Lex and YaccLex and yacc help you write programs that transform structured input. This includes an enormous range of applications—anything from a simple text search program that looks for patterns in its input file to a C compiler that transforms a source program into optimized object code.In programs with structured input, two tasks that occur over and over are dividing the input. Lex and Yacc can generate program fragments that solve the first task. The task of discovering the source structure again is decomposed into subtasks: Split the source file into tokens (Lex). Find the hierarchical structure of the program (Yacc). A First Example: A Simple Interpreter. LEX: Lex - A Lexical Analyzer Generator M. 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).

• Kms Tools
• Online Lex And Yacc Compiler Java
• Online Lex And Yacc Compiler C
• Online Lex And Yacc Compiler Code
• Download Flex For Windows 10

Lex & Yaccfor Compiler Writing

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

%}

//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

Parsing with Yacc/Bison: Practice Problems

Clone the repository

The files below and other example programs are available in a git repository

You must have git and python (3.x) on your system. Once you’ve confirmed this, run this command:

If you have already cloned the repository earlier you canget the new homework files by going to the directorywhere you cloned the repository earlier and then doing:

Then go to the yacc-practice directory

Getting started

Let’s start with a simple lexical analyzer that we will use for our first yacc program.Save the following Lex program to a file called parens.lex

To specify a context-free grammar (CFG) in yacc we write the left-hand sideof the rule just before the : followed by the right-hand side of eachrule. Rules with the same left-hand side are grouped together usingthe alternation symbol |. Save the following Yacc program to a filecalled parens.y:

The above grammar has two CFG rules, which we usually write as:

In Backus-Naur notation we write the two rules together using |:

where <epsilon> is the empty string and in Yacc grammarsthis is represented as having an empty right-hand side. Sothe Yacc grammar equivalent of the above grammar is:

The ‘;’ represents the end of all the S rules in the grammar.

To create a C or C++ program we will use bison (the GNU version ofyacc) and flex (the GNU version of lex). Let’s using bisonand flex to generate a C version of the parser for the CFG above and compile it:

It will accept strings with balanced parentheses where no close parencan precede an open paren:

Similar to how we can attach actions to regular expressions in Lexwe can also attach actions to CFG rules in Yacc:

In the above Yacc grammar, the symbol $2 refers to the valueof the second element in the right-hand side of the rule in thecontext of a parse tree for some input string. In the above ruleS: '(' S ')' the $2 refers to the value S in a parse tree.$$ refers to the value created for the left-hand side and itis passed up the parse tree. After we re-compile the new Yaccfile, we see the following behaviour:

Let’s look at a schematic for the parse tree for (()):

Here are some exercises to test your understanding:

1. Add a new CFG rule TOP: S to the top of the Yacc grammar (the order is important).
2. Does anything change in the strings this new Yacc grammar accepts?
3. What is the value of $1 in the rule TOP: S?
4. Add an action to the rule to print out $1 to see if you could predict the right value.

Simple Expression Interpreter

The yacc program below is a very simple (and incomplete) expression interpreter. The { ... } section contains arbitrary C/C++ code and the %token definitions is a list of tokens provided bythe lexical analyzer.

If you save the above program to a file called simple-expr.y thenthe program bison can be used to convert this parser definitioninto a parser implementation by using the following command:

The -d option produces a header file called simple-expr.tab.hto convey information about the tokens to the lexical analyzer. Examinethe contents of this file.

The lexical analyzer is shown below as a lex program.

Save it to a file called simple-expr.lex.The lex program can be compiled to a C program using flex:

The final binary is created by compiling the outputfrom flex and bison with a C/C++ compiler as follows.

If you see the following error:

Then use -ll instead of -lfl. And if that doesn’t work then youprobably need to install bison and flex on your local machine. TheCSIL Linux machines should have these tools installed already.

