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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);

}

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