A Bison Manual. You build a text file of the production (format in the next section); traditionally this file ends in.y, although bison doesn t care.

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1 A Bison Manual 1 Overview Bison (and its predecessor yacc) is a tool that take a file of the productions for a context-free grammar and converts them into the tables for an LALR(1) parser. Bison produces a file that contains these tables and a routine int yyparse (void) that reads the tables to match an expansion of the start symbol. You do not need to know how to build the LALR(1) tables, although that can help you decode the errors that bison finds in your productions. This parser is a fairly powerful parser the technology is well established (yacc was developed in the mid 1970 s). One advantage is that you do not need to rewrite the production to remove left recursion, contrary to Recursive Descent which require no left recursion. An LALR(1) parser is a Shift/Reduce parser it shifts symbols (terminal and non-terminal) onto a stack until the right hand side of the proper production is on the top of the stack, when it takes that right hand side off the stack and pushes the corresponding left hand side of the production. The complexity is in building the Finite State Machine that tell you when the right hand side of the proper production to use is on the top of the stack. NB: this document describes bison producing a Yes/No parser to get the result to save the results of the parse (z.b.to produce a tree) we will need to add actions to the productions. 2 Running Bison You build a text file of the production (format in the next section) traditionally this file ends in.y, although bison doesn t care. You run bison by typing: bison filename. (For yacc, yacc filename.) If the input filename is x.y, bison produces a file called x.tab.c, which contains the table and the function int yyparse (). (For yacc, the output file is always y.tab.c.) There are several useful options to bison (and yacc): -d This make bison also produce a file x.tab.h which is used to make bison and flex agree on the number representation of the tokens. (In yacc the file is always y.tab.h.) -v This creates a file y.output which contains a description of the recognizer states and their transitions. This can be difficult to read (even if you know how to build the 1

2 states), but it can be useful to debug problems in your grammar. -y (bison only) This makes the output files always be y.tab.c and y.tab.h. (Filenames like yacc) You will always get a line on the screen if there are problems: Shift/Reduce or Reduce/Reduce conflicts. The file y.output is useful to find out what is wrong (but requires knowledge of how the states are generated). 3 The Format of the Input File The.y file is divided into 3 parts, separated by. I will describe them out of order. 3.1 Part 2: Productions The second section is the productions. A production is a left hand side, a colon (:), right hand side(s) separated by vertical bars ( ), and terminated by a semicolon (). E.g., expr : expr Plus term expr Minus term term Bison is free with white space you could put all the right sides on the same line you don t need spaces around the colon, bars, and semicolon. I think that this is a good style all the right hand sides all with the same indentation, so it is easy to read and understand. It is also my convention to name all the non-terminals with names in all lower case and to start the names of the token start with an upper case. I also give even the single character tokens names (like Plus above which is the single character Part 1: Definitions The first section of the.y file contains a variety of declarations used to interpret the productions. 2

3 We need the prototypes of any functions used in the actions. This part starts with %{ and ends with %} on a line by itself. Inbetween we will typically put #includes and protypes for the functions we define in part 3. We need to declare the Token names. We do this with a %token followed by a blank separated list of token names. If this gives too long a line (and probably will), you can more line starting with %token and a list of more token names. The set of non-terminals is the set of all symbols in all the productions minus the set of token names. We should declare the start symbol. We do this with a %start and the start non-terminal. There are other declarations that we will need when we add actions to our productions. 3.3 Part 3: C Code We put any C code (usually functions) that we feel that we need. This code is put at the end of the generated.tab.c file. 4 Dependance on Other Functions The code generated by bison assume that 2 functions exist. There must be a function int yylex (void) that returns another token from the input file. This is usually built by a tool called flex. The function yyparse also assumes that a function int yyerror (char *s) exists to print out error messages. It makes sense to include this function in the scanner input file, since the scanner can easily keep track of line numbers and position on a line. 5 Actions By default bison produces a yes/no parser either it accepts or not. Usually we need to do things as the parser runs: build representation, process macros, execute code [in a interactive language]. To do this we add actions to the productions. Actions are C code enclosed in braces { }. New (local to that action) declarations are also available. Actions at the end of a production are evaluated when the production is finished. It is legal to put actions in the middle of a production doing this can introduce Shift/Reduce or Reduce/Reduce conflict that didn t exist before. [I have rarely found a real need for these internal actions.] 3

