Input means to provide the program with some data to be used in the program
Output means to display data on screen or write the data to a printer or a file.
C programming language provides many built-in functions to read any given input and to display data on screen when there is a need to output the result.
In this tutorial, we will learn about such functions, which can be used in our program to take input from user and to output the result on screen.
All these built-in functions are present in C header file "#include<stdio.h>"
To use the built in functions we have to specify header files at head in which a particular function is defined(written) while discussing about it.

String I/O
gets() and puts()

Till now we have taken input from the scanf() function and we have displayed the data on the screen using printf()
But printf() and scanf() function has one drawback , that they dont work properly on string which contains space
It means printf() and scanf() function takes space " " character as deliminator

See this program to understand it better:

void main()
{
    char s[20];


    printf("Enter the string: ");
    scanf("%s",s);  //%s is use to take string as input.
     //ther is no need of writing & while taking string as input.

    printf("\nString is: %s",s);
}

Output:

Enter the string: learn programming

String is: learn

As you saw that it only print learn , because as space character came scanf() character took space as deliminator and from space character it did not took any character as input.
And , then printf() displayed the input string which was learn on the screen.

Strings I/O

By overcoming from the above problem , C language provides the string Input and Output Functions.
String input and output functions are used to take string as input from the user or to display string on the screen.
This functions does not take " " space character as deliminator.
Because of above functionality strings functions comes in action.

gets() function

Syntax:

   gets(array_name);

Let's take string as input:

void main()
{   //declaring the character arrays
    char a[10],b[10],c[10];
    
    
    printf("Enter the first string: ");
    gets(a);
    
    printf("Enter the second string: ");
    gets(b);
    
    printf("Enter the third string: ");
    gets(c);
}

Output:

Enter the first string: learn programming

Enter the first string: Core programming

Enter the first string: hello world

Now let's see puts() function to display the above strings

puts() function

Syntax:

   puts(array_name);

Let's print the strings:

void main()
{   //declaring the character arrays
    char a[20],b[20],c[20];


    printf("Enter the first string: ");
    gets(a);

    printf("Enter the second string: ");
    gets(b);

    printf("Enter the third string: ");
    gets(c);

//printing the strings a,b,and c.
    printf("First string is ");
    puts(a);

    printf("Second string is ");
    puts(b);

    printf("Third string is ");
    puts(c);


}

Output:

Enter the first string: learn programming

Enter the first string: Core programming

Enter the first string: hello world

First string is learn programming
Second string is Core programming
Third string is hello world.