Pointer Recap

Simply put:
 a pointer is a variable that holds a memory address

 This gives power to using C that other languages don't offer.

To Create a pointer:
  dataType *pointerName;

Pointers need to be defined a specific type so it knows
how to interpret the data at the memory address its pointing to.
For example, is it pointing to an int that uses 4 bytes?
or a char that uses only 1 byte?

To debug your program, you can print out the memory address
that a pointer is pointing to using %p:

  int miles;
  int *ptrMiles;
  ptrMiles = &miles;		// Note: * not used when setting ptr's contents
  *ptrMiles = 45;			//   * IS used when setting contents of where ptr points to
  printf( "Location of miles: %p\n", &miles );			// needs &
  printf( "Location of ptrMiles: %p\n", ptrMiles );		// no needs &


Using char array example


#include <stdio.h>

int main( )
{
   char string[10] = "Hi there.";
   char *ptr;
   ptr = string;		// don't need & on string because array is a ptr
   printf( "at position 4 is %c\n", ptr[4] );
   printf( "at position 4 is %c\n", *(ptr+4) );
   return 0;
}


Check for equality

  int x, y;
  int *ptrMiles1;
  int *ptrMiles2;
  ptrMiles1 = &x;
  ptrMiles2 = &y;
  if ( *ptrMiles1 == *ptrMiles2 )
		...


Cannot do

  int *ptrMiles1;
  *ptrMiles1 = 13;		// not pointing to anything yet!


Functions
Without pointer

#include <stdio.h>

void func( int val );
int main( )
{
   int xyz = 5;
   func( xyz );
   printf( "%d\n", xyz );		// doesn't change
   return 0;
}
void func( int val )
{
   val = 22;
}

Functions
With pointer

#include <stdio.h>

void func( int *val );
int main( )
{
   int xyz = 5;
   func( &xyz );
   printf( "%d\n", xyz );		// DOES change
   return 0;
}
void func( int *val )
{
   *val = 22;
}


Arrays are passed by reference (pointer) automatically to functions
Structs are passed by value to functions, so use pointers if you
want the function to modify it.

Access elements of a struct via a pointer variable:

#include <stdio.h>
struct X
{
   int val;
};

int main( )
{
	struct X stuff = { 77 };
	struct X *Y;
	Y = &stuff;
	printf( "%d\n", (*Y).val );
	printf( "%d\n", Y->val );
}