What s the difference between a null pointer and a void pointer

Question

Whats the difference between a Null pointer  amp  a Void pointer

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Void refers to the type   Basically the type of data that it points to is unknown   Null refers to the value   It s essentially a pointer to nothing  and is invalid to use

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Both void and null pointers point to a memory location in the end    A null pointer points to place  in a memory segment  which is usually the 0th address of memory segment  It is almost all about how the underlying OS treat that  reserved  memory location  an implementation detail  when you try to access it to read write or dereference  For example  that particular memory location can be marked as  non-accessible  and throws an expection when it s accessed   A void pointer can point to anywhere and potentially represent any type  primitive  reference-type  and including null     As someone nicely put above  null pointer represents a value whereas void  represents a type more than a value

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Usually a null pointer  which can be of any type  including a void pointer    points to    the address 0  against which most CPU instructions sets can do a very fast compare-and-branch  to check for uninitialized or invalid pointers  for instance  with optimal code size performance for the ISA  an address that s illegal for user code to access  such as 0x00000000 in many cases   so that if a code actually tries to access data at or near this address  the OS or debugger can easily stop or trap a program with this bug    A void pointer is usually a method of cheating or turning-off compiler type checking  for instance if you want to return a pointer to one type  or an unknown type  to use as another type   For instance malloc   returns a void pointer to a type-less chunk of memory  the type of which you can cast to later use as a pointer to bytes  short ints  double floats  typePotato s  or whatever

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I don t think AnT s answer is correct    NULL is just a pointer constant  otherwise how could we have ptr   NULL   As NULL is a pointer  what s its type  I think the type is just  void     otherwise how could we have both int   ptr   NULL and  user-defined type   ptr   NULL   void type is actually a universal type  Quoted in  C11 ISO IEC 9899 201x    6 3 2 3 Pointers Section 3          An integer constant expression with the value 0  or such an expression cast to type  void    is called a null pointer constant    So simply put  NULL pointer is a void pointer constant

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A null pointer is guaranteed to not compare equal to a pointer to any object  It s actual value is system dependent and may vary depending on the type  To get a null int pointer you would do   int  p   0    A null pointer will be returned by malloc on failure   We can test if a pointer is null  i e  if malloc or some other function failed simply by testing its boolean value   if  p           Pointer is not null      else          Pointer is null        A void pointer can point to any type and it is up to you to handle how much memory the referenced objects consume for the purpose of dereferencing and pointer arithmetic

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NULL is a value that is valid for any pointer type   It represents the absence of a value   A void pointer is a type   Any pointer type is convertible to a void pointer hence it can point to any value   This makes it good for general storage but bad for use   By itself it cannot be used to access a value   The program must have extra context to understand the type of value the void pointer refers to before it can access the value

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Null pointer is a special reserved value of a pointer  A pointer of any type has such a reserved value  Formally  each specific pointer type  int    char   etc   has its own dedicated null-pointer value  Conceptually  when a pointer has that null value it is not pointing anywhere   Void pointer is a specific pointer type - void   - a pointer that points to some data location in storage  which doesn t have any specific type   So  once again  null pointer is a value  while void pointer is a type  These concepts are totally different and non-comparable  That essentially means that your question  as stated  is not exactly valid  It is like asking  for example   What is the difference between a triangle and a car

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A Null pointer has the value 0  void pointer is a generic pointer introduced by ANSI  Generic pointer can hold the address of any data type

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Null pointers and void pointers are completely different from each other  If we request the operating system through malloc   in c langauge  to allocate memory for a particular data type  then the operating system allocates memory in heap  if space is available in heap  and sends the address of the memory which was allocated   When memory is allocated by os in heap then we can assign this address value in any pointer type variable of that data type  This pointer is then called a void pointer until it is not taken for any process   When the space is not available in heap then the operating system certainly allocates memory and sends an address value of that location but this memory is not allocated in heap by the os because there is no space in heap in this case this memory is allocated by the os in the system memory   This memory can not be accessed by the user hence when we assign this address value in a pointer then this pointer is known as null pointer  and we cannot use this pointer  In the case of void pointer we can use it for any process in any programming language

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They are two different concepts   void pointer  is a type  void      null pointer  is a pointer that has a value of zero  NULL   Example   void  pointer   NULL    That s a NULL void pointer

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A null pointer points has the value NULL which is typically 0  but in any case a memory location which is invalid to dereference  A void pointer points at data of type void  The word  void  is not an indication that the data referenced by the pointer is invalid or that the pointer has been nullified

[c] What's the difference between a null pointer and a void pointer?

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