Ceci est le fichier Info bison.info, produit par Makeinfo version 4.0b à partir bison.texinfo. START-INFO-DIR-ENTRY * bison: (bison). GNU Project parser generator (yacc replacement). END-INFO-DIR-ENTRY This file documents the Bison parser generator. Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, 1999, 2000, 2001 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License" and "Conditions for Using Bison" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License", "Conditions for Using Bison" and this permission notice may be included in translations approved by the Free Software Foundation instead of in the original English.  File: bison.info, Node: Value Type, Next: Multiple Types, Up: Semantics Data Types of Semantic Values ----------------------------- In a simple program it may be sufficient to use the same data type for the semantic values of all language constructs. This was true in the RPN and infix calculator examples (*note Reverse Polish Notation Calculator: RPN Calc.). Bison's default is to use type `int' for all semantic values. To specify some other type, define `YYSTYPE' as a macro, like this: #define YYSTYPE double This macro definition must go in the C declarations section of the grammar file (*note Outline of a Bison Grammar: Grammar Outline.).  File: bison.info, Node: Multiple Types, Next: Actions, Prev: Value Type, Up: Semantics More Than One Value Type ------------------------ In most programs, you will need different data types for different kinds of tokens and groupings. For example, a numeric constant may need type `int' or `long', while a string constant needs type `char *', and an identifier might need a pointer to an entry in the symbol table. To use more than one data type for semantic values in one parser, Bison requires you to do two things: * Specify the entire collection of possible data types, with the `%union' Bison declaration (*note The Collection of Value Types: Union Decl.). * Choose one of those types for each symbol (terminal or nonterminal) for which semantic values are used. This is done for tokens with the `%token' Bison declaration (*note Token Type Names: Token Decl.) and for groupings with the `%type' Bison declaration (*note Nonterminal Symbols: Type Decl.).  File: bison.info, Node: Actions, Next: Action Types, Prev: Multiple Types, Up: Semantics Actions ------- An action accompanies a syntactic rule and contains C code to be executed each time an instance of that rule is recognized. The task of most actions is to compute a semantic value for the grouping built by the rule from the semantic values associated with tokens or smaller groupings. An action consists of C statements surrounded by braces, much like a compound statement in C. It can be placed at any position in the rule; it is executed at that position. Most rules have just one action at the end of the rule, following all the components. Actions in the middle of a rule are tricky and used only for special purposes (*note Actions in Mid-Rule: Mid-Rule Actions.). The C code in an action can refer to the semantic values of the components matched by the rule with the construct `$N', which stands for the value of the Nth component. The semantic value for the grouping being constructed is `$$'. (Bison translates both of these constructs into array element references when it copies the actions into the parser file.) Here is a typical example: exp: ... | exp '+' exp { $$ = $1 + $3; } This rule constructs an `exp' from two smaller `exp' groupings connected by a plus-sign token. In the action, `$1' and `$3' refer to the semantic values of the two component `exp' groupings, which are the first and third symbols on the right hand side of the rule. The sum is stored into `$$' so that it becomes the semantic value of the addition-expression just recognized by the rule. If there were a useful semantic value associated with the `+' token, it could be referred to as `$2'. If you don't specify an action for a rule, Bison supplies a default: `$$ = $1'. Thus, the value of the first symbol in the rule becomes the value of the whole rule. Of course, the default rule is valid only if the two data types match. There is no meaningful default action for an empty rule; every empty rule must have an explicit action unless the rule's value does not matter. `$N' with N zero or negative is allowed for reference to tokens and groupings on the stack _before_ those that match the current rule. This is a very risky practice, and to use it reliably you must be certain of the context in which the rule is applied. Here is a case in which you can use this reliably: foo: expr bar '+' expr { ... } | expr bar '-' expr { ... } ; bar: /* empty */ { previous_expr = $0; } ; As long as `bar' is used only in the fashion shown here, `$0' always refers to the `expr' which precedes `bar' in the definition of `foo'.  File: bison.info, Node: Action Types, Next: Mid-Rule Actions, Prev: Actions, Up: Semantics Data Types of Values in Actions ------------------------------- If you have chosen a single data type for semantic values, the `$$' and `$N' constructs always have that data type. If you have used `%union' to specify a variety of data types, then you must declare a choice among these types for each terminal or nonterminal symbol that can have a semantic value. Then each time you use `$$' or `$N', its data type is determined by which symbol it refers to in the rule. In this example, exp: ... | exp '+' exp { $$ = $1 + $3; } `$1' and `$3' refer to instances of `exp', so they all have the data type declared for the nonterminal symbol `exp'. If `$2' were used, it would have the data type declared for the terminal symbol `'+'', whatever that might be. Alternatively, you can specify the data type when you refer to the value, by inserting `' after the `$' at the beginning of the reference. For example, if you have defined types as shown here: %union { int itype; double dtype; } then you can write `$1' to refer to the first subunit of the rule as an integer, or `$1' to refer to it as a double.  