Want to Know About Different Scenario of Memory Leaks

Question

I am just curious about how memory leaks happens when you write a C program.

Are the following are examples of memory leaks?

1. Trying to access the part of the memory whose access is not given to your program or when you are trying to access the location of the array which is not there.

---

#include <stdio.h>

int main(void)
{
     int num[5];

     int i;

      for(i=0;i<5;i++)
           scanf("%d", (num+i));

   printf("%d\n", num[5]); //printing the data stored at loc num[5] which is not present.

      return 0;
}

2. Printing the value stored in an un-initialized variables.

3. Use of void*memcpy(const*dst,void const*src,size_t n) function memory leak occurs when src and dst pointers points to the same memory address or function is undefined when addresses overlaps.

4. Use of free() more than once on the same pointers which has been freed already. For Example:

---

#include <stdio.h>

int main(void)
{

    int*num = NULL;

    int i;

    num = (int*) calloc(sizeof(int), 5);

    for (i = 0; i < 5; i++)
        scanf("%d", (num + i));

    free(num);
    free(num);
    return 0;
}

Answer 1

All of aforesaid scenarios cause undefined behavior.

• printf("%d\n", num[5]); is out-of-bound memory access.

• Printing the value stored in an un-initialized variables, in case of the variable has trap representation, causes UB.

• Source and destination overlap in memcpy(), UB.

• Multiple free() is also UB.

Also, memory leak is not about any invalid access, it is just wastage of memory leading to out of memory scenario for a system. You can read more about that here.