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Basics of C

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The C Language

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Currently, the most commonly-used language for embedded systems
“High-level assembly”
Very portable: compilers exist for virtually every processor
Easy-to-understand compilation
Produces efficient code

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C History

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Developed between 1969 and 1973 along with Unix
Due mostly to Dennis Ritchie
Designed for systems programming

Operating systems
Utility programs
Compilers

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Hello World in C

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#include <stdio.h>

int main()

{

printf(“Hello, world!\n”);

}

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Hello World in C

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#include <stdio.h>

int main()

{

printf(“Hello, world!\n”);

}

Program mostly a collection of functions

“main” function special: the entry point

“int” qualifier indicates function returns an integer

I/O performed by a library function: not included in the language

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6

Name	Description	Size*	Range*
char	Character or small integer	1 byte	signed: -128 to 127�unsigned: 0 to 255
short int (short)	Short integer	2 bytes	signed: -32768 to 32767�unsigned: 0 to 65535
int	Integer	4 bytes	signed: -2147483648 to 2147483647�unsigned: 0 to 4294967295
long int (long)	Long integer	4 bytes	signed: -2147483648 to 2147483647�unsigned: 0 to 4294967295
float	Floating point number	4 bytes	3.4e +/- 38 (7 digits)
double	Double precision floating point number	8 bytes	1.7e +/- 308 (15 digits)
long double	Long double precision floating point number	8 bytes	1.7e +/- 308 (15 digits)

Data types

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Variable types

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Local variable

Local variables are declared within the body of a function, and can only be used within that function.

Static variable

Another class of local variable is the static type. It is specified by the keyword static in the variable declaration.

The most striking difference from a non-static local variable is, a static variable is not destroyed on exit from the function.

Global variable

A global variable declaration looks normal, but is located outside any of the program's functions. So it is accessible to all functions.

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Variables

A variable is a name that represents one or more memory locations used to hold program data
A variable may be thought of as a container that can hold data used in a program

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int myVariable;

myVariable = 5;

myVariable

5

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An example

int global = 10; //global variable

int func (int x)

{

static int stat_var; //static local variable

int temp; //(normal) local variable

int name[50]; //(normal) local variable

……

}

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Variables

Variables are names for storage locations in memory

10

41

5.74532370373175

× 10^-14

0

15

Data Memory (RAM)

int warp_factor;

float length;

char first_letter;

‘A’

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Variables

Variable declarations consist of a unique identifier (name)…

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char first_letter;

int warp_factor;

float length;

41

5.74532370373175

× 10^-14

0

15

Data Memory (RAM)

‘A’

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Variables

…and a data type

Determines size and how values are interpreted

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char first_letter;

float length;

int warp_factor;

41

5.74532370373175

× 10^-44

0

15

Data Memory (RAM)

‘A’

The second part of a variable declaration is its data type. Data types are used to determine how much space is required for a particular variable (how big or small can the variable be?), as well as how the value stored there is to be interpreted. For example, is the value 41 simply the integer value 41? Is it the ASCII representation of the character ‘A’? Or, is it the IEEE-754 representation of a floating point number? All of this must be known for the compiler to be able to handle variable values appropriately. Note that on this slide, all the values shown are represented by the zero padded number 41 (decimal) or 29 (hexadecimal) in each of these memory locations:

The int is 0029h = 41₁₀

The char is 29h = ‘A’ (treated as an 8-bit integer = 41₁₀)

The float is 00000029h = 5.74532370373175e-44 (in IEEE-754 floating point format)

So the type not only determines how much space a variable will occupy in memory, it also affects how it is encoded in memory and interpreted in the program.

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Identifiers

Names given to program variables
Valid characters in identifiers:

Case sensitive!
Only first 31 characters significant

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I

d

e

n

t

i

f

i

e

r

First Character

‘_’ (underscore)

‘A’ to ‘Z’

‘a’ to ‘z’

Remaining Characters

‘_’ (underscore)

‘A’ to ‘Z’

‘a’ to ‘z’

‘0’ to ‘9’

Identifiers in C must be strings of characters from the valid C character set which includes all the letters of the English alphabet (both upper and lower case), the numbers 0 through 9 and the underscore. The first character of an identifier is further restricted in that numbers may not be used. Note that under no circumstances may an identifier contain a space.

All identifiers are case sensitive, so you can have two identical identifiers with the exception of the case of the first character, and they will be considered to completely different identifiers.

