Lesson 11.2. Syntax of C pointers

The ampersand « & »

You probably don't know this, but you have most likely already used pointers:

scanf("%d", &n);

The & (and commercial or ampersand) in front of a variable means that we use its address. The scanf() function receives the address of the variable, which can modify its content to write the user's input.

When you forget the & in a scanf(), the function takes the value of the variable as its address. If the variable contains 0, the function will try to write in the first byte of the memory, which usually leads to a segmentation error.

The example below displays the address of the variable x:

var x;
printf ("%p", &x);

Declaring a pointer

It is possible to declare a pointer, i.e. a variable intended to contain the address of another variable. You just have to add an asterisk in front of the name of the pointer:

int *address1;
char *address2; 

A pointer is always associated with a type. Since the pointer contains only the address of the first byte, the type designation allows the compiler to know the size of the pointed variable. In the following example, the pointer address contains the address of the variable var. We say that var is pointed to by the address pointer or that address points to var :

float var;
float *address = &var;

Access to the pointed variable

The interest of pointers is to be able to access a variable in reading and/or writing by its address. The access to the pointed content is done by adding an asterisk before the pointer :

float var;
float *address = &var;
*address = 5.2;  // Equivalent to var=5.2;

Example

Here are some example of pointer syntaxes:

// Declare a variable
short var=0xABCD;
// Declare a pointer on var
short* address = &var; 

// Display var
printf ("var      : %hX\n", var);

// Displat the value pointed by address (= var)
printf ("*address : %hX\n", *address);

// Display the address of var
printf ("&var     : %p\n", &var);

// Display the pointer address (adress of var)
printf ("address  : %p\n", address);

// Display the pointer address
printf ("&address : %p\n", &address);

Here is a possible output for this program:

var      : ABCD
*address : ABCD
&var     : 0x7ffcda51423a
address  : 0x7ffcda51423a
&address : 0x7ffcda514230

The following image illustrates the representation of the memory for this execution of the program. We can see the two main entities:

• the var variable
• the address pointer

We can see that the content of the pointer is indeed the address of var. The pointer address points to the variable var :

Exercises

Exercise 1

Write a program that

• declares an integer var and initializes it to 18 ;
• declares an integer pointer address ;
• assigns the address of var to the pointer ;
• displays var
• display the variable pointed to by address (*address).

var = 18
*address = 18

#include <stdio.h> int main(void) { // Declares an integer var and initializes it to 18 // COMPLETE HERE // Declares an integer pointer `address` // COMPLETE HERE // Assigns the address of var to the pointer // COMPLETE HERE // Displays var // COMPLETE HERE // Display the variable pointed to by address (`*address`). // COMPLETE HERE return 0; }

Exercise 2

Write a program that

• declares an integer var and initializes it to 27 ;
• declares an integer pointer address ;
• assigns the address of var to the pointer ;
• display the address contained in the pointer;
• add 1 to the pointer ;
• display the address contained in the pointer.

We can see that the pointer has not been increased by one byte, but by 4 bytes, which corresponds to the size of an integer :

Before : 0x7ffe4b0a7e88
After  : 0x7ffe4b0a7e8c

#include <stdio.h> int main(void) { // Declares an integer var and initializes it to 27 // COMPLETE HERE // Declares an integer pointer address // COMPLETE HERE // Assigns the address of `var` to the pointer // COMPLETE HERE // Display the address contained in the pointer // COMPLETE HERE // Add 1 to the pointer // COMPLETE HERE // Display the address contained in the pointer // COMPLETE HERE return 0; }

Quiz

How to get the address of a variable?

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What syntaxes are used to declare a pointer?

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How to display the content of the variable pointed by the pointer ad?

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How to display the address contained in the ad pointer?

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How to display the address of the ad pointer?

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What does the following program display?

int var = 128, *address;
address = &var;
printf ("%d %d", var, *address);

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What does the following program display?

int var = 128, *address;
address = &var;
printf ("%p", address);

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A pointer is associated with a type ...

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See also

• Learn C programming
• Lesson 1.1. History of C programming language
• Lesson 1.2. First program
• Lesson 1.3. Compilation
• Lesson 1.4. Preprocessor directives
• Lesson 1.5. Which compiler to choose?
• Lesson 1.6. Flowchart
• Lesson 2.1. Types and variables
• Lesson 2.2. Integers
• Lesson 2.3. Floating point numbers
• Lesson 2.4. Characters
• Lesson 2.5. Variables initialization
• Lesson 2.6. Ariane flight 501
• Lesson 3.1. Arithmetic operators
• Lesson 3.2. modulo
• Lesson 3.3. Operators and types
• Lesson 3.4. Type casting
• Lesson 3.5. Bitwise operators
• Lesson 3.6. Bitwise operators details
• Lesson 3.7. Shifting operators
• Lesson 3.8. Assignment operators
• Lesson 3.9. Increment and decrement operators
• Lesson 3.10. Comparison operators
• Lesson 3.11. Logical operators
• Lesson 3.12. Operator precedence
• Lesson 4.1. printf
• Lesson 4.2. scanf
• Lesson 4.3. putchar
• Lesson 5.1. Conditional branching (if...else)
• Lesson 5.2. Nested if and indentation
• Lesson 5.3. Checking intervals
• Lesson 5.4. Ternary Operator ?:
• Lesson 5.5. switch..case statement
• Lesson 5.6. Break keyword in switch..case
• Lesson 6.1. do..while loop in C
• Lesson 6.2. While loop in C
• Lesson 6.3. For loop in C
• Lesson 6.4. How to choose the right loop in C ?
• Lesson 6.5. Exercices on loops in C
• Lesson 7.1. Bit masking
• Lesson 7.2. Bit masking, how to clear a bit in C?
• Lesson 7.3. Bit masking, how to set a bit in C?
• Lesson 7.4. Bit masking, how to toggle a bit in C?
• Lesson 7.5. Bit masking, how to check a single bit in C?
• Lesson 7.6. Overview of bit masking
• Lesson 8.1. Basic syntax of C functions
• Lesson 8.2. Calling C functions
• Lesson 8.3. Void keyword
• Lesson 8.4. Return keyword
• Lesson 8.5. Variable scope
• Lesson 8.6. Global variables
• Lesson 8.7. Static variables
• Lesson 8.8. Random numbers in C
• Lesson 8.9. Mathematical functions in C
• Lesson 9.1. Syntax of C arrays
• Lesson 9.2. Array initialization
• Lesson 9.3. Multidimensional arrays
• Lesson 9.4. How C arrays are stored in memory?
• Lesson 9.5. Arrays and functions
• Lesson 9.6. Exercises on C arrays
• Lesson 10.1. Strings in C
• Lesson 10.2. Null terminated string
• Lesson 10.3. The string.h library
• Lesson 10.4. String and functions
• Lesson 11.1. Introduction to pointers in C
• Lesson 11.3. Dynamic memory allocation
• Lesson 11.4. Incrementing pointers
• Lesson 11.5. Pointers as function argument in C
• Lesson 12.1. Introduction to structures in C
• Lesson 12.2. Properties of structures in C
• Lesson 12.3. Structures and pointers
• Lesson 12.4. Structures and functions
• Lesson 13.1. Recursive functions in C
• Lesson 13.2. Depth of recursive functions
• Lesson 13.2. Mutual recursion
• Lesson 14.1. Additional exercises

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