; i<=len1; ++i)
*(r+len1-i)=*(a+len1-i);
}
//case when the other number has more digits
else
{
r=(char*)malloc((len2+1)*sizeof(char));
for (i=0; i<=len1; ++i)
{
*(r+len2-i)=(*(a+len1-i)+*(b+len2-i)+carry)%10;
carry=(*(a+len1-i)+*(b+len2-i))/10;
}
for (i=len1+1; i<=len2; ++i)
*(r+len2-i)=*(b+len2-i);
}
// printing the result
printf("The result is: \n %s \n", r);
}
The problem is when I try to call for the function in main the end result just gives errors, even though there are not any errors while compiling the code. It might be a problem in main so here is how my main looks like.
int main()
{
FILE *fp;
char *fi=NULL, *se=NULL;
char o;
int i=0;
fp= fopen("/CalculatorHistory.txt", "r+");
opening();
fi=(char*)malloc(60*sizeof(char));
se=(char*)malloc(60*sizeof(char));
while(i<2)
{
first();
scanf("%s", fi);
// I am not sure when im saving pointer arrays, if it works like this
// or i should save it in a for loop
fprintf(fp, fi);
oper();
scanf(" %c", &o);
fprintf(fp, o);
second();
scanf("%s", se);
fprintf(fp, se);
switch (o)
{
case '+':
sum(fi, se);
case '-':
sub(fi, se);
case '*':
mul(fi, se);
case '/':
div(fi, se);
case '%':
mod(fi, se);
}
}
return 0;
}
I hope that somebody can help me. I'm pretty lost and still really new to c programming.