c programming session8

C_ProgrammingPart 8

ENG. KEROLES SHENOUDA

1

Access Wrong Bit on the Registerone of the most famous issue for the embedded programmer

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Functionality issue

Bit-FieldsTHINK FIRST THEN ANSWER

HOW TO USE STRUCTURE ON ACCESS

SPECIFIC BIT ON THE REGISTERS ?

3

Bit-Fields

Unlike some other computer languages, C has a built-in

feature, called a bit-field, that allows you toaccess a single bit. Bit-fields can be useful for a number of reasons, such as:

If storage is limited, you can store several Boolean (true/false)

variables in one byte.

Certain devices transmit status information encoded into one or more bits within a byte

Certain encryption routines need to access the bits within a byte.

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Bit-Fields

A bit-field must be a member of a structure or union. It defines how long, in bits, the field is to be.The general form of a bit-field definition is

type name: length; type is the type of the bit-field, and length is the number of bits in

the field. The type of a bitfield must be int, signed, or unsigned. (C99 also allows a bit-field to be of type _Bool.)

Bit-fields are frequently used when analyzing input from a hardware device.

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Bit-FieldsFor example, the status port of a serial communications adapter might

return a status byte organized like this:

Bit Meaning When Set0 Change in clear-to-send line1 Change in data-set-ready2 Trailing edge detected3 Change in receive line4 Clear-to-send5 Data-set-ready6 Telephone ringing7 Received signal

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struct status_type {unsigned char delta_cts:1;unsigned char delta_dsr:1;unsigned char tr_edge:1;unsigned char delta_rec:1;unsigned char cts:1;unsigned char dsr:1;unsigned char ring:1;unsigned char rec_line:1;

} status;

Bit-Fields

You might use statements like the ones shown here to enable a program to determine when it cansend or receive data:

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status = get_port_status();if(status.cts) printf("clear to send");if(status.dsr) printf("data ready");

Bit Meaning When Set0 Change in clear-to-send line1 Change in data-set-ready2 Trailing edge detected3 Change in receive line4 Clear-to-send5 Data-set-ready6 Telephone ringing7 Received signal


} status;

What is the Output ? 8

What is the Output ? 9

Bit-Fields 10


} status;

status7 6 5 4 3 2 1 0

delta_cts

delta_dsr

tr_edge

delta_rec

cts

dsr

rec_line

Bit-Fields

You do not have to name each bit-field. This makes it easy to reach the bit you want, bypassing unused ones.

For example, if you only care about the cts and dsr bits, you could declare thestatus_type structure like this:

Also, notice that the bits after dsr do not need to be specified if they are not used.

11status

7 6 5 4 3 2 1 0

cts

dsr

Bit-Fields

It is valid to mix normal structure members with bit-fields. For example

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What is the Output ?

Bit-Fields

It is valid to mix normal structure members with bit-fields. For example

13

What is the Output ?

Bit-FieldsBit-fields have certain restrictions

you cannot take the address of a bit-field.

Bit-fields cannot be arrayed.

You cannot know, from machine to machine, whether the fields will run from right to left or from left to right;

have some machine dependencies

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Pointers

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What Are Pointers?

A pointer is a variable that holds a memory

address.

This address is the location of another object(typically another variable) in memory. For example, if one variable contains the address of another

variable, the first variable is said to point to the second.

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Why do we need Pointer?

Simply because it’s there!

It is used in some circumstances in C

Remember this?

scanf(“%d”, &i);

Pointers Pointer Variables

If a variable is going to be a pointer, it must be declared as such. A pointer declaration consists of a base type, an *, and the variable name. The general form for declaring a pointer variable istype *name;

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19


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Pointers Pointer Variables 21


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23What is the Output ?


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Pointer ArithmeticThere are only two arithmetic operations

that you can use on pointers:

addition and subtraction.

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26rule govern pointer arithmetic.

All pointer arithmetic is relative to its base type (assume 2-byte integers)

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Pointer to Array 28

LAB Average of Weights

It is required to calculate the summation weight of 5 boxes. The user should enter the boxes

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Pointers and Arrays

There is a close relationship between pointers and arrays.