Also, an identifier may be any length, but only the first 31 characters are significant. So, if you have a 32 character identifiers where all characters but the 32^nd are the same, then they will be considered the same identifier. This is true according to the ANSI C standard, but most modern compilers provide many more significant characters than 31.

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How to Declare a Variable?

A variable must be declared before it can be used
The compiler needs to know how much space to allocate and how the values should be handled

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Syntax

type identifier₁, identifier₂,…,identifier_n;

Examples

int x, y, z;

float warpFactor;

char text_buffer;

unsigned index;

Variables

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�How to Declare a Variable?

Variables may be declared in a few ways:

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Syntax

type identifier;

type identifier = InitialValue;

type identifier₁, identifier₂, identifier₃;

type identifier₁ = Value₁, identifier₂ = Value₂;

One declaration on a line

One declaration on a line with an initial value

Multiple declarations of the same type on a line

Multiple declarations of the same type on a line with initial values

Variables

MAIN IDEA

There are several ways to declare variables
Variables may be initialized when they are declared
Multiple variables may be declared on the same line

Here we can see an expanded list of the various ways you can declare variables.

The first one we saw as an example on the previous slide. It simply declares one variable.

The second one though is new. Here we are declaring a single variable as in the first one, but this time we are providing an initialization value. When our code begins execution, this variable will automatically be assigned the value that you specify.

The third is what we saw on the previous slide where multiple variables of the same type are declared on a single line.

And finally on the last line, we declare multiple variables of the same type all with their own initialization values.

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�How to Declare a Variable

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Examples

int x;

int y = 12;

int a, b, c;

long int myVar = 0x12345678;

long z;

char first = 'a', second, third = 'c';

float big_number = 6.02e+23;

Variables

Here are a few examples of the different ways variables can be declared.

The first one shows the full syntax for creating an unsigned int.

The second one also declares an unsigned int, but uses the shorter notation. It also declares that the variable should be automatically initialized with a value of 12 by the compiler.

The third one declares three ints.

The fourth declares a long int using the full syntax, and initializes it with a value of 0x12345678

The fifth one declares a long variable by itself.

The sixth one declares three char variables and initializes two of them (the one called "second" will be initialized to 0 by the compiler)

The last one declares a float with an initial value of 6.02 X 10²³

On a side note, it is customary to use "camel case" when naming variables that include more than one word as part of their name. Traditionally, variables start with a lower case letter, but subsequent words in the name start with an upper case letter. This makes it easier to distinguish individual words in the name since spaces aren't allowed. Some programmers hate camel case, and use underscore characters in the name to separate words. They are all acceptable, so it is purely a matter of preference which one you decide to use. However, it is considered good programming practice to pick one style and stick with it.

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A Simple C Program

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#include <stdio.h>

int main(void)

{

float radius, area, PI;

//Calculate area of circle

radius = 12.0;

PI = 3.1416;

area = PI * radius * radius;

printf("Area = %f", area);

}

Header File

Function

Variable Declarations

Comment

Preprocessor Directives

To better understand the details we will be seeing in subsequent slides, it may be helpful to take a look at the big picture first. This is a very simple, yet complete C program. It doesn't use every possible feature of C, but it does illustrate some of the bare minimum of items required for a complete program.

The one item that is 100% required in every C program is a function called "main". It is possible to have any number of other functions in a C program, but they will all be called from within the main function. This is where your code will begin to execute immediately after the various system initialization and housekeeping routines get things setup as dictated by your compiler. Notice the curly braces around the function code below the header. Think of these as meaning "begin" and "end". Curly braces are used throughout C to identify blocks of code that belong together as a single unit.

At the very top of the program is a line that you will see in virtually every C program (though not necessarily in embedded C programs), and that is the inclusion of an external header file that contains declarations of various functions that are part of the standard C library and are used to implement common I/O functions that are not an inherent part of the language itself. We will see much more on the subject of header files later on.

Immediately below that, we use the #define directive to define a text substitution label called PI. When we compile our code, a special program known as the preprocessor will go over the code first and replace each instance of "PI" with the text "3.14159". When the compiler runs later, it will be correctly interpreted as a floating point number. The #define directive can play many roles in a C program, and we will see more detail on how it works later on.

Inside the main function itself, we declare two floating point variables so that we may use them later on in the function. In C, you must declare a variable before it may be used. Notice that the two variables are associated with a data type "float". This is an important point that we will cover in detail shortly.

