This is bison.info, produced by makeinfo version 4.1 from 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, 89, 90, 91, 92, 93, 95, 98, 1999 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: Rpcalc Rules, Next: Rpcalc Lexer, Prev: Rpcalc Decls, Up: RPN Calc Grammar Rules for `rpcalc' -------------------------- Here are the grammar rules for the reverse polish notation calculator. input: /* empty */ | input line ; line: '\n' | exp '\n' { printf ("\t%.10g\n", $1); } ; exp: NUM { $$ = $1; } | exp exp '+' { $$ = $1 + $2; } | exp exp '-' { $$ = $1 - $2; } | exp exp '*' { $$ = $1 * $2; } | exp exp '/' { $$ = $1 / $2; } /* Exponentiation */ | exp exp '^' { $$ = pow ($1, $2); } /* Unary minus */ | exp 'n' { $$ = -$1; } ; %% The groupings of the rpcalc "language" defined here are the expression (given the name `exp'), the line of input (`line'), and the complete input transcript (`input'). Each of these nonterminal symbols has several alternate rules, joined by the `|' punctuator which is read as "or". The following sections explain what these rules mean. The semantics of the language is determined by the actions taken when a grouping is recognized. The actions are the C code that appears inside braces. *Note Actions::. You must specify these actions in C, but Bison provides the means for passing semantic values between the rules. In each action, the pseudo-variable `$$' stands for the semantic value for the grouping that the rule is going to construct. Assigning a value to `$$' is the main job of most actions. The semantic values of the components of the rule are referred to as `$1', `$2', and so on. * Menu: * Rpcalc Input:: * Rpcalc Line:: * Rpcalc Expr::  File: bison.info, Node: Rpcalc Input, Next: Rpcalc Line, Up: Rpcalc Rules Explanation of `input' ...................... Consider the definition of `input': input: /* empty */ | input line ; This definition reads as follows: "A complete input is either an empty string, or a complete input followed by an input line". Notice that "complete input" is defined in terms of itself. This definition is said to be "left recursive" since `input' appears always as the leftmost symbol in the sequence. *Note Recursive Rules: Recursion. The first alternative is empty because there are no symbols between the colon and the first `|'; this means that `input' can match an empty string of input (no tokens). We write the rules this way because it is legitimate to type `Ctrl-d' right after you start the calculator. It's conventional to put an empty alternative first and write the comment `/* empty */' in it. The second alternate rule (`input line') handles all nontrivial input. It means, "After reading any number of lines, read one more line if possible." The left recursion makes this rule into a loop. Since the first alternative matches empty input, the loop can be executed zero or more times. The parser function `yyparse' continues to process input until a grammatical error is seen or the lexical analyzer says there are no more input tokens; we will arrange for the latter to happen at end of file.  File: bison.info, Node: Rpcalc Line, Next: Rpcalc Expr, Prev: Rpcalc Input, Up: Rpcalc Rules Explanation of `line' ..................... Now consider the definition of `line': line: '\n' | exp '\n' { printf ("\t%.10g\n", $1); } ; The first alternative is a token which is a newline character; this means that rpcalc accepts a blank line (and ignores it, since there is no action). The second alternative is an expression followed by a newline. This is the alternative that makes rpcalc useful. The semantic value of the `exp' grouping is the value of `$1' because the `exp' in question is the first symbol in the alternative. The action prints this value, which is the result of the computation the user asked for. This action is unusual because it does not assign a value to `$$'. As a consequence, the semantic value associated with the `line' is uninitialized (its value will be unpredictable). This would be a bug if that value were ever used, but we don't use it: once rpcalc has printed the value of the user's input line, that value is no longer needed.  File: bison.info, Node: Rpcalc Expr, Prev: Rpcalc Line, Up: Rpcalc Rules Explanation of `expr' ..................... The `exp' grouping has several rules, one for each kind of expression. The first rule handles the simplest expressions: those that are just numbers. The second handles an addition-expression, which looks like two expressions followed by a plus-sign. The third handles subtraction, and so on. exp: NUM | exp exp '+' { $$ = $1 + $2; } | exp exp '-' { $$ = $1 - $2; } ... ; We have used `|' to join all the rules for `exp', but we could equally well have written them separately: exp: NUM ; exp: exp exp '+' { $$ = $1 + $2; } ; exp: exp exp '-' { $$ = $1 - $2; } ; ... Most of the rules have actions that compute the value of the expression in terms of the value of its parts. For example, in the rule for addition, `$1' refers to the first component `exp' and `$2' refers to the second one. The third component, `'+'', has no meaningful associated semantic value, but if it had one you could refer to it as `$3'. When `yyparse' recognizes a sum expression using this rule, the sum of the two subexpressions' values is produced as the value of the entire expression. *Note Actions::. You don't have to give an action for every rule. When a rule has no action, Bison by default copies the value of `$1' into `$$'. This is what happens in the first rule (the one that uses `NUM'). The formatting shown here is the recommended convention, but Bison does not require it. You can add or change whitespace as much as you wish. For example, this: exp : NUM | exp exp '+' {$$ = $1 + $2; } | ... means the same thing as this: exp: NUM | exp exp '+' { $$ = $1 + $2; } | ... The latter, however, is much more readable.  