File: bison.info, Node: Mid-Rule Actions, Prev: Action Types, Up: Semantics Actions in Mid-Rule ------------------- Occasionally it is useful to put an action in the middle of a rule. These actions are written just like usual end-of-rule actions, but they are executed before the parser even recognizes the following components. A mid-rule action may refer to the components preceding it using `$N', but it may not refer to subsequent components because it is run before they are parsed. The mid-rule action itself counts as one of the components of the rule. This makes a difference when there is another action later in the same rule (and usually there is another at the end): you have to count the actions along with the symbols when working out which number N to use in `$N'. The mid-rule action can also have a semantic value. The action can set its value with an assignment to `$$', and actions later in the rule can refer to the value using `$N'. Since there is no symbol to name the action, there is no way to declare a data type for the value in advance, so you must use the `$<...>N' construct to specify a data type each time you refer to this value. There is no way to set the value of the entire rule with a mid-rule action, because assignments to `$$' do not have that effect. The only way to set the value for the entire rule is with an ordinary action at the end of the rule. Here is an example from a hypothetical compiler, handling a `let' statement that looks like `let (VARIABLE) STATEMENT' and serves to create a variable named VARIABLE temporarily for the duration of STATEMENT. To parse this construct, we must put VARIABLE into the symbol table while STATEMENT is parsed, then remove it afterward. Here is how it is done: stmt: LET '(' var ')' { $$ = push_context (); declare_variable ($3); } stmt { $$ = $6; pop_context ($5); } As soon as `let (VARIABLE)' has been recognized, the first action is run. It saves a copy of the current semantic context (the list of accessible variables) as its semantic value, using alternative `context' in the data-type union. Then it calls `declare_variable' to add the new variable to that list. Once the first action is finished, the embedded statement `stmt' can be parsed. Note that the mid-rule action is component number 5, so the `stmt' is component number 6. After the embedded statement is parsed, its semantic value becomes the value of the entire `let'-statement. Then the semantic value from the earlier action is used to restore the prior list of variables. This removes the temporary `let'-variable from the list so that it won't appear to exist while the rest of the program is parsed. Taking action before a rule is completely recognized often leads to conflicts since the parser must commit to a parse in order to execute the action. For example, the following two rules, without mid-rule actions, can coexist in a working parser because the parser can shift the open-brace token and look at what follows before deciding whether there is a declaration or not: compound: '{' declarations statements '}' | '{' statements '}' ; But when we add a mid-rule action as follows, the rules become nonfunctional: compound: { prepare_for_local_variables (); } '{' declarations statements '}' | '{' statements '}' ; Now the parser is forced to decide whether to run the mid-rule action when it has read no farther than the open-brace. In other words, it must commit to using one rule or the other, without sufficient information to do it correctly. (The open-brace token is what is called the "look-ahead" token at this time, since the parser is still deciding what to do about it. *Note Look-Ahead Tokens: Look-Ahead.) You might think that you could correct the problem by putting identical actions into the two rules, like this: compound: { prepare_for_local_variables (); } '{' declarations statements '}' | { prepare_for_local_variables (); } '{' statements '}' ; But this does not help, because Bison does not realize that the two actions are identical. (Bison never tries to understand the C code in an action.) If the grammar is such that a declaration can be distinguished from a statement by the first token (which is true in C), then one solution which does work is to put the action after the open-brace, like this: compound: '{' { prepare_for_local_variables (); } declarations statements '}' | '{' statements '}' ; Now the first token of the following declaration or statement, which would in any case tell Bison which rule to use, can still do so. Another solution is to bury the action inside a nonterminal symbol which serves as a subroutine: subroutine: /* empty */ { prepare_for_local_variables (); } ; compound: subroutine '{' declarations statements '}' | subroutine '{' statements '}' ; Now Bison can execute the action in the rule for `subroutine' without deciding which rule for `compound' it will eventually use. Note that the action is now at the end of its rule. Any mid-rule action can be converted to an end-of-rule action in this way, and this is what Bison actually does to implement mid-rule actions.  