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Using Memory
 

C is designed to be a low-level language that can easily access memory locations and perform memory-related operations.
For instance, the scanf() function places the value entered by the user at the location, or address, of the variable. This is accomplished by using the & symbol.

For Example: 

int num;
printf(“Enter a number: “);

scanf(“%d”, &num);

printf(“%d”, num); 

 

&num is the address of variable num.

A memory address is given as a hexadecimal number. Hexadecimal, or hex, is a base-16 number system that uses digits 0 through 9 and letters A through F (16 characters) to represent a group of four binary digits that can have a value from 0 to 15.
It’s much easier to read a hex number that is 8 characters long for 32 bits of memory than to try to decipher 32 1s and 0s in binary.
The following program displays the memory addresses for variables i and k:

void test(int k);

int main() {
  int i = 0;
   
  printf(“The address of i is %x\n”, &i);
  test(i);
  printf(“The address of i is %x\n”, &i);
  test(i);

  return 0;
}

void test(int k) {
  printf(“The address of k is %x\n”, &k);

 

In the printf statement, %x is the hex format specifier.
Program output varies from run to run, but looks similar to:

The address of i is 846dd754
The address of k is 846dd758
The address of i is 846dd754
The address of k is 846dd758 [/Su_quote] 

NOTE!
The address of a variable stays the same from the time it is declared until the end of its scope.

What is a Pointer?
 

Pointers are very important in C programming because they allow you to easily work with memory locations.
They are fundamental to arrays, strings, and other data structures and algorithms.
A pointer is a variable that contains the address of another variable. In other words, it “points” to the location assigned to a variable and can indirectly access the variable.
Pointers are declared using the * symbol and take the form:

pointer_type *identifier
 

pointer_type is the type of data the pointer will be pointing to. The actual pointer data type is a hexadecimal number, but when declaring a pointer, you must indicate what type of data it will be pointing to.
Asterisk * declares a pointer and should appear next to the identifier used for the pointer variable.
The following program demonstrates variables, pointers, and addresses:

int j = 63;
int *p = NULL;
p = &j;

printf(“The address of j is %x\n”, &j);
printf(“p contains address %x\n”, p);
printf(“The value of j is %d\n”, j);
printf(“p is pointing to the value %d\n”, *p); 

 

There are several things to notice about this program:
• Pointers should be initialized to NULL until they are assigned a valid location.
• Pointers can be assigned the address of a variable using the ampersand & sign.
• To see what a pointer is pointing to, use the * again, as in *p. In this case the * is called the indirection or dereference operator. The process is called dereferencing.

The program output is similar to:

The address of j is ff3652cc
p contains address ff3652cc
The value of j is 63
p is pointing to the value 63
 

NOTE!
Some algorithms use a pointer to a pointer. This type of variable declaration uses **, and can be assigned the address of another pointer, as in: int x = 12; int *p = NULL int **ptr = NULL; p = &x; ptr = &p;

Pointers in Expressions [/su_quote] 

Pointers can be used in expressions just as any variable. Arithmetic operators can be applied to whatever the pointer is pointing to.
For example:

int x = 5;
int y;
int *p = NULL;
p = &x;

y = *p + 2; /* y is assigned 7 */
y += *p;     /* y is assigned 12 */
*p = y;       /* x is assigned 12 */
(*p)++;      /* x is incremented to 13 */

printf(“p is pointing to the value %d\n”, *p); 

 

Note that parentheses are required for the ++ operator to increment the value being pointed to. The same is true when using the — operator.


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