Clearing Input Buffer in C/C++

In C/C++, an input buffer is a temporary storage area where the program processes the input. Suppose, you are an user and you type some characters using a keyboard but those characters are not passed to the program directly because it involves several layers of handling such as keyboard firmware, OS input queues, and event processing. So, they first proceed with the collection of an input buffer. Next, when the program is ready, it reads from the buffer.

How Input Buffer Affects a Program?

The input buffer affected to the program when we are using the functions like scanf() or getchar(). If such buffer are not cleared then unexpected behavior occurs in the program.

Here are some issues due to input buffer in the program:

• Leftover \n (Newline) Characters: When we press the ENTER key button , the \n is stay in the buffer.
• Skips Data Inputs: When scanf() fails, the bad input stored in the buffer. Next, scanf() or getchar() may read the old data instead of reading new data.
• Infinite Loops: When the buffer is not cleared the loops may reads the old data.

Let us see the practical usage of clearing input buffer in the programs:

1. Below the program skips getchar() as \n was already in buffer.

int num;
char ch;

printf("Enter a number: ");
// User enter "10" and the buffer has "10\n"
scanf("%d", &num);  

printf("Enter a character: ");
// Reads leftover '\n' instead of waiting for input
ch = getchar();     

2. To avoid the bugs from the program use either fgets() or sscanf(). So, it will clear the buffer.

// Clears all leftover characters
while ((getchar()) != '\n');  

Using getchar() and cin.get()

Both these functions are used to clear the input buffer to the programs. The getchar() of C read one character from the input buffer when there is no newline ['\n'] and the loop is reading character continuosly without discard it. The cin.get() of C++ also do the same. Here, it removes the leftover character from the input stream.

Example

In this example, we demonstrates the functions like getchar() [C] and cin.get() [C++] to clear the input buffer.

#include<stdio.h>

int main()
{
   char str[80], ch;
    
   scanf("%s", str);
   
   // Clear input buffer
   while ((getchar()) != '\n');       
   ch = getchar();      
   
   // display the string   
   printf("%s\n", str); 

   // display the characters   
   printf("%c", ch);                 

   return 0;
}

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#include <iostream>
#include <string> 
using namespace std;

int main()
{
   string str;
   char ch;

   // read an input string
   cin >> str;

   // remove leftover characters until newline
   while (cin.get() != '\n');

   // read a single character
   ch = cin.get();

   // Print the string
   cout << str << endl;

   // Print the character
   cout << ch;

   return 0;
}

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Using fgets() Function

The fgets() is C standard library that comes under <stdio.h> header. It is used to read a string from a file and store it in buffer input. This reads the space, specified integers or characters, line of text and stops when newline will be found.

Syntax

It's syntax is as follows:

char *fgets(char *str, int n, FILE *stream);

Here,

• str: This is pointer to a character array where the input will be stored.
• n: This maximum number of characters to read, containing the null terminator \0.
• stream: This is an input stream.

Example

In this example, you will see the usage of fgets() function.

#include <stdio.h>

int main() {
   char str[100];

   // Read a full line from stdin
   fgets(str, sizeof(str), stdin);

   // prints the string including the newline
   printf("You entered: %s", str);  

   return 0;
}

Output

The above program produces the following result:

tutorial
You entered: tutorial

Using getline() Function

The getline() of C++ is used to read an entire line of input from a stream (such as cin). It reads a line of text, including spaces, and stops at a newline (\n). It automatically handle the memory positions for the string and doesn't specify the buffer size.

Syntax

It's syntax is as follows:

istream? getline(istream& is, string& str);

Here,

• is: This is the input stream.
• str: This is the string object where the input will be stored.

Example

This is the C++ program to show the usage of getline() function.

#include <iostream>
#include <string>
using namespace std;

int main() {
    string str;

    // Read a full line from cin
    getline(cin, str);

    cout << "You entered: " << str << endl;

    return 0;
}

Output

The above program produces the following result:

School
You entered: School