Convert the above yacc and lex programs so that it can handlemultiple expressions, exactly one per line.You will need a recursive context-free rule in the yacc definitionin order to handle multiple linesof input. Try different ways of writing this recursive rule.Note that we can assign a value to a variable, e.g b=5+10-5 butwe cannot yet use b in a following expression, e.g. a=b+10(which fails with a syntax error).

You can automate the creation of the binary using a makefile. make is very finicky about whitespace, especially the actions have to be indented using a literal tab character. To avoid headaches download the makefile directly by using the view raw link below.

Run make simple-expr assuming you used the same filenames suggested above. Then you cantest it with various inputs:

The yacc and lex code above does not yet handle assignments tovariables. In order to implement this, we need two different kindsof values to be returned from the lexical analyzer: one for numbers,and another for variable names. The lex code below shows you how to dothat. Save this lex code to the file simple-varexpr.lex.

For numbers it returns yylval.rvalue and for variable names itreturns yylval.lvalue. The two types of return value, rvalueand lvalue are defined in the yacc program simple-varexpr.yshown below.

Kms Tools

The two different return values are represented using the %uniondeclaration which is the same concept as the union type in C and C++. The %union declaration can include complex datatypes.The yacc code defines a type not just for the tokens, but also fornonterminals, which is specified in the %type definition. Thisallows yacc to check that the type of the non-terminal expressionis rvalue, an integer type.

Your Task

Extend this code in two ways:

• First, add the ability to process multiple statements so that the following works: echo “a=10nb=10” | ./simple-varexpr
• Second, properly handle assignments of values to variables. Thevariable on the left hand side of an equation will be an (ell)-valueand the variable used on the right hand side of an equation willbe a r-value.

Adding Functions to your Expression Interpreter

Extend your expression interpreter to include constants of type double,and variables that can hold either integer or double types. Finally, add the functions: exp, sqrt, log so that you can interpretthe following types of input:

To avoid issues with precedence of operators use the yacc grammar providedbelow:

Writing a Decaf Program

Read the Decaf specification and write a Decafprogram that implements the quicksort algorithm to sort a list.Create an array variable list with 100 elements. Then initializeit using the following Decaf loop:

Sort this list using quicksort and then print the sorted list byiteratively calling the print_int library function in the formatshown in the Homework 2 testcases directory.

Expression interpreter

Write down a yacc parser for the following context-free grammar:

The tokens PLUS, TIMES, LPAREN, RPAREN are defined to be +, *, (, ) respectively.And the token ID is defined to be an identifier as in the Decaf specification.These tokens should be defined using a lexical analyzer produced using lex.

For the input string x + y * ( z ) the output produced by theyacc parser should be the parse tree for the input string in theformat shown below. The tree below is indented but you should simplyprint your parse tree as a single line of output text.

Note the backslash preceding each instance of a literal parenthesisto avoid confusion with the parentheses used to denote the treestructure. You may need to augment the grammar to produce the rightoutput.

Marking Precedence and Associativity in Yacc

Use the following expression grammar in Yacc:

If you run it through bison you will get shift/reduce conflicts:

Add precedence declarations just below the %token line in theyacc program. Precedence is added using the %left or %rightassociativity declaration for tokens. The precedence is markedby having a list of such declarations with the highest precedencetoken appearing at the bottom of the list. So if we wish * tohave higher precedence than - we would write:

Two tokens can be declared with the same precedence and leftassociativity by declaring them on the same line:

However, this does not solve the problem of the unary minusproduction. Unary minus should have a precedence higher thanmultiplication or division. To mark this we have to add aprecedence annotation on the rule itself:

Then we can add this to our list of precedence

Online Lex And Yacc Compiler Java

Modify the yacc program above to eliminate all shift/reduce conflictsusing precedence and associativity declarations in yacc.

Online Lex And Yacc Compiler C

Inherited Attributes

Online Lex And Yacc Compiler Code

For the following context-free grammar in yacc format:

Download Flex For Windows 10

Provide a yacc program that passes the type information for each variableby using inherited attributes. The program should enter each variable name into a symbol table along with its type information.