Finding the sum of n big numbers - c++

Question

Question: Given a number k, find the sum of k positive big integers.
This is my code,it works, but our Online Judge is rejecting it, says segfault. Why does it show segfault? I could do it with two strings but why isn't this working?
#include <iostream>
#include <string.h>
using namespace std;
void add(int l,int k);
void append(char a[], int temp);
int o;
int tf=0;
int carry=0;
char b[1000000];
char a[10000][10000];
char c[1000000];
int main()
{
int k,x=0,l=0,m=0;
cin>>k;
while(x<k)
{
cin>>a[x];
if(strlen(a[x])>l)
{
l=strlen(a[x]);
}
x++;
}
x=0;
while(x<k)
{
if(strlen(a[x])<l)
{
int temp=0;
append(a[x],l-strlen(a[x]));
}
x++;
}
add(l,k);
if(carry!=0)
{
cout<<carry;
}
while(o>=0)
{
cout<<(int)b[o];
o--;
}
}
void add(int l,int k)
{
int lb=l-1;
int r=k-1;
int sum=0;
int x=0;
int neg=0;
while(lb>=0)
{
r=k-1;
sum=0;
while(r>=0)
{
sum=sum+a[r][lb]-48;
r--;
}
sum=sum+carry;
lb--;
if(sum>=10)
{
b[x]=sum%10;
carry=sum/10;
}
else
{
b[x]=sum;
carry=0;
}
sum=0;
o=x;
x++;
}
}
void append(char a[], int temp)
{
int l=0,m;
int tempb=temp;
m=strlen(a)-1;
while(temp>0)
{
c[l]='0';
l++;
temp--;
}
int z=0;
while(z<=m)
{
c[l]=a[z];
z++;
l++;
}
z=0;
while(z<=m+tempb)
{
a[z]=c[z];
z++;
}
}
Input Format:
First line contains k , which specifies the number of big numbers. Each of the next k lines contains a
big positive integer.
Output Format:
For each test case print the new big integer in a new line
Sample Input:
3
1331331
1313
453535322
Sample Output:
454867966
Constraints:
1<=k<=10
1<=number of digits in big numbers<=10000

Answer 1

Based on the problem statement:
Each input entry my have at most nmax = 10000 digits
Since each entry is stored as a C-style, zero-terminated string, each character array must have a length of (nmax + 1) = 10001 characters, to accomodate for the C-string terminator '\0'.
As you stored the entries into character arrays without leaving room for the zero terminator, assuming each entry was 10000-characters long:
Each entry with k>=1 overwrote the terminator or entry k-1, thus coalescing the entries together;
You thus ended with one giant string, with l = strlen(a[0]) = 100000;
From then on, all further processings were performed with these incorrect (coalesced) inputs and length, leading to buffer overrun at some point later in the execution.