Input Buffering

In the context of a compiler, the lexical analyzer is responsible for scanning the input source code and breaking it down into individual tokens (such as keywords, identifiers, literals, etc.) that can be processed by the compiler. The lexical analyzer reads the input character by character, and one way to do this is by using two pointers: the begin pointer (bp) and the forward pointer (fp).

Initially, both the bp and fp point to the first character of the input string. The fp moves forward, scanning the input one character at a time, until it encounters a blank space or whitespace, which indicates the end of a lexeme (a lexeme is a sequence of characters representing a token). When the end of a lexeme is identified, the token is recognized based on the characters between bp and fp.

However, reading input character by character directly from secondary storage can be slow and inefficient. Input buffering is a technique that overcomes this issue. Instead of reading one character at a time, a block of data is first read into a buffer, and then the lexical analyzer processes the data from the buffer. This reduces the number of system calls required to read input, which can improve performance since system calls have overhead.

There are two commonly used input buffering methods: the One Buffer Scheme and the Two Buffer Scheme.

1. **One Buffer Scheme**: In this scheme, only one buffer is used to store the input string. The problem with this approach is that if a lexeme is very long and crosses the buffer boundary, the buffer needs to be refilled, which may overwrite the beginning of the lexeme.

2. **Two Buffer Scheme**: To overcome the issue with the One Buffer Scheme, two buffers are used to store the input string. The lexical analyzer scans the first buffer until it reaches the end of the buffer, and then it switches to the second buffer. This way, the entire lexeme can be processed without overwriting it.

The buffers are alternately filled, and the end of a buffer is marked by a special character called a "sentinel." This sentinel helps in identifying the end of the buffer when switching to the next buffer.

Advantages of input buffering include improved performance by reducing system calls and simplifying the compiler design. However, there are potential disadvantages like memory consumption and buffer management errors that need to be considered.

Overall, input buffering is a valuable technique in compiler design that can optimize performance and streamline the compilation process when implemented correctly.