Consider this program fragment:char str[80], *p1;p1 = str;Here, p1 has been set to the address of the first array element in str.

To access the fifth element in str, you could write

str[4]or*(p1+4)

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Pointer to Structure

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->

Pointer to Structure

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Pointers AND Functions

Pointers are used efficiently with functions. Using pointers provides two main features:

Fast data transfer, because only pointers are transferred.

Pointers allow the definition of several outputs for the same function.

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Fast Data Transfer Using Pointers

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Fast Data Transfer Using Pointers

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Passing Arrays and Pointers to FunctionsNormal Array Passing

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Passing Arrays and Pointers to FunctionsArray Passing with Pointers

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Method 2 is completely equivelent to Method 1 which means that:

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Finally we can summarize function parameters types 1. Input Parameters (Calling by Value)

The parameter values is completely transmitted to the function. This gives thefunction the ability to read the transmitted data only.2. Input/Output Parameters (Refrence or Pointer)The parameter pointer (refernce) is transmitted only. This gives the function theability to read from and write to the original parameters.3. Output Parameters (Return Value)

The return data of the function is assumed as an output parameter. Normally C does

not provide other Output parameters except the return value.

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The sort function contains the two types of parameters.

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Pointer with Unknown Type (void*)

Programmer can define general pointer without specifying a linked data type.

This type of pointers called (void pointer).

void pointer can not be used

normally to manipulated data, itis required to type cast each data access operation.

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Example: Universal Compare with void Pointers45

Example: Universal Compare with void Pointers46

The function prototype is:

int Compare(void* value1, void* value2, int type)which means it takes any two pointers with uncknowntype, the third parameter informs thetype of the submitted values (1 means integer, 2 means double).

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Pointer Conversionsvoid * pointers

In C, it is permissible to assign a void * pointer to any other type of pointer. It is also permissible to assign any other type of pointer to a void * pointer. A

void * pointer is called a generic pointer.

The void * pointer is used to specify a pointer whose base type is unknown.

The void * type allows a function to specify a parameter that is capable of receiving any type of pointer argument without reporting a type mismatch.

It is also used to refer to raw memory (such as that returned by themalloc( ) function described later in this chapter)

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Multiple IndirectionPointer to Pointer

You can have a pointer point to another pointer that points to the target value.

This situation is called

multiple indirection, or pointers to pointers

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Using Pointer to Pointer 50

Using Pointer to Pointer 51

NULL and Unassigned Pointers

If the pointer is unassigned it will contain an invalid address, it is unsafe to use an unassigned pointer, normally the program will crash. Fllowingprogram will crash becausethe (pX) pointer is not pointed to a valid address, it contain a memory gurbage

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NULL and Unassigned Pointers

To avoid using unassigned pointers, all pointers must hold a valid address, if not it must holda zero value. Sometimes zero value called (NULL). Above program may be fixed as shownbellow:

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Labs

54

An Illustration

int i = 5, j = 10;

int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;

Data Table

Name Type Description Value

i int integer variable 5

j int integer variable 10

An Illustration

int i = 5, j = 10;

int *ptr; /* declare a pointer-to-integer variable */

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;

Data Table




ptr int * integer pointer variable

An Illustration

int i = 5, j = 10;

int *ptr;

int **pptr; /* declare a pointer-to-pointer-to-integer variable */

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;

Data Table




ptr int * integer pointer variable

pptr int ** integer pointer pointer variable

Double Indirection

An Illustration

int i = 5, j = 10;

int *ptr;

int **pptr;

ptr = &i; /* store address-of i to ptr */

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;

Data Table




ptr int * integer pointer variable address of i

pptr int ** integer pointer pointer variable

*ptr int de-reference of ptr 5

An Illustration

int i = 5, j = 10;

int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr; /* store address-of ptr to pptr */

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;

Data Table





pptr int ** integer pointer pointer variable address of ptr

*pptr int * de-reference of pptr value of ptr

(address of i)

