The code `int * p = realloc(NULL, 4 * sizeof(int));`...

The code int * p = realloc(NULL, 4 * sizeof(int)); allocates memory for 4 integers (each of size sizeof(int)) using realloc. Here's a detailed breakdown of what this code does:

1. realloc with NULL as the first argument:

   • Normally, realloc is used to resize an existing block of memory. However, when the first argument to realloc is NULL, it behaves like malloc.
   • As a result, realloc(NULL, size) is equivalent to malloc(size).

2. 4 * sizeof(int):

   • This calculates the size of memory to allocate. sizeof(int) gives the size (in bytes) of an int on the platform. Multiplying it by 4 requests enough memory to store 4 integers.

3. Assignment:

   • The newly allocated memory is assigned to the pointer p.

Summary:

• The code allocates enough memory to hold 4 integers dynamically and assigns the starting address of this memory block to the pointer p.
• After the execution of this statement:
  • p will point to the start of the newly allocated block of memory.
  • The memory is uninitialized, meaning it contains arbitrary (garbage) data.

Important Note:

• If the memory allocation fails (for example, due to insufficient memory), realloc will return NULL. Therefore, you should always check if p is NULL before using it to avoid undefined behavior.