The symbol table is generated using the hash organisation. Lex and yacc are tools used to generate lexical analyzers and parsers. The output also contains the symbol table which contains tokens and their type. This document explains how to construct a compiler using lex and yacc. Errors in single-line comments, multi-line comments are displayed along with line numbers. The line number is displayed so that it is easier to debug the code for errors. The token, the type of token and the line number of the token in the C code are being displayed. ![]() The tokens displayed as part of the output include keywords, identifiers, signed/unsigned integer/floating point constants, operators, special characters, headers, data-type specifiers, array, single-line comment, multi-line comment, preprocessor directive, pre-defined functions (printf and scanf), user-defined functions and the main function. It takes as input a C code and outputs a stream of tokens. We have designed a lexical analyzer for the C language using lex. Symbol table is used by both the analysis and the synthesis parts of a compiler. Symbol table is an important data structure created and maintained by compilers in order to store information about the occurrence of various entities such as variable names, function names, etc. The lexical analyzer breaks these syntaxes into a series of tokens, by removing any whitespace or comments in the source code. It takes the modified source code from language preprocessors that are written in the form of sentences. Lexical analysis is the first phase of a compiler. The output of the compilation is called object code or sometimes an object module. When executing, the compiler first parses all of the language statements syntactically one after the other and then, in one or more successive stages, builds the output code, making sure that statements that refer to other statements are referred to correctly in the final code. The programmer then runs the appropriate language compiler, specifying the name of the file that contains the source statements. The file used for writing a C-language contains what are called the source statements. This is the accepting configuration.Īt each step of parsing, the parser takes an action resulting in a configuration change.A compiler is a special program that processes statements written in a particular programming language and turns them into machine language or code that a computer's processors use. Note here that expr is the start variable of the parser's context free grammar. On successful completion of parsing, the configuration would be: The contents of the stack and the contents of the input buffer together define the configuration of the parser. When the parsing process begins, the contents of the stack and the input buffer would be as follows: Let us consider parsing of the input 2+2*3 using this grammar. We refer them collectively with one production number for simplicity. Production 4 is actually a collection of 10 productions. The rules part of the YACC program has been shown below: Infix to Postfix converter implemented using YACC. We will explore productions further in detail later in this documentation.įor more about context free grammars refer to this wiki. ![]() Non-terminals (of zero or more length) on the right side (body part). As mentioned earlier, YACC takes this specification and generates a parser for SIL.Ī context free grammar is defined by a four tuple (N,T,P,S) - a set N of non-terminals,Ī set T of terminals (in our project, these are the tokens returned by the lexical analyzer and hence we refer to them as tokens frequently), set P of productions and a start variable S.Įach production consists of a non-terminal on the left side (head part) and a sequence of tokens and ![]() ![]() Johnson at Bell labs.Ī parser is a program that checks whether its input (viewed as a stream of tokens) meets a given grammar specification. The parser checks whether the program satisfies the syntax specification given in the input_file.y file. The source SIL program is fed as the input to the generated parser ( a.out ). This C program when compiled, yields an executable parser. Specifications (input in input_file.y) into a C implementation (y.tab.c) ofĪ corresponding push down automaton (i.e., a finite state machine with a stack). YACC translates a given Context Free Grammar (CFG) This document is a tutorial for the use of YACC to generate a parser for ExpL. YACC (Yet Another Compiler Compiler) is a tool used to generate a parser.
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