Below the variables is a comment line. Comments are ignored by the compiler and may contain virtually any text. The style of comment shown here starting with a double slash was actually borrowed from the C++ language. We'll see the traditional C style comments later on in the presentation. In most cases, the C++ style comments shown here are far more useful since they start with the double slash and go to the end of the line.

After the comment, we have some statements that calculate the area of a circle with a radius of 12 and outputs the results to a text window.

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Variables and Data Types

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Variable

Declarations

Data Types

Variables in use

#include <stdio.h>

int main(void)

{

float radius, area, PI;

//Calculate area of circle

PI = 3.1416;

radius = 12.0;

area = PI * radius * radius;

printf("Area = %f", area);

}

A Simple C Program

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printf()

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The printf() function can be instructed to print integers, floats and string properly.

The general syntax is

printf( “format”, variables);

An example

int stud_id = 5200;

char name[20] = “Mike”; // array discussed in LO4

printf(“%s ‘s ID is %d \n”, name, stud_id);

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printf()

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Format Identifiers

%d decimal integers

%x hex integer

%c character

%f float number

%lf double number

%s string

%p pointer

%e decimal exponent

How to specify display space for a variable?

printf(“The student id is %5d \n”, stud_id);

The value of stud_id will occupy 5 characters space in the print-out.

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Why “\n”

It introduces a new line on the terminal screen.

\a	alert (bell) character	\\	backslash
\b	backspace	\?	question mark
\f	formfeed	\’	single quote
\n	newline	\”	double quote

\t	horizontal tab
\v	vertical tab

escape sequence

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printf()

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scanf()

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Description:

The C library function

int scanf(const char *format, ...) reads formatted input from keyboard. The function returns how many values where read. If scanf returns a 0 it means nothing was read.

Declaration:

Following is the declaration for scanf() function.

int scanf(const char *format, ...)

Example:

int a, result;

result = scanf("%d", &a);

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scanf()

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Example: The following example shows the usage of scanf() function to read strings.

Note: in scanf, we do not use & with array names

#include <stdio.h>

int main()

{

char str1[20], str2[30];

printf("Enter name: ");

scanf("%s", str1);

printf("Enter your website name: ");

scanf("%s", str2);

printf("Entered Name: %s\n", str1);

printf("Entered Website:%s", str2);

}

Enter name: admin

Enter your website name: www.tutorialspoint.com

Entered Name: admin

Entered Website: www.tutorialspoint.com

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scanf()

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What happens in this program? An integer called pin is defined. A prompt to enter in a number is then printed with the first printf statement. The scanf routine, which accepts the response, has a control string and an address list.

In the control string, the format specifier %d shows what data type is expected.

The &pin argument specifies the memory location of the variable the input will be placed in. After the scanf routine completes, the variable pin will be initialized with the input integer. This is confirmed with the second printf statement. The & character has a very special meaning in C. It is the address operator.

Is a function in C which allows the programmer to accept input from a keyboard. The following program illustrates the use of this function.

#include <stdio.h>

main() {

int pin;

printf("Please type in your PIN\n");

scanf("%d",&pin);

printf("Your access code is %d\n",pin);

}

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Negative

- (unary)

Subtraction

-

Positive

+ (unary)

Modulo

%

Addition

+

NOTE - An int divided by an int returns an int:

10/3 = 3

Use modulo to get the remainder:

10%3 = 1

Multiplication

*

Division

/

Operator

Result

Operation

Example

-x

x - y

+x

x % y

x + y

x * y

x / y

Negative value of x

Difference of x and y

Value of x

Remainder of x divided by y

Sum of x and y

Product of x and y

Quotient of x and y

Arithmetic Operations

Here are a few examples of the different ways variables can be declared.

The first one shows the full syntax for creating an unsigned int.

The second one also declares an unsigned int, but uses the shorter notation. It also declares that the variable should be automatically initialized with a value of 12 by the compiler.

The third one declares three ints.

The fourth declares a long int using the full syntax, and initializes it with a value of 0x12345678

The fifth one declares a long variable by itself.

The sixth one declares three char variables and initializes two of them (the one called "second" will be initialized to 0 by the compiler)

The last one declares a float with an initial value of 6.02 X 10²³

On a side note, it is customary to use "camel case" when naming variables that include more than one word as part of their name. Traditionally, variables start with a lower case letter, but subsequent words in the name start with an upper case letter. This makes it easier to distinguish individual words in the name since spaces aren't allowed. Some programmers hate camel case, and use underscore characters in the name to separate words. They are all acceptable, so it is purely a matter of preference which one you decide to use. However, it is considered good programming practice to pick one style and stick with it.

