Lex.yy.c File Download !FULL!

Lex was meant to be used in concert with Yacc. The history and details of this are detailed in Steven Johnson's paper Yacc: Yet Another Compiler Compiler. The Yacc parser uses only names beginning in "yy' - there's no apparent meaning discussed beyond simply desiring a namespace. The "yy" in lex.yy.c indicates that the lex output is intended for a yacc parser.

where lex.l is the file containing your lex specification. (The name lex.l is conventionally the favorite, but you can use whatever name you want. Keep in mind, though, that the .l suffix is a convention recognized by other system tools, make in particular.) The source code is written to an output file called lex.yy.c by default. That file contains the definition of a function called yylex() that returns 1 whenever an expression you have specified is found in the input text, 0 when end of file is encountered. Each call to yylex() parses one token (assuming a return); when yylex() is called again, it picks up where it left off.

Lex.yy.c File Download

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The lexical analyzer code stored in lex.yy.c (or the .c file to which it was redirected) must be compiled to generate the executable object program, or scanner, that performs the lexical analysis of an input text.

For more information about the function yywrap(), see the "Writing lex Source " section. When your driver file is compiled with lex.yy.c, as in the following example, its main() will call yylex() at run time exactly as if the lex library had been loaded:

LEX is a tool used to generate a lexical analyzer.This document is a tutorial for the use of LEX for ExpL Compiler development.Technically, LEX translates a set of regular expression specifications (given as input in input_file.l) into a C implementation of a corresponding finite state machine (lex.yy.c). This C program, when compiled, yields an executable lexical analyzer.

The declarations section consists of two parts, auxiliary declarations and regular definitions. The auxiliary declarations are copied as such by LEX to the output lex.yy.c file. This C code consists of instructions to the C compiler and are not processed by the LEX tool.The auxiliary declarations (which are optional) are written in C language and are enclosed within ' %{ ' and ' %} ' . It is generally used to declare functions, include header files, or define global variables and constants.

LEX obtains the regular expressions of the symbols 'number' and 'op' fromthe declarations section and generates code into a function yylex() in the lex.yy.c file. This function checks the input stream for the first match to one of the patterns specified and executes code in the action part corresponding to the pattern.

LEX generates C code for the rules specified in the Rules section and places this code into a single function called yylex(). (To be discussed in detail later). In addition to this LEX generated code, the programmer may wish to add his own code to the lex.yy.c file. The auxiliary functions section allows the programmer to achieve this.

The following variables are offered by LEX to aid the programmer in designing sophisticated lexical analyzers. These variables are accessible in the LEX program and are automatically declared by LEX in lex.yy.c.

Try to locate this code segment in the file lex.yy.c.What could be the consequences of removing this code segment from lex.yy.c before compiling it for generating thelexical analyzer? The above statement indicates that if the programmer does not define yyin, then yylex() by default sets yyin to the console input. Hence, any re-definition for yyin must be made before invoking yylex(). (This will be explained in detail later).

yylex() is a function of return type int. LEX automatically defines yylex() in lex.yy.c but does not call it. The programmer must call yylex() in the Auxiliary functions section of the LEX program. LEX generates code for the definition of yylex() according to the rules specified in the Rules section.

As LEX does not define yywrap() in lex.yy.c file butmakes a call to it under yylex(),the programmer must define it in the Auxiliary functions sectionor provide %option noyywrap in the declarations section.This options removes the call to yywrap() in the lex.yy.c file. Note that, it is mandatory to either define yywrap()or indicate the absence using the %option feature. If not, LEX will flag an error

Conceptually, LEX constructs a finite state machine to recognize all the regular expression patterns specifiedin the LEX program file. The code written by the programmer in the action part is executed when the machine is in accept state.The lex.yy.c program stores information about the finite state machine in the form of a decision table (transition table). A transition(current_state,input_char) function is used to access the decision table. LEX makes it's decision table visible if we compile the program with the -T flag. The finite state machine used by LEX is deterministic finite state automaton. The lex.yy.c simulates the DFA.