File: bison.info, Node: Rpcalc Lexer, Next: Rpcalc Main, Prev: Rpcalc Rules, Up: RPN Calc The `rpcalc' Lexical Analyzer ----------------------------- The lexical analyzer's job is low-level parsing: converting characters or sequences of characters into tokens. The Bison parser gets its tokens by calling the lexical analyzer. *Note The Lexical Analyzer Function `yylex': Lexical. Only a simple lexical analyzer is needed for the RPN calculator. This lexical analyzer skips blanks and tabs, then reads in numbers as `double' and returns them as `NUM' tokens. Any other character that isn't part of a number is a separate token. Note that the token-code for such a single-character token is the character itself. The return value of the lexical analyzer function is a numeric code which represents a token type. The same text used in Bison rules to stand for this token type is also a C expression for the numeric code for the type. This works in two ways. If the token type is a character literal, then its numeric code is the ASCII code for that character; you can use the same character literal in the lexical analyzer to express the number. If the token type is an identifier, that identifier is defined by Bison as a C macro whose definition is the appropriate number. In this example, therefore, `NUM' becomes a macro for `yylex' to use. The semantic value of the token (if it has one) is stored into the global variable `yylval', which is where the Bison parser will look for it. (The C data type of `yylval' is `YYSTYPE', which was defined at the beginning of the grammar; *note Declarations for `rpcalc': Rpcalc Decls..) A token type code of zero is returned if the end-of-file is encountered. (Bison recognizes any nonpositive value as indicating the end of the input.) Here is the code for the lexical analyzer: /* Lexical analyzer returns a double floating point number on the stack and the token NUM, or the ASCII character read if not a number. Skips all blanks and tabs, returns 0 for EOF. */ #include yylex () { int c; /* skip white space */ while ((c = getchar ()) == ' ' || c == '\t') ; /* process numbers */ if (c == '.' || isdigit (c)) { ungetc (c, stdin); scanf ("%lf", &yylval); return NUM; } /* return end-of-file */ if (c == EOF) return 0; /* return single chars */ return c; }  File: bison.info, Node: Rpcalc Main, Next: Rpcalc Error, Prev: Rpcalc Lexer, Up: RPN Calc The Controlling Function ------------------------ In keeping with the spirit of this example, the controlling function is kept to the bare minimum. The only requirement is that it call `yyparse' to start the process of parsing. main () { yyparse (); }  File: bison.info, Node: Rpcalc Error, Next: Rpcalc Gen, Prev: Rpcalc Main, Up: RPN Calc The Error Reporting Routine --------------------------- When `yyparse' detects a syntax error, it calls the error reporting function `yyerror' to print an error message (usually but not always `"parse error"'). It is up to the programmer to supply `yyerror' (*note Parser C-Language Interface: Interface.), so here is the definition we will use: #include yyerror (s) /* Called by yyparse on error */ char *s; { printf ("%s\n", s); } After `yyerror' returns, the Bison parser may recover from the error and continue parsing if the grammar contains a suitable error rule (*note Error Recovery::). Otherwise, `yyparse' returns nonzero. We have not written any error rules in this example, so any invalid input will cause the calculator program to exit. This is not clean behavior for a real calculator, but it is adequate in the first example.  File: bison.info, Node: Rpcalc Gen, Next: Rpcalc Compile, Prev: Rpcalc Error, Up: RPN Calc Running Bison to Make the Parser -------------------------------- Before running Bison to produce a parser, we need to decide how to arrange all the source code in one or more source files. For such a simple example, the easiest thing is to put everything in one file. The definitions of `yylex', `yyerror' and `main' go at the end, in the "additional C code" section of the file (*note The Overall Layout of a Bison Grammar: Grammar Layout.). For a large project, you would probably have several source files, and use `make' to arrange to recompile them. With all the source in a single file, you use the following command to convert it into a parser file: bison FILE_NAME.y In this example the file was called `rpcalc.y' (for "Reverse Polish CALCulator"). Bison produces a file named `FILE_NAME.tab.c', removing the `.y' from the original file name. The file output by Bison contains the source code for `yyparse'. The additional functions in the input file (`yylex', `yyerror' and `main') are copied verbatim to the output.  