C Programming - Refresher - Part IV

Embedded C – Part IV
Pre-processing & User defined data types
Team Emertxe

Preprocessor: what ?
● Preprocessor is a powerful tool with raw power
●
Preprocessor is often provided as a separate
tool with the C compilers
●
Preprocessor is a program that processes
the code before it passes through the compiler

Preprocessor:
Functionalities
● Inclusion of header files
● Macro expansion
● Conditional compilation
● Removing comments from the source code
● Processing of trigraph sequence.

Preprocessor:
Built-in Defines
__FILE__ : Represents the current source file name in which it appears.
__LINE__ : Represents the current line number during the preprocessing in
the source file.
__DATE__ : Represents the current date during the preprocessing.
__TIME__ : Represents the current time at that point of preprocessing.
__STDC__ : This constant is defined to be true if the compiler conforms to
ANSI/ISO C standard.
__func__ : Represents the current function name in which it appears

Preprocessor:
Built-in Defines Example
Output

Preprocessor:
Directives
● #include
● #define
● #undef
● #ifdef, #ifndef, #else, #endif
● #error
● #line
● #pragma
● Preprocessing directives are lines in
your program that start with `#'.
● The `#' is followed by an identier that
is the directive name.
For example,
#include <stdio.h>

Directive:
#include
Copy of a specified file included in place of the directive
#include <filename>
● Searches standard library for file
● Use for standard library files
#include "filename"
● Searches current directory, then standard library
● Use for user-defined files
Used for:
● Programs with multiple source files to be compiled together
Header file – has common declarations and definitions
(classes, structures, function prototypes)

Directive:
#define
PI is a Macro
● This is used to create symbolic constants and macros
● When program is preprocessed, all occurrences of symbolic constant
replaced with replacement text
#define identifier replacement-textSyntax
Example #define PI 3.14159

Macros with Arguments
● Operation defined in #define
● A macro without arguments is treated like a symbolic constant
● A macro with arguments has its arguments substituted for replacement text,
when the macro is expanded
● Performs a text substitution – no data type checking
Example #define CIRCLE_AREA( x ) ( PI * ( x ) * ( x ) )
Would cause
area = CIRCLE_AREA( 4 );
To become
area = ( 3.14159 * ( 4 ) * ( 4 ) );

Macros with Arguments
DIY
1. Write a macro to find max of two given nos.
2. Write a macro to set nth bit of given character.

Directive:#undefine,
#ifdef, #ifndef, #else, #endif
#include<stdio.h>
#define ABC 25
#ifdef ABC
#undef ABC
#define ABC 50
#else
#define ABC 100
#endif
int main()
{
printf("%d",ABC);
return 0;
}

Directive:
#error
Syntax #error tokens
●
Tokens are sequences of characters separated by spaces "Error cannot process"
has 3 tokens
● Displays a message including the specified tokens as an error message
● Stops preprocessing and prevents program compilation

Directive:
#line
● Renumbers subsequent code lines, starting with integer value
● File name can be included
● #line 100 "myFile.c"
● Lines are numbered from 100 beginning with next source code file
● Compiler messages will think that the error occurred in "myfile.C"
● Makes errors more meaningful
● Line numbers do not appear in source file

Preprocessor Operators
# (stringization operator)
– Causes a replacement text token to be converted to a string surrounded
by quotes
– The statement
#define HELLO( x ) printf( “Hello, ” #x “n” );
would cause
HELLO( John )
to become
printf( “Hello, ” “John” “n” );
Strings separated by whitespace are concatenated
when using printf

## (concatenation operator)
● Concatenates two tokens
● The statement
#define TOKENCONCAT( x, y ) x ## y
would cause
TOKENCONCAT( O, K )
to become
OK

Example

Advantages &
Disadvantages
Macros are not type-checked
Examples:
int k = max_macro(i, j);
/* works with int */
float max_float = max_macro(10.0, 20.0);
/* also works with float constants */
int k = max_function(i, j);
/* works with int */
float max_float = max_function (10.0, 20.0);
/* does not work - you can pass only integral values */

Advantages &
Disadvantages
Macros have side effects during textual replacement whereas
functions does not have
Examples:
int k = max_macro (i++, j);
/* we are in trouble as i++ is evaluated twice */
/* int k = (i++ > j) ? i++ : j */
int k = max_function (i++, j);
/* no problem, as it is not expanded, but a call to max_function is made */
Macros might result in faster code as textual replacement is done
and no function call overhead is involved.

Structures:
What
What ?
● It is user defined data type.
● It is used to group together different types of variables under the same name.
Example
To create the student record, which consist of
✔ Name
✔ Roll number
✔ Age
✔ Marks etc...

