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What is a null macro what is the difference between a null pointer and a null macro?


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2008-02-24 14:20:33
2008-02-24 14:20:33

NULL Macro is simply what is defined as 0 in a macro provided by the library

Null pointer is a pointer which has 0 or NULL value stored and points to nowhwere still it points to 0x00 i.e. the first memory location of the OS

Null pointer != Uninitialized pointer because an uninitialised pointer can point anywhere in the memory location ...but a NULL pointer surely points to no where(but still behind the scene we can say that it only points to 0x00). Never we can retrive a Null pointer location using th"&" operator..neither will malloc/calloc return NULL IF THERE IS SPACE IN THE MEMORY. NULL pointer is unique !!

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A null pointer is a pointer variable with the value 0. A null macro is a macro that defines the NULL symbol (typically 0L on a 32-bit system).

A void pointer is a pointer that has no type information attached to it.A null pointer is a pointer that points to "nothing". A null pointer can be of any type (void included, of course).

If by 'void pointer 0' you mean '(void *)0', then it is equal to NULL.

When a pointer refers initial address in memory or 0th address in memory it is called null pointer.

Using a NULL macro to make C portableI'll assume that you're asking your question for C type language programming. A NULL pointer is a pointer that's guarnteed to point to nothing. This may be 0 in a UNIX/Linux system or some other address in another system. Using the NULL macro to set/initialize your pointers will make your programs more portable among systems than using something like the 0.#include char *c = 0; // initialize to NULL--not portablechar *p = NULL; // initialize to NULL as defined in stdio is portableAddendumThe code:char *c = 0;actually is portable because the compiler converts 0's used in a pointer context (cast to a pointer) to the machine's representation of a NULL pointer, which may or may not be all 0 bits. The NULL macro itself might be defined as something like 0 or (void *)0, and both definitions are portable. As a corollary, the following code is also portable:if (!c) {// do something}because it is equivalent to:if (c != 0) {// do something}and the 0 above is converted to a NULL pointer because it is being compared with a pointer.

A Null pointer has the value 0. void pointer is a generic pointer introduced by ANSI. Before ANSI, char pointers are used as generic pointer. Generic pointer can hold the address of any data type. Pointers point to a memory address, and data can be stored at that address.

The NULL macro is an implementation-defined macro. It is used to symbolise the zero address (0x0) in C programs and older C++ programs. It is not type safe, but is the conventional method of assigning the zero address to a pointer variable. C++11 introduced the type-safe nullptr data type.

For example, if you write NULl instead of NULL.

An uninitialized pointer is a pointer that has not been assigned a value; it's uninitialized.The value of an uninitialized pointer is undefined (it could be anything).A null pointer is simply a pointer that has a value of NULL.A word of advice: initialize your pointers to NULL.This allows you to see that the pointer is invalid; to check if an uninitialized pointer is valid is quite impossible.Examples:Uninitialized:int *myPointer;Null-pointer:int *myNullPointer = NULL;Dereferencing either of these pointers is something you'll like to avoid.It is very common in a program to check for null pointers; some functions return NULL on failure, for example:struct Foo *result = doSomething();if (result == NULL) {printf("Error: The function doSomething() returned NULL);return;} else {printf("The result: %d", result->bar);}

# define pi 3.17 // tihs is a preprocessing directive macro. "pi" is called macro template " 3.17" is the macro value. means in entire program pi value is 3.17. if we declared like # define pi 0 means pi value is zero means null. so the macro template which carries a null value is called a NULL MACRO.

Pointer is a variable that stores address of a variable . A NULL Pointer a pointer that doesn't point to anything, it is a literal zero .Some people ,notably C++ programmers, prefer to use 0 rather than NULL.

