Functions in c programming language
C language functions and types of functions.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 is such
that 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, strcat() to concatenate two strings, memcpy() to copy
one memory location to another location, and many more functions.
A function can also be referred as 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 consists of a function header and a
function 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:
Given below is the source code for a function called max(). This function takes
two parameters num1 and num2 and returns the maximum value 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(),the function declaration is as follows:
int max(int num1, int num2);
Parameter names are not important in function declaration, only their type is
required, so the following is also a 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, the program control is transferred to the called
function. A called function performs a defined task and when its return
statement is executed or when its function-ending closing brace is reached, it
returns the program control back to the main program.
To call a function, you simply need to pass the required parameters along with
the function name, and if the function returns a value, then you can store the
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;
}
We have kept max()along with main() and compiled the source code. While
running the 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.
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 in which arguments can be passed
to a function:
Call Type and 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.
Call by Value
The 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 uses call by value to pass arguments. In general, it
means the 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 the 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
It shows that there are no changes in the values, though they had been changed
inside the function.
Call by Reference
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. It means the
changes made to the parameter affect the passed argument.
To pass a 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 their
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;
}
Let us now 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 i.e. address of variable a and
* &b indicates pointer to b i.e. 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 the above code in a single C file, compile and execute it, to 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
It shows that the change has reflected outside the function as well, unlike call
by value where the changes do not reflect outside the function.
By default, C uses call by value to pass arguments. In general, it means the
code within a function cannot alter the arguments used to call the function.
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