An Illustration

int i = 5, j = 10;

int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;

Data Table







An Illustration

int i = 5, j = 10;

int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;

Data Table






**pptr int de-reference of de-reference of pptr

7

An Illustration

int i = 5, j = 10;

int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;

Data Table




ptr int * integer pointer variable address of j



An Illustration

int i = 5, j = 10;

int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;

Data Table




ptr int * integer pointer variable address of j


**pptr int de-reference of de-reference of pptr

9

An Illustration

int i = 5, j = 10;

int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;

Data Table






*pptr int * de-reference of pptr value of ptr

(address of i)

An Illustration

int i = 5, j = 10;

int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;

Data Table


i int integer variable -2




*ptr int de-reference of ptr -2

Pointer Arithmetic

What’s ptr + 1?

The next memory location!

What’s ptr - 1?

The previous memory location!

What’s ptr * 2 and ptr / 2?

Invalid operations!!!

Pointer Arithmetic and Array

float a[4];

float *ptr;

ptr = &(a[2]);

*ptr = 3.14;

ptr++;

*ptr = 9.0;

ptr = ptr - 3;

*ptr = 6.0;

ptr += 2;

*ptr = 7.0;

Data Table


a[0] float float array element (variable) ?




ptr float * float pointer variable

*ptr float de-reference of float pointer variable

?


float a[4];

float *ptr;

ptr = &(a[2]);

*ptr = 3.14;

ptr++;

*ptr = 9.0;

ptr = ptr - 3;

*ptr = 6.0;

ptr += 2;

*ptr = 7.0;

Data Table






ptr float * float pointer variable address of a[2]


?


float a[4];

float *ptr;

ptr = &(a[2]);

*ptr = 3.14;

ptr++;

*ptr = 9.0;

ptr = ptr - 3;

*ptr = 6.0;

ptr += 2;

*ptr = 7.0;

Data Table




a[2] float float array element (variable) 3.14




3.14


float a[4];

float *ptr;

ptr = &(a[2]);

*ptr = 3.14;

ptr++;

*ptr = 9.0;

ptr = ptr - 3;

*ptr = 6.0;

ptr += 2;

*ptr = 7.0;

Data Table








?


float a[4];

float *ptr;

ptr = &(a[2]);

*ptr = 3.14;

ptr++;

*ptr = 9.0;

ptr = ptr - 3;

*ptr = 6.0;

ptr += 2;

*ptr = 7.0;

Data Table








9.0


float a[4];

float *ptr;

ptr = &(a[2]);

*ptr = 3.14;

ptr++;

*ptr = 9.0;

ptr = ptr - 3;

*ptr = 6.0;

ptr += 2;

*ptr = 7.0;

Data Table








?


float a[4];

float *ptr;

ptr = &(a[2]);

*ptr = 3.14;

ptr++;

*ptr = 9.0;

ptr = ptr - 3;

*ptr = 6.0;

ptr += 2;

*ptr = 7.0;

Data Table








6.0


float a[4];

float *ptr;

ptr = &(a[2]);

*ptr = 3.14;

ptr++;

*ptr = 9.0;

ptr = ptr - 3;

*ptr = 6.0;

ptr += 2;

*ptr = 7.0;

Data Table








3.14


float a[4];

float *ptr;

ptr = &(a[2]);

*ptr = 3.14;

ptr++;

*ptr = 9.0;

ptr = ptr - 3;

*ptr = 6.0;

ptr += 2;

*ptr = 7.0;

Data Table








7.0

Pointer to Function

powerful feature of C is the function pointer.

A function has a physical location in memory that can be assigned to a pointer.

This address is the entry point of the functionand it is the address used when the function is called.

Once a pointer points to a function, thefunction can be called through that pointer.

Function pointers also allow functions to be passed as arguments to other functions

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Pointers Tricks

80

How to Read C complex pointer

double to integer

Modularity

Pointer with Constant

How to Read C complex pointer expression

Operator Precedence Associative

(),[] 1 Left to Right

*,Identifier 2 Right to Left

Data Type 3 –

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Before We Learn How to Read Complex Array we should

first know precedence and associative .

