CERT C: Rule MEM31-C

Memory leak occurs when you do not free a block of memory allocated through malloc, calloc, realloc, or new. If the memory is allocated in a function, the defect does not occur if:

Dynamic memory allocation functions such as malloc allocate memory on the heap. If you do not release the memory after use, you reduce the amount of memory available for another allocation. On embedded systems with limited memory, you might end up exhausting available heap memory even during program execution.

Determine the scope where the dynamically allocated memory is accessed. Free the memory block at the end of this scope.

To free a block of memory, use the free function on the pointer that was used during memory allocation. For instance:

ptr = (int*)malloc(sizeof(int));
//...
free(ptr);

It is a good practice to allocate and free memory in the same module at the same level of abstraction. For instance, in this example, func allocates and frees memory at the same level but func2 does not.

void func() {
  ptr = (int*)malloc(sizeof(int));
  {
    //...
  }
  free(ptr);
}

void func2() {
  {
   ptr = (int*)malloc(sizeof(int));
   //...
  }
  free(ptr);
}

#include<stdlib.h>
#include<stdio.h>

void assign_memory(void)
{
    int* pi = (int*)malloc(sizeof(int));
    if (pi == NULL) 
        {
         printf("Memory allocation failed");
         return;
        }


    *pi = 42;
    /* Defect: pi is not freed */
} //Noncompliant

In this example, pi is dynamically allocated by malloc. The function assign_memory does not free the memory, nor does it return pi.

One possible correction is to free the memory referenced by pi using the free function. The free function must be called before the function assign_memory terminates

#include<stdlib.h>
#include<stdio.h>

void assign_memory(void)
{
    int* pi = (int*)malloc(sizeof(int));
    if (pi == NULL) 
        {
         printf("Memory allocation failed");
         return;
        }
    *pi = 42;

    /* Fix: Free the pointer pi*/
    free(pi);                   
}

Another possible correction is to return the pointer pi. Returning pi allows the function calling assign_memory to free the memory block using pi.

#include<stdlib.h>
#include<stdio.h>

int* assign_memory(void)
{
    int* pi = (int*)malloc(sizeof(int));
    if (pi == NULL) 
        {
            printf("Memory allocation failed");
            return(pi);
        }
    *pi = 42;

    /* Fix: Return the pointer pi*/
    return(pi);                   
}