Lesson 11.1. Introduction to pointers in C

Definition

In C, a pointer is a special variable that contains a memory address, usually the address of another variable, array or function. Pointers have several advantages:

• they allow to allocate memory dynamically, for example to change the size of an array ;
• functions can modify the content of a variable passed as an argument;
• pointers offer a direct access to memory (under certain conditions);
• thanks to pointers, it becomes possible to share memory between several applications.

RAM (Random Access Memory)

RAM stands for Random Access Memory. It is obviously not a matter of having uncertain access to memory, but of being able to access any section of memory directly. A better translation would be direct access memory as opposed to sequential access memory.

A sequential access memory requires to browse from the beginning of the memory to the desired element. The typical example is tape recording:

To provide direct access to the RAM, the technical solution is to associate an address to each byte of the memory. The addresses are generally represented in their hexadecimal form.

Example

Consider the following code which displays the addresses of variables i, f and s :

int i=0;
float f;
char s[]="Hello";

printf ("Addresse of s: %p\n", &s);
printf ("Addresse of f: %p\n", &f);
printf ("Addresse of i: %p\n", &i);

Note in passing that the format code %p is for displaying a pointer.

Here is an illustration of the memory allocation for this program (note that each execution can produce different addresses):

Address of s: 0x7fff57edbf5e
Address of f: 0x7fff57edbf64
Address of i: 0x7fff57edbf68

At the time of the execution of the program (and not during the compilation), a memory space is allocated for each variable. This memory space will consist of one or more bytes. Each variable will be associated with an address that will allow direct access. Here is a representation of the memory :

We distinguish the allocation of each variable in the memory. The addresses in bold represent the base address (i.e. the address of the pointer).

In this example the variables are grouped together. This strategy (which is not systematic) allows to save memory. An analogy would be the minimization of waste in the cutting of a fabric.

Exercise

Exercise 1

Using the example above, write a program that declares an array of 8 characters and displays the address of each cell in the array:

Address of t[0]: 0x7ffeb0c9bc60
Address of t[1]: 0x7ffeb0c9bc61
Address of t[2]: 0x7ffeb0c9bc62
Address of t[3]: 0x7ffeb0c9bc63
Address of t[4]: 0x7ffeb0c9bc64
Address of t[5]: 0x7ffeb0c9bc65
Address of t[6]: 0x7ffeb0c9bc66
Address of t[7]: 0x7ffeb0c9bc67

#include <stdio.h> int main(void) { int i; // Array of 8 characters // COMPLETE HERE // Display address of each cell // COMPLETE HERE return 0; }

Exercise 2

Using the previous example, write a program that declares an array of 8 variables of type double and displays the address of each cell. The cells of the array must be consecutive, but separated by 8 bytes:

Address of t[0] : 0x7ffed37d4f70
Address of t[1] : 0x7ffed37d4f78
Address of t[2] : 0x7ffed37d4f80
Address of t[3] : 0x7ffed37d4f88
Address of t[4] : 0x7ffed37d4f90
Address of t[5] : 0x7ffed37d4f98
Address of t[6] : 0x7ffed37d4fa0
Address of t[7] : 0x7ffed37d4fa8

#include <stdio.h> int main(void) { int i; // Create an array of 8 doubles // COMPLETE HERE // Display the address of each cell // COMPLETE HERE return 0; }

Quiz

In computer science, what does RAM stand for?

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In computer science, what can be a pointer?

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When is the memory allocated?

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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 11.2. Syntax of C pointers
• 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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