What is the difference between NULL 0 and 0

Question

In C  there appear to be differences between various values of zero -- NULL  NUL and 0   I know that the ASCII character  0  evaluates to 48 or 0x30   The NULL pointer is usually defined as    define NULL 0   Or   define NULL  void   0   In addition  there is the NUL character   0  which seems to evaluate to 0 as well   Are there times when these three values can not be equal   Is this also true on 64 bit systems

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void   0 is NULL  and   0  represents the end of a string

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If NULL and 0 are equivalent as null pointer constants  which should I use  in the C FAQ list addresses this issue as well      C programmers must understand that   NULL and 0 are interchangeable in   pointer contexts  and that an uncast 0   is perfectly acceptable  Any usage of   NULL  as opposed to 0  should be   considered a gentle reminder that a   pointer is involved  programmers   should not depend on it  either for   their own understanding or the   compiler s  for distinguishing pointer   0 s from integer 0 s       It is only in pointer contexts that   NULL and 0 are equivalent  NULL should   not be used when another kind of 0 is   required  even though it might work    because doing so sends the wrong   stylistic message   Furthermore  ANSI   allows the definition of NULL to be     void   0   which will not work at   all in non-pointer contexts   In   particular  do not use NULL when the   ASCII null character  NUL  is desired    Provide your own definition    define NUL   0       if you must

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A one-L NUL  it ends a string    A two-L NULL points to no thing    And I will bet a golden bull   That there is no three-L NULLL   How do you deal with NUL

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One good piece which helps me when starting with C  taken from the Expert C Programming by Linden  The One  l  nul and the Two  l  null Memorize this little rhyme to recall the correct terminology for pointers and ASCII zero  The one  quot l quot  NUL ends an ASCII string   The two  quot l quot  NULL points to no thing   Apologies to Ogden Nash  but the three  quot l quot  nulll means check your spelling     The ASCII character with the bit pattern of zero is termed a  quot NUL quot   The special pointer value that means the pointer points nowhere is  quot NULL quot   The two terms are not interchangeable in meaning

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A byte with a value of 0x00 is  on the ASCII table  the special character called NUL or NULL  In C  since you shouldn t embed control characters in your source code  this is represented in C strings with an escaped 0  i e    0   But a true NULL is not a value  It is the absence of a value  For a pointer  it means the pointer has nothing to point to  In a database  it means there is no value in a field  which is not the same thing as saying the field is blank  0  or filled with spaces    The actual value a given system or database file format uses to represent a NULL isn t necessarily 0x00

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NUL  is not 0  but refers to the ASCII NUL character  At least  that s how I ve seen it used   The null pointer is often defined as 0  but this depends on the environment you are running in  and the specification of whatever operating system or language you are using   In ANSI C  the null pointer is specified as the integer value 0   So any world where that s not true is not ANSI C compliant

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What is the difference between NULL      0    and 0    null character  NUL   is easiest to rule out     0  is a character literal  In C  it is implemented as int  so  it s the same as 0  which is of INT TYPE SIZE  In C    character literal is implemented as char  which is 1 byte  This is normally different from NULL or 0   Next  NULL is a pointer value that specifies that a variable does not point to any address space  Set aside the fact that it is usually implemented as zeros  it must be able to express the full address space of the architecture  Thus  on a 32-bit architecture NULL  likely  is 4-byte and on 64-bit architecture 8-byte  This is up to the implementation of C   Finally  the literal 0 is of type int  which is of size INT TYPE SIZE  The default value of INT TYPE SIZE could be different depending on architecture   Apple wrote      The 64-bit data model used by Mac OS X is known as  LP64   This is the common data model used by other 64-bit UNIX systems from Sun and SGI as well as 64-bit Linux  The LP64 data model defines the primitive types as follows          ints are 32-bit   longs are 64-bit   long-longs are also 64-bit   pointers are 64-bit      Wikipedia 64-bit      Microsoft s VC   compiler uses the LLP64 model    64-bit data models Data model short int long  long long pointers Sample operating systems LLP64      16    32  32    64        64       Microsoft Win64  X64 IA64  LP64       16    32  64    64        64       Most Unix and Unix-like systems  Solaris  Linux  etc   ILP64      16    64  64    64        64       HAL SILP64     64    64  64    64        64           Edit  Added more on the character literal    include  lt stdio h gt   int main void        printf   d   sizeof   0         return 0      The above code returns 4 on gcc and 1 on g