This DFA represents the regular expression provided as a specification(i.e. pattern to be matched) in the first rule of the tokensimulator program in section 9. When the DFA is in the final state i.e. III,then the corresponding action is executed as instructed in the lex.yy.c file.The constructed DFA is simulated using a simulation algorithm.

The lex utility shall generate C programs to be used in lexical processing of character input, and that can be used as aninterface to yacc. The C programs shall be generated from lex source code andconform to the ISO C standard, without depending on any undefined, unspecified, or implementation-defined behavior, except incases where the code is copied directly from the supplied source, or in cases that are documented by the implementation. Usually,the lex utility shall write the program it generates to the file lex.yy.c; the state of this file is unspecified iflex exits with a non-zero exit status. See the EXTENDED DESCRIPTION section for a complete description of the lexinput language.

When lex.yy.c is compiled and linked with the lex library (using the -l l operand with c99), the resulting program shall read character input from the standard input and shallpartition it into strings that match the given expressions.

Any line in the Definitions section beginning with a shall be assumed to be a C program fragment and shallbe copied to the external definition area of the lex.yy.c file. Similarly, anything in the Definitions sectionincluded between delimiter lines containing only "%{" and "%}" shall also be copied unchanged to the externaldefinition area of the lex.yy.c file.

Any such input (beginning with a or within "%{" and "%}" delimiter lines) appearing at thebeginning of the Rules section before any rules are specified shall be written to lex.yy.c after the declarations ofvariables for the yylex() function and before the first line of code in yylex(). Thus, user variables local toyylex() can be declared here, as well as application code to execute upon entry to yylex().

The action to be taken when an ERE is matched can be a C program fragment or the special actions described below; the programfragment can contain one or more C statements, and can also include special actions. The empty C statement ';' shall be avalid action; any string in the lex.yy.c input that matches the pattern portion of such a rule is effectively ignored orskipped. However, the absence of an action shall not be valid, and the action lex takes in such a condition isundefined.

The default action when a string in the input to a lex.yy.c program is not matched by any expression shall be to copy thestring to the output. Because the default behavior of a program generated by lex is to read the input and copy it to theoutput, a minimal lex source program that has just "%%" shall generate a C program that simply copies the input tothe output unchanged.

The intention in breaking the list of functions into those that may appear in lex.yy.c versus those that onlyappear in libl.a is that only those functions in libl.a can be reliably redefined by a conforming application.

Step 1: An input file describes the lexical analyzer to be generated named lex.l is written in lex language. The lex compiler transforms lex.l to C program, in a file that is always named lex.yy.c.

Step 2: The C compiler compile lex.yy.c file into an executable file called a.out.

Step 3: The output file a.out take a stream of input characters and produce a stream of tokens.

Lex translates the lex specification into a C source file called lex.yy.c which we compiled and linked with the lex library -ll. We then execute the resulting program to check that it works as we expect, as we saw earlier in this section. Try it to convince yourself that this simple description really does recognize exactly the verbs we decided to recognize.

The first line runs lex over the lex specification and generates a file, lex.yy.c, which contains C code for the lexer. In the second line, we use yacc to generate both y.tab.c and y.tab.h (the latter is the file of token definitions created by the -d switch.) The next line compiles each of the two C files. The final line links them together and uses the routines in the lex library libl.a, normally in /usr/lib/libl.a on most UNIX systems. If you are not using AT&T lex and yacc, but one of the other implementations, you may be able to simply substitute the command names and little else will change. (In particular, Berkeley yacc and flex will work merely by changing the lex and yacc commands to byacc and flex, and removing the -ll linker flag.) However, we know of far too many differences to assure the reader that this is true. For example, if we use the GNU replacement bison instead of yacc, it would generate two files called ch1-M.tab.c and ch1-M.tab.h. On systems with more restrictive naming, such as MS-DOS, these names will change (typically ytab.c and ytab.h.) See Appendices A through H for details on the various lex and yacc implementations. 2351a5e196