# V2582. MISRA. Block of memory should only be freed if it was allocated by a Standard Library function.

This diagnostic rule is based on the MISRA (Motor Industry Software Reliability Association) software development guide.

This rule only applies to programs written in C. The 'free' function releases memory that was dynamically allocated by the 'malloc', 'calloc', or 'realloc' functions. Using the 'free' function twice on the same memory block causes undefined behavior.

Take a look at the following example:

    void foo()
    {
      int arr[50];
      // ....
      free(arr);
    }

The analyzer found an error here. The developer calls the 'free' function to delete an array. This is unnecessary and results in undefined behavior. The array is stored on the stack, and the memory is freed automatically when the 'foo' function exits.

Here's another example:

    void foo()
    {
      float *p1 = (float *)malloc(N * sizeof(float));
      float *p2 = (float *)malloc(K * sizeof(float));
      // ....
      free(p1);
      free(p1);
    }

The code contains a typo. The 'free' function is called twice for the memory block with the same pointer, 'p1'. This causes two problems. First, the buffer whose address is stored in the 'p2' variable, is not freed and causes a memory leak. Second, this code produces undefined behavior because the same buffer is released twice.

The fixed code:

    void foo()
    {
      float *p1 = (float *)malloc(N * sizeof(float));
      float *p2 = (float *)malloc(K * sizeof(float));
      // ....
      free(p1);
      free(p2);
    }

This diagnostic is classified as:

MISRA-C-2012-22.2 MISRA-C-2023-22.2