A function is a group of statements that together perform a
task. Every C program has at least one function, which is main(), and all the
most trivial programs can define additional functions.
You can divide up your code into separate functions. How you
divide up your code among different functions is up to you, but logically the
division usually is so each function performs a specific task.
A function declaration tells the compiler about a
function's name, return type, and parameters. A function definition provides the actual body of the
function.
The C standard library provides numerous built-in functions
that your program can call. For example, function strcat() to
concatenate two strings, function memcpy() to
copy one memory location to another location and many more functions.
A function is known with various names like a method or a
sub-routine or a procedure, etc.
Defining a Function:
The general form of a function definition in C programming
language is as follows:
return_type function_name( parameter list )
{
body of the function
}
A function definition in C programming language consists of
a function header and afunction body. Here are
all the parts of a function:
·
Return Type: A function may return a value. The return_type is the data type of the value the
function returns. Some functions perform the desired operations without
returning a value. In this case, the return_type is the keyword void.
·
Function Name: This
is the actual name of the function. The function name and the parameter list
together constitute the function signature.
·
Parameters: A
parameter is like a placeholder. When a function is invoked, you pass a value
to the parameter. This value is referred to as actual parameter or argument.
The parameter list refers to the type, order, and number of the parameters of a
function. Parameters are optional; that is, a function may contain no
parameters.
·
Function Body: The
function body contains a collection of statements that define what the function
does.
Example:
Following is the source code for a function called max(). This function
takes two parameters num1 and num2 and returns the maximum between the two:
/* function returning the max between two
numbers */
int max(int num1, int num2)
{
/*
local variable declaration */
int result;
if
(num1 > num2)
result = num1;
else
result = num2;
return result;
}
Function Declarations:
A function declaration tells the compiler about a function
name and how to call the function. The actual body of the function can be
defined separately.
A function declaration has the following parts:
return_type function_name( parameter list
);
For the above defined function max(), following is the function
declaration:
int max(int num1, int num2);
Parameter names are not important in function declaration
only their type is required, so following is also valid declaration:
int max(int, int);
Function declaration is required when you define a function
in one source file and you call that function in another file. In such case you
should declare the function at the top of the file calling the function.
Calling a Function:
While creating a C function, you give a definition of what
the function has to do. To use a function, you will have to call that function
to perform the defined task.
When a program calls a function, program control is
transferred to the called function. A called function performs defined task and
when its return statement is executed or when its function-ending closing brace
is reached, it returns program control back to the main program.
To call a function, you simply need to pass the required
parameters along with function name, and if function returns a value, then you
can store returned value. For example:
#include <stdio.h>
/* function declaration */
int max(int num1, int num2);
int main ()
{
/* local variable
definition */
int a = 100;
int b = 200;
int ret;
/* calling a function
to get max value */
ret = max(a, b);
printf( "Max value
is : %d\n", ret );
return 0;
}
/* function returning the max between two numbers */
int max(int num1, int num2)
{
/* local variable
declaration */
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
I kept max() function along with main() function and
compiled the source code. While running final executable, it would produce the
following result:
Max value is : 200
Function Arguments:
If a function is to use arguments, it must declare variables
that accept the values of the arguments. These variables are called the formal parameters of the function.
The formal parameters behave like other local variables
inside the function and are created upon entry into the function and destroyed
upon exit.
While calling a function, there are two ways that arguments
can be passed to a function:
Call Type
|
Description
|
Call
by value
|
This method copies the actual
value of an argument into the formal parameter of the function. In this case,
changes made to the parameter inside the function have no effect on the
argument.
|
Call
by reference
|
This method copies the address of
an argument into the formal parameter. Inside the function, the address is
used to access the actual argument used in the call. This means that changes
made to the parameter affect the argument.
|
By default, C uses call by value to
pass arguments. In general, this means that code within a function cannot alter
the arguments used to call the function and above mentioned example while
calling max() function used the same method.
Function call by value in C
he call
by value method of passing
arguments to a function copies the actual value of an argument into the formal
parameter of the function. In this case, changes made to the parameter inside
the function have no effect on the argument.
By default, C programming language uses call by value method to pass arguments. In general,
this means that code within a function cannot alter the arguments used to call
the function. Consider the function swap() definition as follows.
/* function definition to swap the values */
void swap(int x, int y)
{
int temp;
temp = x; /* save the value of x */
x = y; /* put y into x */
y = temp; /* put temp into y */
return;
}
Now, let us call the function swap() by passing actual values as in the
following example:
#include <stdio.h>
/* function declaration */
void swap(int x, int y);
int main ()
{
/* local variable definition */
int a = 100;
int b = 200;
printf("Before swap, value of a : %d\n", a );
printf("Before swap, value of b : %d\n", b );
/* calling a function to swap the values */
swap(a, b);
printf("After swap, value of a : %d\n", a );
printf("After swap, value of b : %d\n", b );
return 0;
}
Let us put above code in a single C file,
compile and execute it, it will produce the following result:
Before swap, value of a :100
Before swap, value of b :200
After swap, value of a :100
After swap, value of b :200
Function call by reference in C
The call
by reference method of
passing arguments to a function copies the address of an argument into the
formal parameter. Inside the function, the address is used to access the actual
argument used in the call. This means that changes made to the parameter affect
the passed argument.
To pass the value by reference, argument
pointers are passed to the functions just like any other value. So accordingly
you need to declare the function parameters as pointer types as in the
following function swap(),
which exchanges the values of the two integer variables pointed to by its
arguments.
/* function definition to swap the values */
void swap(int *x, int *y)
{
int temp;
temp = *x; /* save the value at address x */
*x = *y; /* put y into x */
*y = temp; /* put temp into y */
return;
}
For now, let us call the function swap() by passing values by reference as in
the following example:
#include <stdio.h>
/* function declaration */
void swap(int *x, int *y);
int main ()
{
/* local variable definition */
int a = 100;
int b = 200;
printf("Before swap, value of a : %d\n", a );
printf("Before swap, value of b : %d\n", b );
/* calling a function to swap the values.
* &a indicates pointer to a ie. address of variable a and
* &b indicates pointer to b ie. address of variable b.
*/
swap(&a, &b);
printf("After swap, value of a : %d\n", a );
printf("After swap, value of b : %d\n", b );
return 0;
}
Let us put above code in a single C file,
compile and execute it, it will produce the following result:
Before swap, value of a :100
Before swap, value of b :200
After swap, value of a :200
After swap, value of b :100
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