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It appears that a number of people misunderstand what the differences between NULL    0  and 0 are  So  to explain  and in attempt to avoid repeating things said earlier   A constant expression of type int with the value 0  or an expression of this type  cast to type void   is a null pointer constant  which if converted to a pointer becomes a null pointer  It is guaranteed by the standard to compare unequal to any pointer to any object or function   NULL is a macro  defined in  as a null pointer constant    0 is a construction used to represent the null character  used to terminate a string    A null character is a byte which has all its bits set to 0

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NULL is not guaranteed to be 0 -- its exact value is architecture-dependent  Most major architectures define it to  void  0     0  will always equal 0  because that is how byte 0 is encoded in a character literal   I don t remember whether C compilers are required to use ASCII -- if not   0  might not always equal 48  Regardless  it s unlikely you ll ever encounter a system which uses an alternative character set like EBCDIC unless you re working on very obscure systems   The sizes of the various types will differ on 64-bit systems  but the integer values will be the same     Some commenters have expressed doubt that NULL be equal to 0  but not be zero  Here is an example program  along with expected output on such a system    include  lt stdio h gt   int main          size t ii      int  ptr   NULL      unsigned long  null value    unsigned long    amp ptr      if  NULL    0            printf   NULL    0 n          printf   NULL   0x        for  ii   0  ii  lt  sizeof  ptr   ii              printf    02X   null value ii          printf    n        return 0      That program could print   NULL    0 NULL   0x00000001

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All three define the meaning of zero in different context    pointer context - NULL is used and means the value of the pointer is 0  independent of whether it is 32bit or 64bit  one case 4 bytes the other 8 bytes of zeroes   string context - the character representing the digit zero has a hex value of 0x30  whereas the NUL character has hex value of 0x00  used for terminating strings     These three are always different when you look at the memory   NULL - 0x00000000 or 0x00000000 00000000  32 vs 64 bit  NUL - 0x00 or 0x0000  ascii vs 2byte unicode   0  - 0x20   I hope this clarifies it

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Note  This answer applies to the C language  not C     Null Pointers The integer constant literal 0 has different meanings depending upon the context in which it s used  In all cases  it is still an integer constant with the value 0  it is just described in different ways  If a pointer is being compared to the constant literal 0  then this is a check to see if the pointer is a null pointer  This 0 is then referred to as a null pointer constant  The C standard defines that 0 cast to the type void   is both a null pointer and a null pointer constant  Additionally  to help readability  the macro NULL is provided in the header file stddef h  Depending upon your compiler it might be possible to  undef NULL and redefine it to something wacky  Therefore  here are some valid ways to check for a null pointer  if  pointer    NULL   NULL is defined to compare equal to a null pointer   It is implementation defined what the actual definition of NULL is  as long as it is a valid null pointer constant  if  pointer    0   0 is another representation of the null pointer constant  if   pointer   This if statement implicitly checks  quot is not 0 quot   so we reverse that to mean  quot is 0 quot   The following are INVALID ways to check for a null pointer  int mynull   0   lt some code gt  if  pointer    mynull   To the compiler this is not a check for a null pointer  but an equality check on two variables  This might work if mynull never changes in the code and the compiler optimizations constant fold the 0 into the if statement  but this is not guaranteed and the compiler has to produce at least one diagnostic message  warning or error  according to the C Standard  Note that the value of a null pointer in the C language does not matter on the underlying architecture  If the underlying architecture has a null pointer value defined as address 0xDEADBEEF  then it is up to the compiler to sort this mess out  As such  even on this funny architecture  the following ways are still valid ways to check for a null pointer  if   pointer  if  pointer    NULL  if  pointer    0   The following are INVALID ways to check for a null pointer   define MYNULL  void    0xDEADBEEF if  pointer    MYNULL  if  pointer    0xDEADBEEF   as these are seen by a compiler as normal comparisons  Null Characters   0  is defined to be a null character - that is a character with all bits set to zero    0  is  like all character literals  an integer constant  in this case with the value zero   So   0  is completely equivalent to an unadorned 0 integer constant - the only difference is in the intent that it conveys to a human reader   quot I m using this as a null character  quot      0  has nothing to do with pointers  However  you may see something similar to this code  if    char pointer   checks if the char pointer is pointing at a null character  if   char pointer   checks if the char pointer is pointing at a non-null character  Don t get these confused with null pointers  Just because the bit representation is the same  and this allows for some convenient cross over cases  they are not really the same thing  References See Question 5 3 of the comp lang c FAQ for more  See this pdf for the C standard  Check out sections 6 3 2 3 Pointers  paragraph 3

[c] What is the difference between NULL, '\0' and 0?

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