Heap Overflow and Stack Overflow Explained

Heap and stack overflows are both types of buffer overflows that occur when a program attempts to write data beyond the allocated boundary of a buffer.

Heap Overflow

Heap is used to store dynamic variables. It is a region of process's memory. malloc(), calloc(), resize() all these inbuilt functions are generally used to store dynamic variables.

Following are the regions where heap overflow occurs ?

If we allocate dynamic large number of variables

int main() {
   float *ptr = (int *)malloc(sizeof(float)*1000000.0));
}

If we continuously allocate memory and do not free after using it.

int main() {
   for (int i=0; i<100000000000; i++) {
      int *p = (int *)malloc(sizeof(int));
   }
}

Stack Overflow

The stack is a memory region used for storing function call information, local variables and returns addresses. An stack overflow occurs when the program attempts to write data beyond the allocated size of a buffer on the stack.

If a program consumes more memory space, then stack overflow will occur as stack size is limited in computer memory.

Following are the regions where stack overflow occurs ?

If a function is called recursively by itself infinite times then stack will be unable to store large number of local variables, so stack overflow will occur.

#include<stdio.h>
void calculate(int a) {
   if (a== 0)
      return;
      a = 6;
      calculate(a);
}
int main() {
   int a = 5;
   calculate(a);
}

If we declare a large number of local variables or declare a large dimensional array or matrix can result in stack overflow.

int main() {
   A[20000][20000];
}

Heap vs Stack Overflow

Here, is the comparison between heap overflow and stack overflow: