What EXACTLY is meant by de-referencing a NULL pointer

Question

I am a complete novice to C  and during my university work I ve come across comments in code that often refer to de-referencing a NULL pointer   I do have a background in C   I ve been getting by that this might be similar to a  NullReferenceException  that you get in  Net  but now I am having serious doubts     Can someone please explain to me in laymans terms exactly what this is and why it is bad

User · Accepted Answer

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    amp 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

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Quoting from wikipedia      A pointer references a location in   memory  and obtaining the value at the   location a pointer refers to is known   as dereferencing the pointer    Dereferencing is done by applying the unary   operator on the pointer   int x   5  int   p          pointer declaration p    amp x           pointer assignment  p   7           pointer dereferencing  example 1 int y    p       pointer dereferencing  example 2    Dereferencing a NULL pointer  means performing  p when the p is NULL

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From wiki     A null pointer has a reserved value  often but not necessarily the value zero  indicating that it refers to no object           Since a null-valued pointer does not refer to a meaningful object  an attempt to dereference a null pointer usually causes a run-time error    int val  1  int  p   NULL   p   val     Whooosh

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A NULL pointer points to memory that doesn t exist  and will raise Segmentation fault  There s an easier way to de-reference a NULL pointer  take a look  int main int argc  char const  argv            int   0   0     Segmentation fault  core dumped      return 0     Since 0 is never a valid pointer value  a fault occurs  SIGSEGV  si signo SIGSEGV  si code SEGV MAPERR  si addr NULL

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It means   myclass  p   NULL   p            illegal  dereferencing NULL pointer        p      illegal  dereferencing NULL pointer p- gt meth       illegal  equivalent to   p  meth    which is dereferencing NULL pointer  myclass  p      some legal  non-NULL pointer      p            Ok        p      Ok p- gt meth       Ok  if myclass  meth   exists   basically  almost anything involving   p  or implicitly involving   p   e g  p- gt     which is a shorthand for   p        except for pointer declaration

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Dereferencing just means reading the memory value at a given address   So when you have a pointer to something  to dereference the pointer means to read or write the data that the pointer points to   In C  the unary   operator is the dereferencing operator   If x is a pointer  then  x is what x points to   The unary  amp  operator is the address-of operator   If x is anything  then  amp x is the address at which x is stored in memory   The   and  amp  operators are inverses of each other  if x is any data  and y is any pointer  then these equations are always true      amp x     x  amp   y     y   A null pointer is a pointer that does not point to any valid data  but it is not the only such pointer    The C standard says that it is undefined behavior to dereference a null pointer   This means that absolutely anything could happen  the program could crash  it could continue working silently  or it could erase your hard drive  although that s rather unlikely    In most implementations  you will get a  segmentation fault  or  access violation  if you try to do so  which will almost always result in your program being terminated by the operating system   Here s one way a null pointer could be dereferenced   int  x   NULL      x is a null pointer int y    x         CRASH  dereference x  trying to read it  x   0             CRASH  dereference x  trying to write it   And yes  dereferencing a null pointer is pretty much exactly like a NullReferenceException in C   or a NullPointerException in Java   except that the langauge standard is a little more helpful here   In C   dereferencing a null reference has well-defined behavior  it always throws a NullReferenceException   There s no way that your program could continue working silently or erase your hard drive like in C  unless there s a bug in the language runtime  but again that s incredibly unlikely as well

[c] What EXACTLY is meant by "de-referencing a NULL pointer"?

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