How to Create a Memory Pointer in C

C makes extensive use of memory pointers for handling arrays, function calls, data structures and much more. Knowing how to work directly with pointers is key to efficient C coding.

Difficulty: Moderately Easy

Instructions

Create Memory Pointer Variables

1. Understand that memory pointer variables always point to data of a particular type. For instance, a pointer to an int is different from a pointer to a char. However, C will not stop you from freely mixing them up. Do so only if you're sure you know what you're doing.

2. Create a memory pointer variable by using the syntax you'd use to create a variable of the desired type, but with an asterisk (*) before the variable name, like this:
   

   int *x;

3. Consider NULL. Pointers can always be NULL (0), and this typically is used to refer to a pointer that has not yet been set to point anywhere.

Reference and Dereference

4. Get to know Referencing. Referencing refers to the process of finding the pointer to an existing variable. In C, the referencing operator is the ampersand (&). For instance:
   

   int color = 5;
   int *pointer_to_color;
   pointer_to_color = &color;

5. Utilize Dereferencing. Dereferencing is the process of following a pointer to its value, the opposite of referencing. In C the asterisk (*) is used for dereferencing, as follows:
   

   printf("Color is %d\n", *pointer_to_color); /* prints 5 */

Use Memory Pointer Variables

6. Pass in a pointer to the variable when you need a function to be able to change a variable, instead of passing the variable's value. This lets the function use dereferencing to change the value:
   

   void convert_color_to_RGB(int color, int *red, int *green, int *blue) {
    *red = redpart(color);
    *green = greenpart(color);
    *blue = bluepart(color);
   }
   convert_color_to_RGB(15, &myred, &mygreen, &myblue);
   

7. Work around C's limits. Whenever you want to pass an array or structure into a function, you must pass a pointer instead, because C only permits single data types to be passed to functions:
   

   int subtotal(int *scores, int howmany) {
    int total = 0, i;
    for (i = 0; i < howmany; i++) total += scores[i];
    return total;
   }

8. Create a loop. Since strings are actually arrays of characters, you can create a pointer to a character to loop through a string:
   

   void replace_character(char *s, char from, char to) {
    char *cp;
    for (cp = s; cp && *cp; cp++) if (*cp == from) *cp = to;
   }
   
   							

9. Understand the way C views arrays. Arrays are handled by C as pointers, using pointer arithmetic. C will automatically multiply what you add to a pointer by the size of elements it points to. That means
   

   scores[5] = 17;

   is exactly the same as
   

   *(scores + 5) = 17;

   . You can use pointers as a shorthand for array dereferences. For instance,
   

   *scores = 17;

   always refers to the 0th element of the array.

Tips & Warnings

• In C, there is a fine line between memory pointers and arrays. Essentially, an array is just a memory pointer pointing to a block of memory that gets automatically allocated.
• Dereferencing a null pointer is a fatal run-time error. Even if you've coded to ensure a pointer is never null by the time of the dereference, you should code defensively to avoid the error.
• The actual value of a pointer is unimportant, because the only purpose of a pointer is to dereference it. However, for debugging purposes, you can print the value of a pointer using the formatting code %p. It's a bad idea to attribute much significance to it; what it means depends on the operating system's internal workings.