Functions in C

Functions
 A function is a self contained block of code that performs a
particular task.
 Any C program can be seen as a collection/group of these
functions.
 A functions takes some data as input, perform some
operation on that data and then return a value.
 Any C program must contain at least one function, which is
main().
 There is no limit on the number of functions that might be
present in a C program.

 For ex.
main()
{
message();
printf(“I am in main”);
}
message()
{
printf(“I am in message”);
}

Function Prototype
 All Identifiers in C must be declared before they are used. This is true
for functions as well as variables.
 For functions, the declarations needs to be done before the first call of
the function.
 A function declaration specifies the name, return type, and arguments
of a function. This is also called the function prototype.
 To be a prototype, a function declaration must establish types for the
function’s arguments.
 Having the prototype available before the first use of the function
allows the compiler to check that the correct number and types of
arguments are used in the function call.

 The prototype has the same syntax as the function
definition, except that it is terminated by a semicolon
following the closing parenthesis and therefore has no
body.

 Although, functions that return int values do not require
prototypes, prototypes are recommended.

Function Definition
 General form of any function definition is:
return-type function-name(argument declarations)
{
declarations and statements
}
 Return-type refers to the data type of the value being
returned from the function. If the return type is omitted,
int is assumed.
 The values provided to a function for processing are the
arguments.
 The set of statements between the braces is called as the
function body.

Arguments – Call by Value
 In C, all functions are passed “by value” by default.
 This means that the called function is given the values of its
arguments in temporary variables rather than the originals.
 For ex. main()
{
int a=4,b=5;
sum(a,b);
printf(“Sum = %d”,a+b);
}
sum(int a,int b)
{
a++;
b++;
}

Arguments – Call by Reference
 When necessary, it is possible to arrange a function which can modify a
variable in a calling routine.
 The caller must provide the address of the variable to be set (generally, a
pointer to a variable), and the called function must declare the parameter to
be a pointer and access the variable indirectly through it.
 For ex: main()
{
int a=4,b=5;
sum(&a,&b);
}
sum(int *a,int *b)
{
(*a)++;
(*b)++;
printf(“Sum = %d”,(*a)+(*b));
}

Recursion
 Recursion defines a function in terms of itself.
 It is a programming technique in which a function calls
itself.
 A recursive function calls itself repeatedly, with different
argument values each time.
 Some argument values cause the recursive method to
return without calling itself. This is the base case.
 Either omitting the base case or writing the recursion step
incorrectly will cause infinite recursion (stack overflow
error).

 Recursion to find the factorial of any number:
int factorial(int x)
{
if(x<=1)
return 1;
else
return(x*factorial(x-1));
}

External Variables
 See the below example:
main()
{
extern int a;
printf(“Value of a = %d”,a);
}
int a=5;

Output: Value of a = 5

Scope Rules
 Example:
int num1 = 100; //Global Scope
static int num2 = 200; //File Scope
int main()
{
int num3 = 300; //Local Scope
if(num2>num1)
{
int i=0; //Block Scope
printf(“Value of i = %dn”,i++);
}
printf(“Value of num1 = %dn”,num1);
}

Static Variables
 Example:
static int min = 10;
int setmax()
{
static int max = 100;
return ++max;
}
main()
{
min++;
printf(“Value of max = %d”, setmax());
printf(“Value of max = %d”, setmax());
printf(“Value of min = %d”, min);
}

Functions in C

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