4 To communicate information about the symbols to the actions, each symbol (non-terminals, tokens, even the internal action) can have a single value. You can choose a type for each symbol they do not all need to be the same type, and not all symbols need have values if one is not needed. 5.1 Additions to Section 1 of the Description File The set of possible value types is declared in the first section of the input file as a union of fields, one for every value type. You do this with a %union, an open brace, the types and a field identifier each terminated by a semicolon, and a closing brace. [This looks like a typical C union except for the opening %.] Bison will put the corresponding C union into the y.tab.h file. You must also tell bison the value type for each symbol that has one. For tokens you do this by putting the appropriate field identifier (not the actual type) encloed in angle brackets [< >] between the %token and the first of the tokens in its line. The value type applies to all the tokens in that line. For non-terminals you do this by putting in a line: %type, the field identifier inside angle brackets, and a blank separated list of the non-terminals whose value type is that of the field identifier. The value type applies to all the tokens in that line. If you do these things, bison turns on type checking that the value you give to symbols matches the types used for those symbols. [NB, the value of actions in the middle of a productions need to be handled a little differently, but this case is fairly rarely needed.] 5.2 Writing the Actions in Section 2 An action is C code enclosed in braces an action can use the values of any right-hand symbol to its left. For an action at the end of a production (the usual case) this means all the symbols on the right hand side of the production. To do this in each production we number the symbols (tokens, non-terminals, and internal actions) by positive integers the first symbol is 1, the second is 2. Do this whether a symbol has a value or not. In an actions we can get the value attached to a symbol to the left of the action by a $ notation: the value of the nth symbol in a production is represented by $n. Bison checks whether that symbol actually has a value. The result of an action is represented by $$ the value of the action at the end of the production becomes the value of the non-terminal on the left side of the production. All the $ constructs behave as ordinary variables. 4

5 An Example (with no actions) %{ #include <stdio.h> %} %token Ident IntConst %token While Do End %token Assign Plus Minus Star Slash Semicolon LPar RPar %start prog prog : stmts stmts : stmt stmt Semicolon stmts stmt : Ident Assign expr While expr Do stmts End expr : expr Plus term expr Minus term term term : term Star factor term Slash factor factor factor : Ident IntConst LPar expr RPar 5

6 An Example (with actions) %{ #include <stdio.h> #include "tree.h" extern tree root //declared in the driver program tree mktree (int lab, tree l, tree r) tree MkIntTree (int lab, int v) %} %token <i> Ident IntConst %token While Do End %token Assign Plus Minus Star Slash Semicolon LPar RPar %start prog %union {int i tree t} %type <t> stmts stmt expr term factor prog : stmts { root = $1 } stmts : stmt { $$ = $1 } stmt Semicolon stmts { $1->next = $3 $$ = $1 } stmt : Ident Assign expr { $$ = mktree (Assign, mkinttree (Ident, $1), $3 } While expr Do stmts End { $$ = mktree (While, $2, $4) } expr : expr Plus term { $$ = mktree (Plus, $1, $3) } expr Minus term { $$ = mktree (Minus, $1, $3) } term { $$ = $1 } term : term Star factor { $$ = mktree (Star, $1, $3) } term Slash factor { $$ = mktree (Slash, $1, $3) } factor { $$ = $1 } 6

7 factor : Ident { $$ = mkinttree (Ident, $1) } IntConst { $$ = mkinttree (IntConst, $1) } LPar expr RPar { $$ = $2 } tree mktree (int lab, tree l, tree r) { tree p = (tree) malloc (sizeof (tree_node)) p->label = lab // the node type p->left = l p->right = r // the node children p->next = NULL // the neighbor in a list return p } tree MkIntTree (int lab, int v) { tree p = (tree) malloc (sizeof (tree_node)) p->label = lab // the node type p->value = v // the specific value of this kind of node p->next = NULL // the neighbor in a list return p } 7

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