What is Input Buffering in Compiler Design?

An interesting factor of coding is its execution–and to get executed it needs a compiler. A compiler helps to transform a program written in a particular language into a machine language. it brings out the efficiency of the code end, making the program run perfectly.

What is Input Buffering in Compiler Design?

Under the compiler concept, there are various related aspects that are mandatory to be studied. That said, one of the concepts that we will be looking at is the input buffering in a compiler design, its sub-aspects and its pros and cons. Let’s get ahead. 

What are the phases of compiler

To understand input buffering in compiler design, we need to know the phases of compiler, to figure at which phase it is needed. These phases are,

  • Lexical analysis
  • Syntax analysis
  • Semantic analysis
  • Intermediate code generation
  • Code optimisation
  • Target code generation

All these phases together are called a single pass compiler. 

Single pass compiler is a type of a compiler that turns the source code into a compiler. It is in this compiler design type, the input buffering is used.

What is buffering and its types?

To get a clear knowledge of input buffering, let us first know what “buffering” means. In the main memory, there will be a particular place to hold data for the time being. That place is known as a buffer, and the process of temporarily storing the data is known as buffering. 

Under buffering, there are three types, namely,

  • Single buffer
  • Double buffer
  • Circular buffer

Note: Input buffering falls under the double buffer category as it makes use of two (double) buffer points to produce the result. Moreover, a double buffer gives a temporary memory for items to be saved until the result is produced. 

What is input buffering?

First, let’s know what a lexical analyzer is. The lexical analyzer reads the input characters and then makes them into lexemes which further gives an output of token sequence for every lexeme.

Input buffering looks up one or more characters beyond a lexeme to find the right and required lexeme. If lexemes are found, the output of the token sequence gets produced. To get the lexemes, input buffering uses a two-buffer scheme and employs sentinels to speed up the process and mark the buffer end.

What are buffer pairs?

Buffer pairs is a unique buffering method that is employed to deduct overhead amounts, which is vital to run input characters in moving characters. 

  • Buffer pairs have two buffers and each has the size of N-character. These N characters are alternatively reloaded.
  • Lexeme’s two pointers are looked after (Begin and Forward)
  • Begin pointer denotes the unfound current lexeme’s beginning
  • Forward pointer helps to find a match for the required pattern.
  • When a lexeme is found, the Begin pointer is added to that character.
  • The Forward pointer is applied to the same character at its end (right side)
  • Technically, a current lexeme means the set of characters which are between the two pointers.

What are Sentinels?

Whenever a Forward pointer is progressed, a sentinel checks it to ensure that a buffer’s half hasn’t moved off. If it has been done, then the other half of the buffer should get reloaded. 

Hence, each end of the buffer’s halves need to undergo two tests for every move of the forward pointer. The first test is called “For end of buffer” and the second test is to see which character has been read. When we use a sentinel, these two tests are turned into one. Moreover, a sentinel cannot be a source program’s part.

The approaches for the lexical analyzer:

There are three approaches that are used to implement lexical analyzer. They are:

  • Usage of lexical-analyzer generator which helps to give reading and buffering a routine.
  • Using I/O facilities, transforming lexical analyzer into conventional systems-programming language.
  • To manage the input’s reading, lexical analyzer can be transformed and written in assembly language.

Input buffering in compiler design

Now we can clearly understand the purpose of the input buffering in compiler design. To figure out tokens, lexical analyzers must go to the secondary memory every time. Now for it to go to the secondary memory each time, it will consume too much money and time. 

Hence, the input strings are initially buffered before it gets tested by the lexical analyzer. 

As we previously read, lexical analyzer reads one character at a time and detects tokens and to detect tokens we need the Begin Lexeme and Forward Lexeme. This process is what we denote as input buffering in compiler design.

Additionally, input buffering reduces the amount of overhead and design the compiler in a way that the process of transferring input characters in moving characters gets fastened up.

Advantages and disadvantages of input buffering

Input buffering helps the source code majorly but it also equal negatives. Here are the advantages and disadvantages of input buffering in compiler designs.


Input buffering not only provides a way to find the right lexemes but also provides the following benefits:

  • Only one test is needed to check whether the Forward pointer is moved.
  • Until the characters read a buffer’s half, the tests will be run.
  • As N characters are between eofs, the number of tests per input is only 1.


Along with benefits, input buffering also brings in certain disadvantages including,

  • While the time consumed gets lessened, the lookahead amounts are limited and restricted.
  • The restricted lookahead will make it difficult to find tokens.
  • There is a point where the program waits until the character which runs by the right parenthesis figures out whether the DECLARE is the name of an array or just a keyword.

Final thoughts

Input buffering in compiler design is necessary as it finds lexemes and wraps up the whole process quickly. We hope you’ve got some insight into compiler phases, buffering, its types and additional information, all surrounding input buffering.