•Priority : Operator precedence describes the order in

which C reads expressions

•order : Order operators of equal precedence in an

expression are applied

Before Reading Article You Should know Precedence

and Associative Table


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char (* ptr)[5]

Step 1 :

•Observe Above Precedence Table

•[] and () have Same Priority

•Using Associativity , Highest Priority Element is decided

•Associativity is from Left to Right First Priority is Given to “()”

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Step 2 :

•Between Pair of Brackets again we have to decide which one has highest priority ? ‘*’ or ‘ptr’ ?

•* and Identifier ptr have Same Priority

•Associativity is from Right to Left First Priority is Given to “ptr”

http://4.bp.blogspot.com/_ynkoLGkpwXY/TFweojDVchI/AAAAAAAAAl8/z0GWHe-X_dI/s1600/pointer_how_to_2.bmp




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Read it as –

ptr is pointer to a one dimensional array having

size five which can store data of type char

How to Read C complex pointer expressionexamples

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88


89


Test Your self

90

Pointers Tricks

91


Pointers on Embedded C

Modularity


Pointers on Embedded C 92

93What isthe Output ?

94What isthe Output ?

Pointers Tricks

95



Modularity


Modularity 96

For example “UART” (send data)

Send_data(u16 data)

{

USART_Init ();

USART_Trans_Int_Enable();

USART_Trans_Enable();

USART_Transmit(u16 data);

}

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USART_Init (){…}

USART_Trans_Int_Enable (){…}USART_Trans_Enable (){…}

USART_Transmit(u16 data);{…}

UART


Send_data(u16 data)

{

USART_Init ();




}

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USART_Init (){…}



UARTSend dataByte Byte

Once the UART sent the Data,

The TX interrupt will insert and

the CPU will call ISR(USART_TXC_vect){ …….. }


Send_data(u16 data)

{

USART_Init ();




}

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USART_Init (){…}




Once the UART sent the Data,

The TX interrupt will insert and

the CPU will call ISR(USART_TXC_vect){ …….. }

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void (*Ptr_To_Trans_Int) (void);

extern void USART_callback_Trans_Int(void (*Ptr_to_Func)(void))

{

Ptr_To_Trans_Int = Ptr_to_Func;

}

USART_Init (){…}

ISR(USART_TXC_vect){ …….. }

ISR(USART_TXC_vect)

{

(*Ptr_To_Trans_Int)();

}


Pointers Tricks

101



Modularity


Variable Quantifiers - const

const is used with a datatype declaration or definition to specify an unchanging value

const objects may not be changed

102

Variable Quantifiers - const

const is used with a datatype declaration or definition to specify an unchanging value

const objects may not be changed

103

Constant with pointer 104

Constant with pointer 105

Complete the table yes/no … 106

107

int* ptr = &a;

int* const ptr = &a;int const *ptr = &a;

const int *ptr = &a;


Try Now

References 111

C The Complete Reference 4th Ed Herbert Schildt

A Tutorial on Data Representation

std::printf, std::fprintf, std::sprintf, std::snprintf…..

C Programming for Engineers, Dr. Mohamed Sobh

C programming expert.

fresh2refresh.com/c-programming

C programming Interview questions and answers

C – Preprocessor directives

Constant Pointers and Pointers to Constant A Subtle Difference in C Programming

https://www3.ntu.edu.sg/home/ehchua/programming/java/datarepresentation.html

http://en.cppreference.com/w/cpp/io/c/fprintf

https://drive.google.com/file/d/0B7kUsgpvTWFLaEZDWlZEZm0xS0U/view

http://www.cprogrammingexpert.com/

http://fresh2refresh.com/c-programming/

http://www.cquestions.com/

http://fresh2refresh.com/c-programming/c-preprocessor-directives/

http://opensourceforu.com/2015/01/constant-pointers-and-pointers-to-constant-a-subtle-difference-in-c-programming/

c programming session8

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