0 Address (NULL Pointer)

1. What is "Address 0"?

• Definition: In C, address 0 is a special memory address reserved by the system.

• Meaning: It represents "Nowhere" or "Invalid Location."

• The Macro: In code, it is usually represented by the macro NULL.

  • NULL is essentially defined as (void *)0 or simply 0.

2. Key Characteristics

• Reserved Area: The Operating System typically reserves the lowest segment of memory (including address 0) so programs cannot use it.

• No Access: You cannot read from or write to address 0.

• The Consequence: If you try to access data at address 0 (dereferencing a NULL pointer), the program will crash.

  • Error Message: usually "Segmentation Fault" (Segfault) on Linux/Unix or "Access Violation" on Windows.

3. Usage Patterns

We use address 0 (NULL) intentionally to mark pointers as "empty" or "safe".

• Initialization: Always initialize pointers to NULL if they don't point to a valid object yet.

  c int *ptr = NULL; // Good practice

• Error Checking: Functions often return NULL to indicate failure.

  c FILE *fp = fopen("file.txt", "r"); if (fp == NULL) { printf("Failed to open file.\n"); }

• Sentinel Value: It marks the end of data structures (like Linked Lists).