File: bison.info, Node: Locations, Next: Declarations, Prev: Semantics, Up: Grammar File Tracking Locations ================== Though grammar rules and semantic actions are enough to write a fully functional parser, it can be useful to process some additionnal informations, especially symbol locations. The way locations are handled is defined by providing a data type, and actions to take when rules are matched. * Menu: * Location Type:: Specifying a data type for locations. * Actions and Locations:: Using locations in actions. * Location Default Action:: Defining a general way to compute locations.  File: bison.info, Node: Location Type, Next: Actions and Locations, Up: Locations Data Type of Locations ---------------------- Defining a data type for locations is much simpler than for semantic values, since all tokens and groupings always use the same type. The type of locations is specified by defining a macro called `YYLTYPE'. When `YYLTYPE' is not defined, Bison uses a default structure type with four members: struct { int first_line; int first_column; int last_line; int last_column; }  File: bison.info, Node: Actions and Locations, Next: Location Default Action, Prev: Location Type, Up: Locations Actions and Locations --------------------- Actions are not only useful for defining language semantics, but also for describing the behavior of the output parser with locations. The most obvious way for building locations of syntactic groupings is very similar to the way semantic values are computed. In a given rule, several constructs can be used to access the locations of the elements being matched. The location of the Nth component of the right hand side is `@N', while the location of the left hand side grouping is `@$'. Here is a basic example using the default data type for locations: exp: ... | exp '/' exp { @$.first_column = @1.first_column; @$.first_line = @1.first_line; @$.last_column = @3.last_column; @$.last_line = @3.last_line; if ($3) $$ = $1 / $3; else { $$ = 1; printf("Division by zero, l%d,c%d-l%d,c%d", @3.first_line, @3.first_column, @3.last_line, @3.last_column); } } As for semantic values, there is a default action for locations that is run each time a rule is matched. It sets the beginning of `@$' to the beginning of the first symbol, and the end of `@$' to the end of the last symbol. With this default action, the location tracking can be fully automatic. The example above simply rewrites this way: exp: ... | exp '/' exp { if ($3) $$ = $1 / $3; else { $$ = 1; printf("Division by zero, l%d,c%d-l%d,c%d", @3.first_line, @3.first_column, @3.last_line, @3.last_column); } }  File: bison.info, Node: Location Default Action, Prev: Actions and Locations, Up: Locations Default Action for Locations ---------------------------- Actually, actions are not the best place to compute locations. Since locations are much more general than semantic values, there is room in the output parser to redefine the default action to take for each rule. The `YYLLOC_DEFAULT' macro is called each time a rule is matched, before the associated action is run. Most of the time, this macro is general enough to suppress location dedicated code from semantic actions. The `YYLLOC_DEFAULT' macro takes three parameters. The first one is the location of the grouping (the result of the computation). The second one is an array holding locations of all right hand side elements of the rule being matched. The last one is the size of the right hand side rule. By default, it is defined this way: #define YYLLOC_DEFAULT(Current, Rhs, N) \ Current.last_line = Rhs[N].last_line; \ Current.last_column = Rhs[N].last_column; When defining `YYLLOC_DEFAULT', you should consider that: * All arguments are free of side-effects. However, only the first one (the result) should be modified by `YYLLOC_DEFAULT'. * Before `YYLLOC_DEFAULT' is executed, the output parser sets `@$' to `@1'. * For consistency with semantic actions, valid indexes for the location array range from 1 to N.  File: bison.info, Node: Declarations, Next: Multiple Parsers, Prev: Locations, Up: Grammar File Bison Declarations ================== The "Bison declarations" section of a Bison grammar defines the symbols used in formulating the grammar and the data types of semantic values. *Note Symbols::. All token type names (but not single-character literal tokens such as `'+'' and `'*'') must be declared. Nonterminal symbols must be declared if you need to specify which data type to use for the semantic value (*note More Than One Value Type: Multiple Types.). The first rule in the file also specifies the start symbol, by default. If you want some other symbol to be the start symbol, you must declare it explicitly (*note Languages and Context-Free Grammars: Language and Grammar.). * Menu: * Token Decl:: Declaring terminal symbols. * Precedence Decl:: Declaring terminals with precedence and associativity. * Union Decl:: Declaring the set of all semantic value types. * Type Decl:: Declaring the choice of type for a nonterminal symbol. * Expect Decl:: Suppressing warnings about shift/reduce conflicts. * Start Decl:: Specifying the start symbol. * Pure Decl:: Requesting a reentrant parser. * Decl Summary:: Table of all Bison declarations.  