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Arithmetic Assignment Operators

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Operator

Result (FALSE = 0, TRUE ≠ 0)

Operation

Example

Equal to

==

Not equal to

!=

Greater than

>

Greater than or equal to

>=

Less than

<

Less than or

equal to

<=

x == y

x != y

x > y

x >= y

x < y

x <= y

True if x equal to y, else false

True if x not equal to y, else false

True if x greater than y, else

false

True if x greater than or equal to y, else false

True if x less than y, else false

True if x less than or equal to y, else false

Relational Operations

Here are a few examples of the different ways variables can be declared.

The first one shows the full syntax for creating an unsigned int.

The second one also declares an unsigned int, but uses the shorter notation. It also declares that the variable should be automatically initialized with a value of 12 by the compiler.

The third one declares three ints.

The fourth declares a long int using the full syntax, and initializes it with a value of 0x12345678

The fifth one declares a long variable by itself.

The sixth one declares three char variables and initializes two of them (the one called "second" will be initialized to 0 by the compiler)

The last one declares a float with an initial value of 6.02 X 10²³

On a side note, it is customary to use "camel case" when naming variables that include more than one word as part of their name. Traditionally, variables start with a lower case letter, but subsequent words in the name start with an upper case letter. This makes it easier to distinguish individual words in the name since spaces aren't allowed. Some programmers hate camel case, and use underscore characters in the name to separate words. They are all acceptable, so it is purely a matter of preference which one you decide to use. However, it is considered good programming practice to pick one style and stick with it.

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Increment and Decrement Operators

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awkward	easy	easiest
x = x+1;	x += 1	x++
x = x-1;	x -= 1	x--

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Example

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Arithmetic operators

int i = 10;

int j = 15;

int add = i + j; //25

int diff = j – i; //5

int product = i * j; // 150

int quotient = j / i; // 1

int residual = j % i; // 5

i++; //Increase by 1

i--; //Decrease by 1

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Example

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Comparing them

int i = 10;

int j = 15;

float k = 15.0;

j / i = ?

j % i = ?

k / i = ?

k % i = ?

The Answer

j / i = 1;

j % i = 5;

k / i = 1.5;

k % i It is illegal.

Note: For %, the operands can only be integers.

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Logical Operations

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What is “true” and “false” in C

In C, there is no specific data type to represent “true” and “false”. C uses value “0” to represent “false”, and uses non-zero value to stand for “true”.

Logical Operators

A && B => A and B

A || B => A or B

A == B => Is A equal to B?

A != B => Is A not equal to B?

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Logical Operations

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A > B => Is A greater than B?

A >= B => Is A greater than or equal to B?

A < B => Is A less than B?

A <= B => Is A less than or equal to B?

Don’t be confused

&& and || have different meanings from & and |.

& and | are bitwise operators.

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No operator for exponent in C

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OPERATOR PRECEDENCE

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Precedence and Associativity of C Operators
Symbol	Type of Operation	Associativity
[ ] ( ) . –> postfix ++ and postfix ––	Expression	Left to right
prefix ++ and prefix –– sizeof & * + – ~ !	Unary	Right to left
typecasts	Unary	Right to left
* / %	Multiplicative	Left to right
+ –	Additive	Left to right
<< >>	Bitwise shift	Left to right
< > <= >=	Relational	Left to right
== !=	Equality	Left to right
&	Bitwise-AND	Left to right
^	Bitwise-exclusive-OR	Left to right
\|	Bitwise-inclusive-OR	Left to right
&&	Logical-AND	Left to right
\|\|	Logical-OR	Left to right
? :	Conditional-expression	Right to left
= *= /= %= += –= <<= >>= &= ^= \|=	Simple and compound assignment	Right to left
,	Sequential evaluation	Left to right

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Some practices

Compute the value of the following logical expressions?

int i = 10; int j = 15; int k = 15; int m = 0;

if( i < j && j < k) =>

if( i != j || k < j) =>

if( j<= k || i > k) =>

if( j == k && m) =>

if(i) =>

if(m || j && i ) =>

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Answers

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int i = 10; int j = 15; int k = 15; int m = 0;

if( i < j && j < k) => false

if( i != j || k < j) => true

if( j<= k || i > k) => true

if( j == k && m) => false

if(i) => true

if(m || j && i ) => true