Lex & Yaccfor Compiler Writing

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Online Lex And Yacc Compiler

Open Command prompt and switch to your working directory where you have stored your lex file (“.l “) and yacc file (“.y “) Let your lex and yacc files be “ hello.l ” and “ hello.y “. Now, follow the preceding steps to compile and run your program. For Compiling Lex file only: flex hello.l; gcc lex.yy.c; For Compiling Lex & Yacc. Lex and Yacc can do anything, but calling them easy to use, I wouldn't dare. I'd like to be able to embed the lexer in my program eventually, so I don't want to depend on JVM or something which may not be installed. Falken May 20 '12 at 16:47.

Then Lex compiler runs the lex.1 program and produces a C program lex.yy.c. Finally C compiler runs the lex.yy.c program and produces an object program a.out. A.out is lexical analyzer that transforms an input stream into a sequence of tokens. I'm having Lex and YACC files to parse my files (.l file and.y file).

Some of the most time consuming and tedious parts of writinga compiler involve the lexical scanning and syntax analysis. Luckily there isfreely available software to assist in these functions. While they will not doeverything for you, they will enable faster implementation of the basicfunctions. Lex and Yacc arethe most commonly used packages with Lex managing thetoken recognition and Yacc handling the syntax. Theywork well together, but conceivably can be used individually as well.

Both operate in a similar manner in which instructions fortoken recognition or grammar are written in a special file format. The textfiles are then read by lex and/or yaccto produce c code. This resulting source code is compiled to make the finalapplication. In practice the lexical instruction file has a“.l” suffix and the grammar file has a “.y” suffix. This process is shown inFigure 1.

Figure 1.Lex and Yacc Process (based on adiagram on page 5 of “A Compact Guide to Lex & Yacc” by Thomas Niemann)

The file format for a lex fileconsists of (4) basic sections

The first is an area for c code that will be place verbatim at the beginning of the generated source code. Typically is will be used for things like #include, #defines, and variable declarations.
The next section is for definitions of token types to be recognized. These are not mandatory, but in general makes the next section easier to read and shorter.
The third section set the pattern for each token that is to be recognized, and can also include c code to be called when that token is identified
The last section is for more c code (generally subroutines) that will be appended to the end of the generated c code. This would typically include a main function if lex is to be used by itself.
The format is applied as follows (the use and placement of the % symbols are necessary):

%{

//header c code Idrac7 license generator.

%}

//definitions

%%

//rules

%%

//subroutines

The format for a yacc file issimilar, but includes a few extras.

The first area (preceded by a %token) is a list of terminal symbols. You do not need to list single character ASCII symbols, but anything else including multiple ASCII symbols need to be in this list (i.e. “”).
The next is an area for c code that will be place verbatim at the beginning of the generated source code. Typically is will be used for things like #include, #defines, and variable declarations.
The next section is for definitions- none of the following examples utilize this area
The fourth section set the pattern for each token that is to be recognized, and can also include c code to be called when that token is identified
The last section is for more c code (generally subroutines) that will be appended to the end of the generated c code. This would typically include a main function if lex is to be used by itself.
The format is applied as follows (the use and placement of the % symbols are necessary):

%tokens RESERVED, WORDS, GO,HERE

%{

//header c code

%}

//definitions

%%

//rules

%%

//subroutines

These formats and general usage will be covered in greaterdetail in the following (4) sections. In general it is best not to modify theresulting c code as it is overwritten each time lexor yacc is run. Most desired functionality can be handledwithin the lexical and grammar files, but there are some things that aredifficult to achieve that may require editing of the c file.

As a side note, the functionality of these programs has beenduplicated by the GNU open source projects Flex and Bison. These can be usedinterchangeably with Lex and Yaccfor everything this document will cover and most other uses as well.

Here are some good references for further study:

The Lex & Yaccpage – has great links to references for lex, yacc, Flex, and Bison http://dinosaur.compilertools.net

Nice tutorial for use of lex &yacc together

http://epaperpress.com/lexandyacc

Assignment implement syntax analyzer for sample program

PROBLEM STATEMENT:

1. Write a YACC program to check whether the given grammar is valid or not. Consider an input expression and convert it to postfix form.

2. Write a program to Implement YACC for Subset of C (for loop) statement.

OBJECTIVES:

To understand Second phase of compiler: Syntax Analysis.
To learn and use compiler writing tools.
Understand the importance and usage of YACC automated tool.
THEORY:
Introduction
Parser generator facilitates the construction of the front end of a compiler. YACC is LALR parser generator. It is used to implement hundreds of compilers. YACC is command (utility) of the UNIX system. YACC stands for “Yet Another Compiler Complier”. File in which parser generated is with .y extension. e.g. parser.y, which is containing YACC specification of the translator. After complete specification UNIX command. YACC transforms parser.y into a C program called y.tab.c using LR parser. The program y.tab.c is automatically generated. We can use command with –d option as yacc –d parser.y By using –d option two files will get generated namely y.tab.c and y.tab.h. The header file y.tab.h will store all the token information and so you need not have to create y.tab.h explicitly. The program y.tab.c is a representation of an LALR parser written in C, along with other C routines that the user may have prepared. By compiling y.tab.c with the ly library that contains the LR parsing program using the command. $ gcc y.tab.c
We obtain the desired object program aout that perform the translation specified by the original program. If procedure is needed, they can be compiled or loaded with y.tab.c, just as with any C program.

LEX recognizes regular expressions, whereas YACC recognizes entire grammar. LEX divides the input stream into tokens, while YACC uses these tokens and groups them together logically.LEX and YACC wo

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