File: bison.info, Node: Token Decl, Next: Precedence Decl, Up: Declarations Token Type Names ---------------- The basic way to declare a token type name (terminal symbol) is as follows: %token NAME Bison will convert this into a `#define' directive in the parser, so that the function `yylex' (if it is in this file) can use the name NAME to stand for this token type's code. Alternatively, you can use `%left', `%right', or `%nonassoc' instead of `%token', if you wish to specify associativity and precedence. *Note Operator Precedence: Precedence Decl. You can explicitly specify the numeric code for a token type by appending an integer value in the field immediately following the token name: %token NUM 300 It is generally best, however, to let Bison choose the numeric codes for all token types. Bison will automatically select codes that don't conflict with each other or with ASCII characters. In the event that the stack type is a union, you must augment the `%token' or other token declaration to include the data type alternative delimited by angle-brackets (*note More Than One Value Type: Multiple Types.). For example: %union { /* define stack type */ double val; symrec *tptr; } %token NUM /* define token NUM and its type */ You can associate a literal string token with a token type name by writing the literal string at the end of a `%token' declaration which declares the name. For example: %token arrow "=>" For example, a grammar for the C language might specify these names with equivalent literal string tokens: %token OR "||" %token LE 134 "<=" %left OR "<=" Once you equate the literal string and the token name, you can use them interchangeably in further declarations or the grammar rules. The `yylex' function can use the token name or the literal string to obtain the token type code number (*note Calling Convention::).  File: bison.info, Node: Precedence Decl, Next: Union Decl, Prev: Token Decl, Up: Declarations Operator Precedence ------------------- Use the `%left', `%right' or `%nonassoc' declaration to declare a token and specify its precedence and associativity, all at once. These are called "precedence declarations". *Note Operator Precedence: Precedence, for general information on operator precedence. The syntax of a precedence declaration is the same as that of `%token': either %left SYMBOLS... or %left SYMBOLS... And indeed any of these declarations serves the purposes of `%token'. But in addition, they specify the associativity and relative precedence for all the SYMBOLS: * The associativity of an operator OP determines how repeated uses of the operator nest: whether `X OP Y OP Z' is parsed by grouping X with Y first or by grouping Y with Z first. `%left' specifies left-associativity (grouping X with Y first) and `%right' specifies right-associativity (grouping Y with Z first). `%nonassoc' specifies no associativity, which means that `X OP Y OP Z' is considered a syntax error. * The precedence of an operator determines how it nests with other operators. All the tokens declared in a single precedence declaration have equal precedence and nest together according to their associativity. When two tokens declared in different precedence declarations associate, the one declared later has the higher precedence and is grouped first.  File: bison.info, Node: Union Decl, Next: Type Decl, Prev: Precedence Decl, Up: Declarations The Collection of Value Types ----------------------------- The `%union' declaration specifies the entire collection of possible data types for semantic values. The keyword `%union' is followed by a pair of braces containing the same thing that goes inside a `union' in C. For example: %union { double val; symrec *tptr; } This says that the two alternative types are `double' and `symrec *'. They are given names `val' and `tptr'; these names are used in the `%token' and `%type' declarations to pick one of the types for a terminal or nonterminal symbol (*note Nonterminal Symbols: Type Decl.). Note that, unlike making a `union' declaration in C, you do not write a semicolon after the closing brace.  File: bison.info, Node: Type Decl, Next: Expect Decl, Prev: Union Decl, Up: Declarations Nonterminal Symbols ------------------- When you use `%union' to specify multiple value types, you must declare the value type of each nonterminal symbol for which values are used. This is done with a `%type' declaration, like this: %type NONTERMINAL... Here NONTERMINAL is the name of a nonterminal symbol, and TYPE is the name given in the `%union' to the alternative that you want (*note The Collection of Value Types: Union Decl.). You can give any number of nonterminal symbols in the same `%type' declaration, if they have the same value type. Use spaces to separate the symbol names. You can also declare the value type of a terminal symbol. To do this, use the same `' construction in a declaration for the terminal symbol. All kinds of token declarations allow `'.  