What exactly is meant by “de-referencing a NULL pointer”?

A NULL pointer points to memory that doesn’t exist. This may be address 0x00000000 or any other implementation-defined value (as long as it can never be a real address). Dereferencing it means trying to access whatever is pointed to by the pointer. The * operator is the dereferencing operator:

int a, b, c; // some integers
int *pi;     // a pointer to an integer

a = 5;
pi = &a; // pi points to a
b = *pi; // b is now 5
pi = NULL;
c = *pi; // this is a NULL pointer dereference

This is exactly the same thing as a NullReferenceException in C#, except that pointers in C can point to any data object, even elements inside an array.

Related Posts:

  1. Warning: assignment from incompatible pointer type
  2. Returning an array using C
  3. Why do I get “cast from pointer to integer of different size” error?
  4. Excess elements of scalar initializer for pointer to array of ints
  5. Realloc Invalid Pointer in C
  6. Pointer to 2D arrays in C
  7. Pointer to a string in C?
  8. Using pointer to char array, values in that array can be accessed?
  9. Arrow operator (->) usage in C
  10. What does “dereferencing” a pointer mean?
  11. char *array and char array[]
  12. What is the difference between const int*, const int * const, and int const *?
  13. What does ** do in C language?
  14. Invalid pointer error on invoking free() after malloc in C
  15. expression must have integral type
  16. How do you pass a function as a parameter in C?
  17. How do you pass a function as a parameter in C?
  18. C free(): invalid pointer
  19. What’s wrong with my code? What is argv[1]?
  20. Valgrind: Invalid read of size 1
  21. Warning: return from incompatible pointer type in C
  22. How to do scanf for single char in C
  23. Break statement not within loop or switch in C
  24. How to properly malloc for array of struct in C
  25. Cache Simulator in C
  26. strcmp giving segmentation fault
  27. Where is the C auto keyword used?
  28. Why am I getting this error: “data definition has no type or storage class”?
  29. Initialization from incompatible pointer type warning when assigning to a pointer
  30. Typedef function pointer?
  31. Convert char array to a int number in C
  32. Bind failed: Address already in use
  33. Warning/error “function declaration isn’t a prototype”
  34. How to prevent multiple definitions in C?
  35. How to pass 2D array (matrix) in a function in C?
  36. How can you print multiple variables inside a string using printf?
  37. warning: passing argument ’from incompatible pointer type [enabled by default]’
  38. C dynamically growing array
  39. Why should we check WIFEXITED after wait in order to kill child processes in Linux system call?
  40. How to compile makefile using MinGW?
  41. Mapping a numeric range onto another
  42. Why are hexadecimal numbers prefixed with 0x?
  43. too many arguments for format [-Wformat-extra-args]
  44. What is the difference between char array and char pointer in C?
  45. How to convert const char* to char* in C?
  46. How to copy a char array in C?
  47. cast to pointer from integer of different size, pthread code
  48. Invalid type argument of unary ‘*’ (have ‘int’) Error in C
  49. C subscripted value is neither array nor pointer nor vector when assigning an array element value
  50. error: too few arguments to function `printDay’ (C language)
  51. Why use pointers?
  52. Where to find the complete definition of off_t type?
  53. struct has no member named
  54. printf not printing to screen
  55. Still Reachable Leak detected by Valgrind
  56. Assembly x86 – “leave” Instruction
  57. What is signed integer overflow?
  58. When to use const char * and when to use const char []
  59. What do numbers using 0x notation mean?
  60. Can I create an Array of Char pointers in C?
  61. Does stack grow upward or downward?
  62. Convert Little Endian to Big Endian
  63. error C2371: ‘functionname’ redefinition: different basic types
  64. How does one represent the empty char?
  65. How to write to a file using open() and printf()?
  66. note: previous implicit declaration of ‘point_forward’ was here
  67. Can I define a function inside a C structure?
  68. Where is the header file on Linux? Why can’t I find ?
  69. getc() vs fgetc() – What are the major differences?
  70. Casting a pointer to an int
  71. typedef struct pointer definition
  72. Tokenizing strings in C
  73. Difference between char *argv[] and char **argv for the second argument to main()
  74. Warning: comparison of distinct pointer types
  75. Dereference void pointer
  76. “-bash: gcc: command not found” using cygwin when compiling c?
  77. How to format strings using printf() to get equal length in the output
  78. What do \t and \b do?
  79. Why and when to use static structures in C programming?
  80. Where does linux store my syslog?
  81. Example of waitpid() in use?
  82. Can a function return two values?
  83. C: warning: excess elements in array initializer; near initialization for ‘xxx’ ; expects ‘char *’, but has type ‘int’
  84. write() to stdout and printf output not interleaved?
  85. How to know what the ‘errno’ means?
  86. Split string with multiple delimiters using strtok in C
  87. Printf was not declared in this scope
  88. warning: missing terminating ” character [enabled by default]
  89. How to free memory from char array in C
  90. Can I get Unix’s pthread.h to compile in Windows?
  91. Initializing 2D char array in C
  92. FFT in a single C-file 
  93. Process exited with return value 3221225477
  94. How do I check if a string contains a certain character?
  95. malloc(sizeof(int)) vs malloc(sizeof(int *)) vs (int *)malloc(sizeof(int))
  96. %p Format specifier in c
  97. Level vs Edge Trigger Network Event Mechanisms
  98. “Nothing to be done for makefile” message
  99. Convert Char to String in C
  100. What is the difference between signed and unsigned int

Leave a Comment