A pointer variable which is declared but not initialized is called a NULL POINTER.ex: int *p;Please don't use the above. A NULL pointer is a specific value assigned to a pointer, just like any other value. NULL is a language-specific designation, and is guaranteed to be comparable to, unlike uninitialized variables, which can have any value.That is:int *a;int *b = NULL;int *c = (int *) malloc(sizeof(char));( a c) is NEVER true.NULL is a reserved word in most high-level languages, and indicates a specific value for assignment. It is commonly used to indicate that something has not yet been assigned a "real" value, or has had its contents deleted. It is an EXPLICIT value, and not just "undefined".In the context of pointers (which, remember, are really memory location addresses), a NULL pointer is one which has NO value, and thus does NOT point to any memory location. The difference between an uninitialized pointer and a NULL pointer is that most common languages do not specify what value an uninitialized pointer has upon creation (many, such as C, are assigned a random value), while a NULL pointer explicitly has NO value (which is the meaning of NULL).Many modern languages and compilers will assign NULL to a pointer upon initialization, but don't count on it. It is sloppy programming to do so, and can lead to many hard-to-find errors.

A null pointer is a pointer which does not point to any valid memory location, and usually contains the binary value "0" to represent this (this is language dependent). The ASCII null character is a character-sized zero value (in ASCII, it is an unsigned byte with a value of 0), and typically represents the end of a string (esp. as in C and C++). A null string is one that is zero characters of usable string data; in a length-based string, this means the length parameter is set to 0, and in an ASCII null-terminated string, means the first character is set to 0.

You haven't assigned the pointer yet, so it's initialized as NULL, or you're trying to assign NULL to the value of the pointer. You have to check if the value is NULL before you use it, or you'll end up with errors just like this.

A null pointer exception in java comes when you are trying to perform any action on an object that isnt initialized/has a value i.e., is a NULL Value Ex: private String s; //declare a string if(s.equals("test")){ //do something.. } You will get a null pointer in the if condition because you are checking a value that is null which is not allowed..

Unitialized pointer may contain any value, so if you should use them, the results are unpredictable; NULL value on the other hands explicitly means that the pointer does not point anywhere. e.g.: size_t ListLen (const List *p) { size_t len= 0; while (p != NULL) { ++len; p= p->Next; } return len; }

Nothing - 0, Zero and null are the same things

Putting it very simply a null value is empty and a not null value contains something.

A NULL pointer has the same size as a non NULL pointer. NULL means that the pointer has been set to the NULL value that is usually zero (0) but the NULL value is at the digression of the compiler manufacture (and may have a value other than zero) so a pointer should always be set to the NULL value and not zero. Current compilers (32 and 64 bit, Intel chip) have a pointer size of 4 (8 bit) bytes. It should be noted that the number of bits in any data type is at the compiler manufactures digression but is heavily influenced by the computer hardware. void *p= NULL; printf ("%d\n", sizeof (p)); or printf ("%d\n", sizeof (void *));

A NULL pointer is a pointer variable that stores the value zero. Although there is a memory address at offset 0, this value is reserved to indicate that the pointer is not pointing at any valid memory. A void pointer is a pointer variable that has no specific type. For instance, int * p; tells the compiler that p will point to a memory location that will be treated as if it were an int type (even if it is a char, or some other type), whereas void * p; tells the compiler that the memory being pointed at has no specific type, and that the type will be determined at runtime by casting the pointer to the appropriate type. This allows the pointer to point to anything, including NULL. An int pointer can also point at anything, but its type is known at runtime and will be treated as such at runtime. The type of a void pointer can only be determined at runtime.

its just an programming error, null definition in Programming is to represent the thing referred to by an uninitialised pointer.

#define NULL ((void *)0) /* defined in <stddef.h> */ const char *mynullvar = NULL;

A NULL in C is a pointer with 0 value, which cannot be a valid address. A null in Oracle is the condition of not having a value, such as a field in a row being null, meaning that it does not have a value. This is not the same as zero - zero and null are two different things. Note, however, that Oracle does not differentiate between a null and a zero length string. This was an error in non-ANSI implementation made many years ago, but it has persisted because fixing it would impact too much "running" code.

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