File: bison.info, Node: Expect Decl, Next: Start Decl, Prev: Type Decl, Up: Declarations Suppressing Conflict Warnings ----------------------------- Bison normally warns if there are any conflicts in the grammar (*note Shift/Reduce Conflicts: Shift/Reduce.), but most real grammars have harmless shift/reduce conflicts which are resolved in a predictable way and would be difficult to eliminate. It is desirable to suppress the warning about these conflicts unless the number of conflicts changes. You can do this with the `%expect' declaration. The declaration looks like this: %expect N Here N is a decimal integer. The declaration says there should be no warning if there are N shift/reduce conflicts and no reduce/reduce conflicts. An error, instead of the usual warning, is given if there are either more or fewer conflicts, or if there are any reduce/reduce conflicts. In general, using `%expect' involves these steps: * Compile your grammar without `%expect'. Use the `-v' option to get a verbose list of where the conflicts occur. Bison will also print the number of conflicts. * Check each of the conflicts to make sure that Bison's default resolution is what you really want. If not, rewrite the grammar and go back to the beginning. * Add an `%expect' declaration, copying the number N from the number which Bison printed. Now Bison will stop annoying you about the conflicts you have checked, but it will warn you again if changes in the grammar result in additional conflicts.  File: bison.info, Node: Start Decl, Next: Pure Decl, Prev: Expect Decl, Up: Declarations The Start-Symbol ---------------- Bison assumes by default that the start symbol for the grammar is the first nonterminal specified in the grammar specification section. The programmer may override this restriction with the `%start' declaration as follows: %start SYMBOL  File: bison.info, Node: Pure Decl, Next: Decl Summary, Prev: Start Decl, Up: Declarations A Pure (Reentrant) Parser ------------------------- A "reentrant" program is one which does not alter in the course of execution; in other words, it consists entirely of "pure" (read-only) code. Reentrancy is important whenever asynchronous execution is possible; for example, a non-reentrant program may not be safe to call from a signal handler. In systems with multiple threads of control, a non-reentrant program must be called only within interlocks. Normally, Bison generates a parser which is not reentrant. This is suitable for most uses, and it permits compatibility with YACC. (The standard YACC interfaces are inherently nonreentrant, because they use statically allocated variables for communication with `yylex', including `yylval' and `yylloc'.) Alternatively, you can generate a pure, reentrant parser. The Bison declaration `%pure_parser' says that you want the parser to be reentrant. It looks like this: %pure_parser The result is that the communication variables `yylval' and `yylloc' become local variables in `yyparse', and a different calling convention is used for the lexical analyzer function `yylex'. *Note Calling Conventions for Pure Parsers: Pure Calling, for the details of this. The variable `yynerrs' also becomes local in `yyparse' (*note The Error Reporting Function `yyerror': Error Reporting.). The convention for calling `yyparse' itself is unchanged. Whether the parser is pure has nothing to do with the grammar rules. You can generate either a pure parser or a nonreentrant parser from any valid grammar.  File: bison.info, Node: Decl Summary, Prev: Pure Decl, Up: Declarations Bison Declaration Summary ------------------------- Here is a summary of the declarations used to define a grammar: `%union' Declare the collection of data types that semantic values may have (*note The Collection of Value Types: Union Decl.). `%token' Declare a terminal symbol (token type name) with no precedence or associativity specified (*note Token Type Names: Token Decl.). `%right' Declare a terminal symbol (token type name) that is right-associative (*note Operator Precedence: Precedence Decl.). `%left' Declare a terminal symbol (token type name) that is left-associative (*note Operator Precedence: Precedence Decl.). `%nonassoc' Declare a terminal symbol (token type name) that is nonassociative (using it in a way that would be associative is a syntax error) (*note Operator Precedence: Precedence Decl.). `%type' Declare the type of semantic values for a nonterminal symbol (*note Nonterminal Symbols: Type Decl.). `%start' Specify the grammar's start symbol (*note The Start-Symbol: Start Decl.). `%expect' Declare the expected number of shift-reduce conflicts (*note Suppressing Conflict Warnings: Expect Decl.). In order to change the behavior of `bison', use the following directives: `%debug' Output a definition of the macro `YYDEBUG' into the parser file, so that the debugging facilities are compiled. *Note Debugging Your Parser: Debugging. `%defines' Write an extra output file containing macro definitions for the token type names defined in the grammar and the semantic value type `YYSTYPE', as well as a few `extern' variable declarations. If the parser output file is named `NAME.c' then this file is named `NAME.h'. This output file is essential if you wish to put the definition of `yylex' in a separate source file, because `yylex' needs to be able to refer to token type codes and the variable `yylval'. *Note Semantic Values of Tokens: Token Values. `%file-prefix="PREFIX"' Specify a prefix to use for all Bison output file names. The names are chosen as if the input file were named `PREFIX.y'. `%locations' Generate the code processing the locations (*note Special Features for Use in Actions: Action Features.). This mode is enabled as soon as the grammar uses the special `@N' tokens, but if your grammar does not use it, using `%locations' allows for more accurate parse error messages. `%name-prefix="PREFIX"' Rename the external symbols used in the parser so that they