As a programming student, you must have come across compiler design subjects in your curriculum.

The subject deals with how a compiler operates and works and is quite interesting for budding programmers.

One such term related to compiler design is input buffering. Input bufferingis a part of the lexical analysis process and is carried out to make sure that the correct lexeme is found.

Do not get confused if these terms are not making sense now. We have explained all these terms and input buffering in compiler design in detail.

Let’s First Understand What Is Lexical Analysis

Lexical analysis is a part of the compiler designer process. The lexical analyzer reads the input characters of your source program and then divides them into lexemes. It will then produce an output of a sequence of tokens assigned to each lexeme.

Now, when a lexeme observes that a lexeme has an identifier, it adds that lexeme to the symbol table.

Not only this, but the lexical analyzer also processes your source text and eliminates white space, and comments from the text.

The lexical analysis process is divided into two parts:


In the scanning process, there is no need for tokenization. This process consists of simple processes like removing white spaces and deleting comments.

Lexical Analysis

In this process, the analyzer will produce a token sequence as the output.

Some Commonly Used Terms

Before we get into the technicalities of input buffering in compiler design, there are certain terms that you need to get hold of. These terms are often used in the concept and understanding them will help you grasp the concept properly.


Tokens are defined as abstract symbols which represent what type of lexical unit is in use.

For example: for = symbol, the token will be EQUAL_OP.

It completely represents the kind of lexeme.


A lexeme is a sequence of characters present in the source program that matches the pattern for a token. The lexeme is also identified as that token’s instance through the lexical analyzer.


A pattern is defined as the description of the form which the lexeme of a token takes. So, if there is a keyword present in the form of a token, the pattern will simply be the character sequence from the keyword only.

Common Approaches For Implementing Lexical Analyser

Below we have listed three common approaches which are followed while implementing a lexical analyzer.

One method can be to write the lexical analyzer in the system programming language and use the I/O functions of the language to analyze the input.

A lexical analyzer generator can be used to provide routine buffering and reading of the input.

The lexical analyzer can also be written in the assembly language and then the input can be managed explicitly.

Input Buffering In Compiler Design

Input buffering is a part of lexical analyses. In lexical analyses, the analyzer has to access the secondary memory every time to identify the tokens. This is a quite costly and time-consuming process.

Therefore, these input strings are stored in a buffer. From this buffer, the lexical analyzer will access the tokens. This whole process is known as input buffering in compiler design.

To identify the correct token, characters are scanned from left to right character by character. For this, lexical analyses employ two pointers for this:

Begin pointer: This pointer is represented as bptr. It represents the beginning of the string.

Look Ahead pointer: This pointer is represented as Iptr. This pointer shifts ahead to find the end of the pointer.

To make sure that the correct lexeme is found, it is important to look up to one or multiple characters. Therefore, two buffer schemes were introduced to manage the larger lookaheads.

Buffer Pairs

This specialized buffering scheme can reduce the overhead amount. It needs to process input characters in transferring characters.

=> This method consists of two different buffers. Each of these buffers will have N-size and be loaded again alternatively.

=> In this, two pointers: forward and lexemeBegin will be managed.

=> LexemeBegin pointer will point to the starting of your current lexeme which is not found yet.

=> Whereas, the forward pointer will scan ahead until it finds a pattern.

=> When the lexeme will be found, the lexemeBegin pointer will be set to the very next character of the found lexeme. The forward pointer will then be set to the right of the lexemeBegin.

=> The current lexeme will be defined as the sequence of characters between these two pointers.


These are used for checking. Whenever the forward pointer is moved, sentinels provide a check that one-half of the buffer is not converted. Only when the first half buffer is checked, another half will be loaded.

Sentinel is a character that is not a part of your main source code or program. This scheme is mostly used to reduce the usage of two test cases.

Advantages Of Sentinels

=> With sentinels, the requirement of using two tests has been reduced. Only one test needs to be performed to ensure whether the forward pointer is pointing to an eof or not.

=> It requires you to complete half the buffer first and then it will proceed with the process.

=> Because N characters are present between eofs, the average of tests performed per character is almost 1.

Disadvantages Of Sentinels

=> Most of the time, the scheme works great but the lookahead amount is limited.

Preliminary Scanning

Some processes are best performed when the characters are shifted from source files to buffer. For instance, it may remove comments. In lexical analysis, pre-processing the stream of characters saves a lot of computations required to transfer the forward pointer between the blank strings.


As a programmer, you need to understand and grasp the fundamentals of core concepts. Compiler design is one such concept.

Different topics like a DAG in compiler designor input buffering are often asked in many interviews.

With that said, Input buffering in compiler design is simply a part of the lexical analysis process where the input is stored in the buffer rather than the main memory. The process is introduced to make lexical analysis faster and better.

So, no matter if you have your exam or you are going for an interview, make sure to brush up on your basics in the core subjects like compiler design as well.