SIGABRT error in C++ - c++

Question

I am getting a SIGABRT error when I compile the following code.(PRIME1 problem of spoj).
Link of the problem is http://www.spoj.com/problems/PRIME1/. It runs well on codeblocks but spoj returns SIGABRT error. Can someone explain the reason?
int main()
{
long long k,x,j=0,size,l=0,p=0,q=0,r=0,s;
cin>>size;
int a[(2*size)];
cout<<endl;
for(int i=0; i< (2*size); i++)
{
cin>>a[i];
}
if( size == 1)
{
p=a[1];
}
else
{
do
{
if(a[l+3]>a[l+1])
{
p=a[l+3];
}
else
{
p=a[l+1];
}
l=l+2;
}while(l<2*(size-1));
}
cout<<p;
long * b = new long [p-1];
for(long long i=0;i<p-1;i++)
{
b[i]=1;
}
b[0]=b[1]=0;
s=sqrt(p)
for(long long i = 2; i <= s; i++)
{
if(b[i] == 1)
{
for(long long j = i*i; j <= p; j = j + i)
{
b[j] = 0;
}
}
}
while(r<(2*size))
{
for(long long i = a[r];i < a[r+1];i++)
{
if(b[i] == 1 )
{
cout << i << "\n";
}
}
cout<<endl;
r=r+2;
}
delete [] b;
}

Answer 1

You are accessing array element accessing outside bounds
Array size 2*size-1 So elements from 0 to 2*size-2
But in your for loop you are going upto 2*size thus accessing 2*size-1 which is outside bounds

Answer 2

int a[(2*size)-1];
This is not legal C++ code (it's using a GCC extension), but it obviously compiled, so we'll let that slide. You are accessing your array out of bounds in the following loop and all over the place later on, which is undefined behavior - you need an array of size 2 * size to read in all the supplied parameters. Although given that they guarantee that size <= 10, you might as well just declare it as int a[20];
But that probably didn't cause the crash. What caused the crash is probably this line:
long * b = new long [p-1];
What's p? Well, let's just consider the easy case of size = 1 where you set p to a[1], or the second number you read in. What's the bounds on that number?
The questions says that the bound is n <= 1000000000, or 109. Your new can be requesting as much as 8GB of memory, depending on the value of sizeof(long) in the system you are using. The allocation is almost certainly going to fail, throwing a std::bad_alloc exception that causes std::abort() to be called as you don't have any exception handling code.

Answer 3

You initialize a to 2 * size - 1 elements...
int a[(2*size)-1];
Yet you write 2 * size elements.
for(int i=0; i< (2*size); i++)
// ...
Your loop should be:
for(int i=0; i< (2*size-1); i++)
Next...
if(size == 1)
{
p=a[1];
}
If size == 1 then you allocated an array of 2 * 1 - 1 = 1 element, so a[1] is an invalid access (you only have a[0] as arrays are 0-indexed).
You then have stuff like this:
if(a[l+3]>a[l+1])
Which loops until l == 2*size-1, so l+3 is invalid as soon as you hit 2 * size - 1 - 3.
Basically you just have a lot of places where you're reading or writing past the end of an array or not ensuring proper initialization and invoking undefined behavior.