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Compiler Design 3. Lexical Analyzer, Flex

Kanat Bolazar January 26, 2010. Compiler Design 3. Lexical Analyzer, Flex. Lexical Analyzer. The main task of the lexical analyzer is to read the input source program, scanning the characters, and produce a sequence of tokens that the parser can use for syntactic analysis.

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Compiler Design 3. Lexical Analyzer, Flex

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  1. Kanat Bolazar January 26, 2010 Compiler Design3. Lexical Analyzer, Flex

  2. Lexical Analyzer • The main task of the lexical analyzer is to read the input source program, scanning the characters, and produce a sequence of tokens that the parser can use for syntactic analysis. • The interface may be to be called by the parser to produce one token at a time • Maintain internal state of reading the input program (with lines) • Have a function “getNextToken” that will read some characters at the current state of the input and return a token to the parser • Other tasks of the lexical analyzer include • Skipping or hiding whitespace and comments • Keeping track of line numbers for error reporting • Sometimes it can also produce the annotated lines for error reports • Produce the value of the token • Optional: Insert identifiers into the symbol table

  3. Character Level Scanning • The lexical analyzer needs to have a well-defined valid character set • Produce invalid character errors • Delete invalid characters from token stream so as not to be used in the parser analysis • E.g. don’t want invisible characters in error messages • For every end-of-line, keep track of line numbers for error reporting • Skip over or hide whitespace and comments • If comments are nested (not common), must keep track of nesting to find end of comments • May produce hidden tokens, for convenience of scanner structure • Always produce an end-of-file token • Important that quoted strings and comments don’t get stuck if an unexpected end of file occurs

  4. Tokens, Token Types and Values • The set of tokens is typically something like the following table • Or may have separate token types for different operators or reserved words • May want to keep line number with each token

  5. Token Actions • Each token recognized can have an action function • Many token types produce a value • In the case of numeric values, make sure property numeric errors produced, e.g. integer overflow • Put identifiers in the symbol table • Note that at this time, no effort is made to distinguish scope; there will be one symbol table entry for each identifier • Later, separate scope instances will be produced • Other types of actions • End-of-line (can be treated as a token type that doesn’t output to the parser) • Increment line number • Get next line of input to scan

  6. Testing • Execute lexical analyzer with test cases and compare results with expected results • Test cases • Exercise every part of lexical analyzer code • Produce every error message • Don’t have to be valid programs – just valid sequence of tokens

  7. Lex and Yacc • Two classical tools for compilers: • Lex: A Lexical Analyzer Generator • Yacc: “Yet Another Compiler Compiler” • Lex creates programs that scan your tokens one by one. • Yacc takes a grammar (sentence structure) and generates a parser. Lexical Rules Grammar Rules Lex Yacc Input yylex() yyparse() Parsed Input

  8. Flex: A Fast Scanner Generator • Often, instead of the standard Lex and Yacc, Flex and Bison are used: • Flex: A fast lexical analyzer • (GNU) Bison: A drop-in replacement for (backwards compatible with) Yacc • Resources: • http://en.wikipedia.org/wiki/Flex_lexical_analyser • http://en.wikipedia.org/wiki/GNU_Bison • http://dinosaur.compilertools.net/ • (the Lex & Yacc Page)

  9. Flex Example 1: Delete This • Shortest Flex example, “deletethis.l”: • %% • deletethis • This scanner will match and not echo (default behavior) the word “deletethis”. • Compile and run it: $ flex deletethis.l # creates lex.yy.c $ gcc -o scan lex.yy.c -lfl # fl: flex library $ ./scan This deletethis is not deletethis useful. This is not useful. ^D

  10. Flex Example 2: Replace This • Another very short Flex example, “replacer.l”: • %% • replacethis printf(“replaced”); • This scanner will match “replacethis” and replace it with “replaced”. • Compile and run it: $ flex -o replacer.yy.c replacer.l $ gcc -o replacer replacer.yy.c -lfl $ ./replacer This replacethis is not very replacethis useful. This replaced is not very replaced useful. Please dontreplacethisatall. Please dontreplacedatall.

  11. Flex Example 3: Common Errors • Let's replace “the the” with “the”: • %% • the the printf(“the”); • uhh • Unfortunately,this does not work: The second “the” is considered part of C code: • %% • the the printf(“the”); • Also, the open and close matching double quotes used in documents will give errors, so you must always replace: • “the” →"the"

  12. Flex Example 3: Common Errors, cont'd • You discover such errors when you compile the C code, not when you use flex: • $ flex -o errors.yy.c errors.l • $ gcc -o errors errors.yy.c -lfl • errors.l: In function ‘yylex’: • errors.l:2: error: ‘the’ undeclared • ... • The error is reported back in our errors.l file, but we can also find it in errors.yy.c: case 1: YY_RULE_SETUP #line 2 "errors.l" <-- For error reporting the printf("the"); <-- the ? not C code YY_BREAK case 2:

  13. Flex Example 4: Replace Duplicate • Let's replace “the the” with “the”: • %% • "the the" printf("the"); • This time, it works: $ flex -o duplicate.yy.c duplicate.l $ gcc -o duplicate duplicate.yy.c -lfl $ ./duplicate This is the the file. This is the file. This is the the the file. This is the the file. Lathe theory Latheory

  14. Flex Example 4: Replace And Delete • Let's replace “the the” with “the” and delete “uhh”: • %% • "the the" printf("the"); • uhh • Run as before: This uhh is the the uhhh file. This is the h file. • Generally, lexical rules are pattern-action pairs: • %% • pattern1 action1 (C code) • pattern2 action2 • ... • Tokens almost never go across space chars as in "the the" above. • Regular expressions are often needed and used.

  15. Flex File Structure • In Lex and Flex, the general rule file structure is: • definitions • %% • rules • %% • user code • Definitions: • DIGIT [0-9] • ID [a-z][a-z0-9]* • can be used later in rules with {DIGIT}, {ID}, etc: • {DIGIT}+"."{DIGIT}* • This is the same as: • ([0-9])+"."([0-9])*

  16. Flex Example 5: Count Lines • int num_lines = 0, num_chars = 0; • %% • \n ++num_lines; ++num_chars; • . ++num_chars; • %% • main() • { • yylex(); • printf( "# of lines = %d, # of chars = %d\n", • num_lines, num_chars ); • }

  17. Some Regular Expressions for Flex • \"[^"]*\" string • "\t"|"\n"\" " whitespace (most common forms) • [a-zA-Z] • [a-zA-Z_][a-zA-Z0-9_]* identifier: allows a, aX, a45__ • [0-9]*"."[0-9]+ allows .5 but not 5. • [0-9]+"."[0-9]* allows 5. but not .5 • [0-9]*"."[0-9]* allows . by itself !!

  18. Resources • Aho, Lam, Sethi, and Ullman, Compilers: Principles, Techniques, and Tools, 2nd ed. Addison-Wesley, 2006. (The “purple dragon book”) • Flex Manual. Available as single postscript file at the Lex and Yacc page online: • http://dinosaur.compilertools.net/#flex • http://en.wikipedia.org/wiki/Flex_lexical_analyser

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