start with PREFIX instead of `yy'. The precise list of symbols renamed is `yyparse', `yylex', `yyerror', `yynerrs', `yylval', `yychar' and `yydebug'. For example, if you use `%name-prefix="c_"', the names become `c_parse', `c_lex', and so on. *Note Multiple Parsers in the Same Program: Multiple Parsers. `%no-parser' Do not include any C code in the parser file; generate tables only. The parser file contains just `#define' directives and static variable declarations. This option also tells Bison to write the C code for the grammar actions into a file named `FILENAME.act', in the form of a brace-surrounded body fit for a `switch' statement. `%no-lines' Don't generate any `#line' preprocessor commands in the parser file. Ordinarily Bison writes these commands in the parser file so that the C compiler and debuggers will associate errors and object code with your source file (the grammar file). This directive causes them to associate errors with the parser file, treating it an independent source file in its own right. `%output="FILENAME"' Specify the FILENAME for the parser file. `%pure-parser' Request a pure (reentrant) parser program (*note A Pure (Reentrant) Parser: Pure Decl.). `%token_table' Generate an array of token names in the parser file. The name of the array is `yytname'; `yytname[I]' is the name of the token whose internal Bison token code number is I. The first three elements of `yytname' are always `"$"', `"error"', and `"$illegal"'; after these come the symbols defined in the grammar file. For single-character literal tokens and literal string tokens, the name in the table includes the single-quote or double-quote characters: for example, `"'+'"' is a single-character literal and `"\"<=\""' is a literal string token. All the characters of the literal string token appear verbatim in the string found in the table; even double-quote characters are not escaped. For example, if the token consists of three characters `*"*', its string in `yytname' contains `"*"*"'. (In C, that would be written as `"\"*\"*\""'). When you specify `%token_table', Bison also generates macro definitions for macros `YYNTOKENS', `YYNNTS', and `YYNRULES', and `YYNSTATES': `YYNTOKENS' The highest token number, plus one. `YYNNTS' The number of nonterminal symbols. `YYNRULES' The number of grammar rules, `YYNSTATES' The number of parser states (*note Parser States::). `%verbose' Write an extra output file containing verbose descriptions of the parser states and what is done for each type of look-ahead token in that state. This file also describes all the conflicts, both those resolved by operator precedence and the unresolved ones. The file's name is made by removing `.tab.c' or `.c' from the parser output file name, and adding `.output' instead. Therefore, if the input file is `foo.y', then the parser file is called `foo.tab.c' by default. As a consequence, the verbose output file is called `foo.output'. `%yacc' `%fixed-output-files' Pretend the option `--yacc' was given, i.e., imitate Yacc, including its naming conventions. *Note Bison Options::, for more.  File: bison.info, Node: Multiple Parsers, Prev: Declarations, Up: Grammar File Multiple Parsers in the Same Program ==================================== Most programs that use Bison parse only one language and therefore contain only one Bison parser. But what if you want to parse more than one language with the same program? Then you need to avoid a name conflict between different definitions of `yyparse', `yylval', and so on. The easy way to do this is to use the option `-p PREFIX' (*note Invoking Bison: Invocation.). This renames the interface functions and variables of the Bison parser to start with PREFIX instead of `yy'. You can use this to give each parser distinct names that do not conflict. The precise list of symbols renamed is `yyparse', `yylex', `yyerror', `yynerrs', `yylval', `yychar' and `yydebug'. For example, if you use `-p c', the names become `cparse', `clex', and so on. *All the other variables and macros associated with Bison are not renamed.* These others are not global; there is no conflict if the same name is used in different parsers. For example, `YYSTYPE' is not renamed, but defining this in different ways in different parsers causes no trouble (*note Data Types of Semantic Values: Value Type.). The `-p' option works by adding macro definitions to the beginning of the parser source file, defining `yyparse' as `PREFIXparse', and so on. This effectively substitutes one name for the other in the entire parser file.  File: bison.info, Node: Interface, Next: Algorithm, Prev: Grammar File, Up: Top Parser C-Language Interface *************************** The Bison parser is actually a C function named `yyparse'. Here we describe the interface conventions of `yyparse' and the other functions that it needs to use. Keep in mind that the parser uses many C identifiers starting with `yy' and `YY' for internal purposes. If you use such an identifier (aside from those in this manual) in an action or in additional C code in the grammar file, you are likely to run into trouble. * Menu: * Parser Function:: How to call `yyparse' and what it returns. * Lexical:: You must supply a function `yylex' which reads tokens. * Error Reporting:: You must supply a function `yyerror'. * Action Features:: Special features for use in actions.  