Structures:
why
Why ?
● It helps to construct a complex data type in more meaningful manner.
● Organising the data in a better & efficient way.
Arrays & Structures
Arrays Structures
Collection of similar data types Collections of different data types

Structures:
Declaration
Declaration Example
DIY:
Declare the structure called date, which contains the memebers
day, month, year

Structures:
Declaration of variables
Declaration of structures variables can be done in two ways
● With structure declaration
● Using structure tag
With structure declaration Using structure tag
Declaring the structure variable reserves the
space in memory

Structures:
Initialization of variables
The number, order, type of these variables should be same
as in the structure template
With structure declaration
Using structure tags

Structures:
Initialization of variables
Members of the structures cannot be initialize while defining
the structure
Invalid Initialization

Structures:
Partial Initialization
Example

Structures:
Accessing the members
Format for accessing the members
variable.member
Example
s1.name
s1.age
s1.roll_num
Dot( . ) operator is also called as period or
membership operator

Structures:
Assignment of structure variables
We can assign values of a structure variable to another structure variable, if both
variables are of the same structure type.
Example

Structures:
Storage Allocation & Padding
Let us consider an example,
ch num
Padded bytes
Total : 8 bytes
sizeof() operator can be used to find the total size of this structure
4 bytes

Structures:
Array of structures
All the structures of an array are stored in consecutive
memory location
Example:
S [ 0 ] S [ 1 ] S [ 2 ]
name age roll_
num
marks name age roll_num marks name age roll_num marks
Kenneth 25 43 67.25 John 24 35 76.50 Richie 26 36 80.00
Total : 32 * 3 = 96 bytes of memory

Structures:
Nested structures
Structures within structure Example
Accessing the members within structure
var1.var2.member1 Example: dob.t.day;

Structures:
Pointers to structures
Example
Defining the pointer Accessing the members thru' ptr struct
student *sptr = &s1;
pointer -> member;
sptr->name, sptr->age
Use -> operator to access the members using pointers
syntax
Example

Structures:
Functions & structures
Passing structure members as Arguments
Calling function
Called function

Structures:
Passing structure variables as Arguments
Calling function
Called function

Structures:
Passing pointers to structures as Arguments
Calling function
Called function

Structures:
Returning structure variable from function
Calling function
Called function

Structures:
Returning a pointer to structure from function
Calling function
Called function

Structures:
Self referential structures
A structure that contains pointers to structures of its own type.
Syntax: Example:
struct tag
{
datatype member1;
datatype member2;
:
:
struct tag *ptr1;
struct tag *ptr2;
};
More details will be dealt in Data structures

Unions:
Introduction
●
User defined data type
●
The syntax used for declaration of a union, declaration of variables and
accessing the members are all similar to that of structures, except the keyword
'union' intead of 'struct'
●
The main difference between union & structures is the way the memory is
allocated for the members.
● In structures, each member has its own memory, whereas in the union members
share the same memory locations.
●
Compiler allocates sufficient memory to hold the largest member in the union.

Unions:
Examples
●
Example to compare the memory allocation for the members of the unions &
structures.
structures unions
sizeof(struct sample) = 12 bytes sizeof(union sample) = 4 bytes
Since highest data type among the
members is 'float'

Unions:
Union inside structure
Example
This structure has three members,
1. Name
2. Roll number
3. Union member performance
Union will take only one value at a
time, either a percent rounded of to
whole number or gpa

typedefs:
Introduction
● The purpose of typedef is to form complex types from more-basic machine types
and assign simpler names to such combinations.
● They are most often used when a standard declaration is cumbersome, potentially
confusing, or likely to vary from one implementation to another.
● Under C convention (such as in the C standard library or POSIX), types declared
with typedef end with '_t' (e.g., size_t, time_t). Such type names are reserved by
POSIX for future extensions and should generally be avoided for user defined
types.

typedefs:
Examples
Syntax
typedef data_type new_name;
Identifier
Existing data type
Keyword
Example
typdef unsigned int long ul_t;
Now, ul_t type can be used to declare the variables of the type unsigned int long.

Typedefs & pointers
Examples
typedef int* iptr;
Now, iptr is synonym for the int * or pointer to int.
iptr p, q; // p, q are of type int*
iptr *p; // Here, p is declared as pointer to pointer to int

Typedefs & arrays
Examples
typedef int intarr [ 10 ];
Now, intarr is synonym for the integer array of size 10.
intarr a;
Above example is equivalent to int a [ 10 ];
intarr a [ 5 ];
Above example is equivalent to int a [ 5][ 10 ];

Enums:
Introduction
● Set of named integer constants
Syntax:
enum tag { member1, member2, member3,...};
Example:
enum month { Jan, Feb, Mar, Apr, May, June};
● Internally the compiler treats these enumerators as integer constant.
● Enumerators are automatically assigned integer values begining from 0,1,2,...
● In the above example, Jan-0, Feb-1, Mar-2,...

Bit-fields
Introduction
● A bit field is set up with a structure declaration that labels each field and
determines its width.
Syntax:
Example:

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C Programming - Refresher - Part IV

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