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Chapter 3 Functions
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3.1 Designing Structured Programs

Structured programming is a problem solving
strategy and a programming methodology that
includes the following guidelines:
◦ The program uses only the sequence,
selection, and repetition control structures.
◦ The flow of control in the program should be
as simple as possible.
◦ The construction of a program embodies topdown design.
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3.1 Designing Structured Programs
Involves repeatedly decomposing a
problem into smaller problems
 Eventually leads to a collection of small
problems or tasks each of which can be
easily coded
 The function construct in C is used to
write code for these small, simple
problems.

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3.2 Functions



A C program is made up of one or more functions, one of which
is main( ).
Execution always begins with main( ), no matter where it is
placed in the program. By convention, main( ) is located before
all other functions.
When program control encounters a function name, the
function is called (invoked).
◦ Program control passes to the function.
◦ The function is executed.
◦ Control is passed back to the calling function.
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3.2 Functions
Standard library functions
 Collection of functions for performing useful
operation ( mostly written by computer
vendors )


Functions such as printf and scanf are standard
library functions
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3.3 User Defined Functions

Declarations:
returnType functionName ( parametetList )
{
declarations
statements
}
void printGreeting ( void )
{
printf ("Hello World!");
}
Header
Body
Example
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3.3 User Defined Functions

Declarations:
returnType functionName ( parametetList )

Function
prototype
Functions must be declared before they are
called
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3.4 Standard Library Functions
Collection of functions for performing useful
operation ( mostly written by computer
vendors ).
 Functions such as printf, scanf,and getchar and
are standard library functions.



Programmers can write their own functions.
C function names follow the same naming
rules as C variables.
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3.4 Standard Library Functions

Informs the compiler that there will be a function defined later
that:
returns this type
has this name
takes these arguments
void printMessage (void) ;

Needed because the function call is made before the definition
-- the compiler uses it to see if the call is made properly
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3.4 Standard Library Functions
Passes program control to the function
 Must match the prototype in name, number of
arguments, and types of arguments

void printMessage (void) ;
int main ( ) same name
{
printMessage ( ) ;
return 0 ;
}
no arguments
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3.5 Scope

File scope
◦ Identifier defined outside function, known in all
functions
◦ Used for global variables, function definitions, function
prototypes

Function scope
◦ Can only be referenced inside a function body
◦ Used only for labels (start:, case: , etc.)
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3.5 Scope

Block scope
◦ Identifier declared inside a block
 Block scope begins at declaration, ends at right brace
◦ Used for variables, function parameters (local
variables of function)
◦ Outer blocks "hidden" from inner blocks if there is a
variable with the same name in the inner block

Function prototype scope
◦ Used for identifiers in parameter list
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