File: bison.info, Node: Parser Function, Next: Lexical, Up: Interface The Parser Function `yyparse' ============================= You call the function `yyparse' to cause parsing to occur. This function reads tokens, executes actions, and ultimately returns when it encounters end-of-input or an unrecoverable syntax error. You can also write an action which directs `yyparse' to return immediately without reading further. The value returned by `yyparse' is 0 if parsing was successful (return is due to end-of-input). The value is 1 if parsing failed (return is due to a syntax error). In an action, you can cause immediate return from `yyparse' by using these macros: `YYACCEPT' Return immediately with value 0 (to report success). `YYABORT' Return immediately with value 1 (to report failure).  File: bison.info, Node: Lexical, Next: Error Reporting, Prev: Parser Function, Up: Interface The Lexical Analyzer Function `yylex' ===================================== The "lexical analyzer" function, `yylex', recognizes tokens from the input stream and returns them to the parser. Bison does not create this function automatically; you must write it so that `yyparse' can call it. The function is sometimes referred to as a lexical scanner. In simple programs, `yylex' is often defined at the end of the Bison grammar file. If `yylex' is defined in a separate source file, you need to arrange for the token-type macro definitions to be available there. To do this, use the `-d' option when you run Bison, so that it will write these macro definitions into a separate header file `NAME.tab.h' which you can include in the other source files that need it. *Note Invoking Bison: Invocation. * Menu: * Calling Convention:: How `yyparse' calls `yylex'. * Token Values:: How `yylex' must return the semantic value of the token it has read. * Token Positions:: How `yylex' must return the text position (line number, etc.) of the token, if the actions want that. * Pure Calling:: How the calling convention differs in a pure parser (*note A Pure (Reentrant) Parser: Pure Decl.).  File: bison.info, Node: Calling Convention, Next: Token Values, Up: Lexical Calling Convention for `yylex' ------------------------------ The value that `yylex' returns must be the numeric code for the type of token it has just found, or 0 for end-of-input. When a token is referred to in the grammar rules by a name, that name in the parser file becomes a C macro whose definition is the proper numeric code for that token type. So `yylex' can use the name to indicate that type. *Note Symbols::. When a token is referred to in the grammar rules by a character literal, the numeric code for that character is also the code for the token type. So `yylex' can simply return that character code. The null character must not be used this way, because its code is zero and that is what signifies end-of-input. Here is an example showing these things: int yylex (void) { ... if (c == EOF) /* Detect end of file. */ return 0; ... if (c == '+' || c == '-') return c; /* Assume token type for `+' is '+'. */ ... return INT; /* Return the type of the token. */ ... } This interface has been designed so that the output from the `lex' utility can be used without change as the definition of `yylex'. If the grammar uses literal string tokens, there are two ways that `yylex' can determine the token type codes for them: * If the grammar defines symbolic token names as aliases for the literal string tokens, `yylex' can use these symbolic names like all others. In this case, the use of the literal string tokens in the grammar file has no effect on `yylex'. * `yylex' can find the multicharacter token in the `yytname' table. The index of the token in the table is the token type's code. The name of a multicharacter token is recorded in `yytname' with a double-quote, the token's characters, and another double-quote. The token's characters are not escaped in any way; they appear verbatim in the contents of the string in the table. Here's code for looking up a token in `yytname', assuming that the characters of the token are stored in `token_buffer'. for (i = 0; i < YYNTOKENS; i++) { if (yytname[i] != 0 && yytname[i][0] == '"' && strncmp (yytname[i] + 1, token_buffer, strlen (token_buffer)) && yytname[i][strlen (token_buffer) + 1] == '"' && yytname[i][strlen (token_buffer) + 2] == 0) break; } The `yytname' table is generated only if you use the `%token_table' declaration. *Note Decl Summary::.  File: bison.info, Node: Token Values, Next: Token Positions, Prev: Calling Convention, Up: Lexical Semantic Values of Tokens ------------------------- In an ordinary (non-reentrant) parser, the semantic value of the token must be stored into the global variable `yylval'. When you are using just one data type for semantic values, `yylval' has that type. Thus, if the type is `int' (the default), you might write this in `yylex': ... yylval = value; /* Put value onto Bison stack. */ return INT; /* Return the type of the token. */ ... When you are using multiple data types, `yylval''s type is a union made from the `%union' declaration (*note The Collection of Value Types: Union Decl.). So when you store a token's value, you must use the proper member of the union. If the `%union' declaration looks like this: %union { int intval; double val; symrec *tptr; } then the code in `yylex' might look like this: ... yylval.intval = value; /* Put value onto Bison stack. */ return INT; /* Return the type of the token. */ ...  