• When LEX and YACC work together lexical analyzer using yylex produce pairs consisting of a token and its associated attribute value. If a token such as DIGIT is returned, the token value associated with a token is communicated to the parser through a YACC defined variable yylval.
• Using yacc: 1) Generates a C function called yyparse 2) yyparse may include calls to yylex 3) Compile this function to obtain the compiler yacc Parser Generator yacc source yacc #include lex.yy.c Compiler implementers have a love-hate relationship with source-code-generating tools such as Lex 9 (which generates lexers from regular.
• Online Lex And Yacc Compiler online, free Software Foundations. I used lex and yacc, and in less than a week my compiler was up and running Later, the Free Software Foundations GNU project produced improved versions of lex and yacc - named flex and bison - for use on platforms that did not run a derivative of the Unix operating system.
• 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).

• Lex is a program that generates lexical analyzer. It is used with YACC parser generator.
• The lexical analyzer is a program that transforms an input stream into a sequence of tokens.
• It reads the input stream and produces the source code as output through implementing the lexical analyzer in the C program.

The following descriptions assume that the calc.lex and calc.yacc example programs are located in your current directory. Compiling the example program. To create the desk calculator example program, do the following: Process the yacc grammar file using the -d optional flag (which informs the yacc command to create a file that defines the tokens used in addition to the C language source code).

The function of Lex is as follows:

• Firstly lexical analyzer creates a program lex.1 in the Lex language. 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.

Lex file format

A Lex program is separated into three sections by %% delimiters. The formal of Lex source is as follows:

Definitions include declarations of constant, variable and regular definitions.

Rules define the statement of form p1 {action1} p2 {action2}....pn {action}.

Where pi describes the regular expression and action1 describes the actions what action the lexical analyzer should take when pattern pi matches a lexeme.

User subroutines are auxiliary procedures needed by the actions. The subroutine can be loaded with the lexical analyzer and compiled separately.

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 work together to analyze the program syntactically. The YACC can report conflicts or ambiguities (if at all) in the form of error messages.

YACC Specifications:

The Structure of YACC programs consists of three parts:

%{

Definitions Section

%}

%%

Rules Section (Context Free Grammar)

%%

Auxiliary Function

———————————————————————————–

Definition Section:

The definitions and programs section are optional. Definition section handles control information for the YACC-generated parser and generally set up the execution environment in which the parser will operate.

In declaration section, %{ and %} symbol used for C declaration. This section is used for definition of token, union, type, start, associativity and precedence of operator. Token declared in this section can then be used in second and third parts of Yacc specification.

Translation Rule Section:

In the part of the Yacc specification after the first %% pair, we put the translation rules. Each rule consists of a grammar production and the associated semantic action. A set of productions that we have been writing: <left side> <alt 1> | <alt 2> | … <alt n> Would be written in YACC as <left side> : <alt 1> {action 1} | <alt 2> {action 2} … … … … … … … … … … | <alt n> {action n} ; In a Yacc production, unquoted strings of letters and digits not declared to be tokens are taken to be nonterminals. A quoted single character, e.g. ‘c’, is taken to be the terminal symbol c, as well as the integer code for the token represented by that character (i.e., Lex would return the character code for ‘ c’ to the parser, as an integer). Alternative bodies can be separated by a vertical bar, and a semicolon follows each head with its alternatives and their semantic actions. The first head is taken to be the start symbol. A Yacc semantic action is a sequence of C statements. In a semantic action, the symbol $$ refers to the attribute value associated with the nonterminal of the head, while $i refers to the value associated with the ith grammar symbol (terminal or nonterminal) of the body. The semantic action is performed whenever we reduce by the associated production, so normally the semantic action computes a value for $$ in terms of the $i’s. In the Yacc specification, we have written the two E-productions.

E-> E + T/T

and their associated semantic action as:

exp : exp “+” term {$$ = $1 + $3;}

| term ;

In above production exp is $1, „+‟ is $2 and term is $3. The semantic action associated with first production adds values of exp and term and result of addition copying in $$ (exp) left hand side. For above second number production, we have omitted the semantic action since it is just copying the value. In general {$$ = $1;} is the default semantic action.

Supporting C-Routines Section:

The third part of a Yacc specification consists of supporting C-routines. YACC generates a single function called yyparse(). This function requires no parameters and returns either a 0 on success, or 1 on failure. If syntax error over its return 1.The special function yyerror() is called when YACC encounters an invalid syntax. The yyerror() is passed a single string (char) argument. This function just prints user defined message like:

yyerror (char err)

{

printf (“Divide by zero”);

}

When LEX and YACC work together lexical analyzer using yylex () produce pairs consisting of a token and its associated attribute value. If a token such as DIGIT is returned, the token value associated with a token is communicated to the parser through a YACC defined variable yylval. We have to return tokens from LEX to YACC, where its declaration is in YACC. To link this LEX program include a y.tab.h file, which is generated after YACC compiler the program using –d option.

Steps to Execute the program

Online Lex And Yacc Compiler Software

$ lex filename.l (eg: cal.l)

$ yacc -d filename.y (eg: cal.y)

$gcc lex.yy.c y.tab.c

Online Lex And Yacc Compiler Java

$./a .out