File: bison.info, Node: Token Positions, Next: Pure Calling, Prev: Token Values, Up: Lexical Textual Positions of Tokens --------------------------- If you are using the `@N'-feature (*note Tracking Locations: Locations.) in actions to keep track of the textual locations of tokens and groupings, then you must provide this information in `yylex'. The function `yyparse' expects to find the textual location of a token just parsed in the global variable `yylloc'. So `yylex' must store the proper data in that variable. By default, the value of `yylloc' is a structure and you need only initialize the members that are going to be used by the actions. The four members are called `first_line', `first_column', `last_line' and `last_column'. Note that the use of this feature makes the parser noticeably slower. The data type of `yylloc' has the name `YYLTYPE'.  File: bison.info, Node: Pure Calling, Prev: Token Positions, Up: Lexical Calling Conventions for Pure Parsers ------------------------------------ When you use the Bison declaration `%pure_parser' to request a pure, reentrant parser, the global communication variables `yylval' and `yylloc' cannot be used. (*Note A Pure (Reentrant) Parser: Pure Decl.) In such parsers the two global variables are replaced by pointers passed as arguments to `yylex'. You must declare them as shown here, and pass the information back by storing it through those pointers. int yylex (YYSTYPE *lvalp, YYLTYPE *llocp) { ... *lvalp = value; /* Put value onto Bison stack. */ return INT; /* Return the type of the token. */ ... } If the grammar file does not use the `@' constructs to refer to textual positions, then the type `YYLTYPE' will not be defined. In this case, omit the second argument; `yylex' will be called with only one argument. If you use a reentrant parser, you can optionally pass additional parameter information to it in a reentrant way. To do so, define the macro `YYPARSE_PARAM' as a variable name. This modifies the `yyparse' function to accept one argument, of type `void *', with that name. When you call `yyparse', pass the address of an object, casting the address to `void *'. The grammar actions can refer to the contents of the object by casting the pointer value back to its proper type and then dereferencing it. Here's an example. Write this in the parser: %{ struct parser_control { int nastiness; int randomness; }; #define YYPARSE_PARAM parm %} Then call the parser like this: struct parser_control { int nastiness; int randomness; }; ... { struct parser_control foo; ... /* Store proper data in `foo'. */ value = yyparse ((void *) &foo); ... } In the grammar actions, use expressions like this to refer to the data: ((struct parser_control *) parm)->randomness If you wish to pass the additional parameter data to `yylex', define the macro `YYLEX_PARAM' just like `YYPARSE_PARAM', as shown here: %{ struct parser_control { int nastiness; int randomness; }; #define YYPARSE_PARAM parm #define YYLEX_PARAM parm %} You should then define `yylex' to accept one additional argument--the value of `parm'. (This makes either two or three arguments in total, depending on whether an argument of type `YYLTYPE' is passed.) You can declare the argument as a pointer to the proper object type, or you can declare it as `void *' and access the contents as shown above. You can use `%pure_parser' to request a reentrant parser without also using `YYPARSE_PARAM'. Then you should call `yyparse' with no arguments, as usual.  File: bison.info, Node: Error Reporting, Next: Action Features, Prev: Lexical, Up: Interface The Error Reporting Function `yyerror' ====================================== The Bison parser detects a "parse error" or "syntax error" whenever it reads a token which cannot satisfy any syntax rule. An action in the grammar can also explicitly proclaim an error, using the macro `YYERROR' (*note Special Features for Use in Actions: Action Features.). The Bison parser expects to report the error by calling an error reporting function named `yyerror', which you must supply. It is called by `yyparse' whenever a syntax error is found, and it receives one argument. For a parse error, the string is normally `"parse error"'. If you define the macro `YYERROR_VERBOSE' in the Bison declarations section (*note The Bison Declarations Section: Bison Declarations.), then Bison provides a more verbose and specific error message string instead of just plain `"parse error"'. It doesn't matter what definition you use for `YYERROR_VERBOSE', just whether you define it. The parser can detect one other kind of error: stack overflow. This happens when the input contains constructions that are very deeply nested. It isn't likely you will encounter this, since the Bison parser extends its stack automatically up to a very large limit. But if overflow happens, `yyparse' calls `yyerror' in the usual fashion, except that the argument string is `"parser stack overflow"'. The following definition suffices in simple programs: void yyerror (char *s) { fprintf (stderr, "%s\n", s); } After `yyerror' returns to `yyparse', the latter will attempt error recovery if you have written suitable error recovery grammar rules (*note Error Recovery::). If recovery is impossible, `yyparse' will immediately return 1. The variable `yynerrs' contains the number of syntax errors encountered so far. Normally this variable is global; but if you request a pure parser (*note A Pure (Reentrant) Parser: Pure Decl.) then it is a local variable which only the actions can access.