File: bison.info, Node: Rpcalc Compile, Prev: Rpcalc Gen, Up: RPN Calc Compiling the Parser File ------------------------- Here is how to compile and run the parser file: # List files in current directory. % ls rpcalc.tab.c rpcalc.y # Compile the Bison parser. # `-lm' tells compiler to search math library for `pow'. % cc rpcalc.tab.c -lm -o rpcalc # List files again. % ls rpcalc rpcalc.tab.c rpcalc.y The file `rpcalc' now contains the executable code. Here is an example session using `rpcalc'. % rpcalc 4 9 + 13 3 7 + 3 4 5 *+- -13 3 7 + 3 4 5 * + - n Note the unary minus, `n' 13 5 6 / 4 n + -3.166666667 3 4 ^ Exponentiation 81 ^D End-of-file indicator %  File: bison.info, Node: Infix Calc, Next: Simple Error Recovery, Prev: RPN Calc, Up: Examples Infix Notation Calculator: `calc' ================================= We now modify rpcalc to handle infix operators instead of postfix. Infix notation involves the concept of operator precedence and the need for parentheses nested to arbitrary depth. Here is the Bison code for `calc.y', an infix desk-top calculator. /* Infix notation calculator--calc */ %{ #define YYSTYPE double #include %} /* BISON Declarations */ %token NUM %left '-' '+' %left '*' '/' %left NEG /* negation--unary minus */ %right '^' /* exponentiation */ /* Grammar follows */ %% input: /* empty string */ | input line ; line: '\n' | exp '\n' { printf ("\t%.10g\n", $1); } ; exp: NUM { $$ = $1; } | exp '+' exp { $$ = $1 + $3; } | exp '-' exp { $$ = $1 - $3; } | exp '*' exp { $$ = $1 * $3; } | exp '/' exp { $$ = $1 / $3; } | '-' exp %prec NEG { $$ = -$2; } | exp '^' exp { $$ = pow ($1, $3); } | '(' exp ')' { $$ = $2; } ; %% The functions `yylex', `yyerror' and `main' can be the same as before. There are two important new features shown in this code. In the second section (Bison declarations), `%left' declares token types and says they are left-associative operators. The declarations `%left' and `%right' (right associativity) take the place of `%token' which is used to declare a token type name without associativity. (These tokens are single-character literals, which ordinarily don't need to be declared. We declare them here to specify the associativity.) Operator precedence is determined by the line ordering of the declarations; the higher the line number of the declaration (lower on the page or screen), the higher the precedence. Hence, exponentiation has the highest precedence, unary minus (`NEG') is next, followed by `*' and `/', and so on. *Note Operator Precedence: Precedence. The other important new feature is the `%prec' in the grammar section for the unary minus operator. The `%prec' simply instructs Bison that the rule `| '-' exp' has the same precedence as `NEG'--in this case the next-to-highest. *Note Context-Dependent Precedence: Contextual Precedence. Here is a sample run of `calc.y': % calc 4 + 4.5 - (34/(8*3+-3)) 6.880952381 -56 + 2 -54 3 ^ 2 9  File: bison.info, Node: Simple Error Recovery, Next: Multi-function Calc, Prev: Infix Calc, Up: Examples Simple Error Recovery ===================== Up to this point, this manual has not addressed the issue of "error recovery"--how to continue parsing after the parser detects a syntax error. All we have handled is error reporting with `yyerror'. Recall that by default `yyparse' returns after calling `yyerror'. This means that an erroneous input line causes the calculator program to exit. Now we show how to rectify this deficiency. The Bison language itself includes the reserved word `error', which may be included in the grammar rules. In the example below it has been added to one of the alternatives for `line': line: '\n' | exp '\n' { printf ("\t%.10g\n", $1); } | error '\n' { yyerrok; } ; This addition to the grammar allows for simple error recovery in the event of a parse error. If an expression that cannot be evaluated is read, the error will be recognized by the third rule for `line', and parsing will continue. (The `yyerror' function is still called upon to print its message as well.) The action executes the statement `yyerrok', a macro defined automatically by Bison; its meaning is that error recovery is complete (*note Error Recovery::). Note the difference between `yyerrok' and `yyerror'; neither one is a misprint. This form of error recovery deals with syntax errors. There are other kinds of errors; for example, division by zero, which raises an exception signal that is normally fatal. A real calculator program must handle this signal and use `longjmp' to return to `main' and resume parsing input lines; it would also have to discard the rest of the current line of input. We won't discuss this issue further because it is not specific to Bison programs.  File: bison.info, Node: Multi-function Calc, Next: Exercises, Prev: Simple Error Recovery, Up: Examples Multi-Function Calculator: `mfcalc' =================================== Now that the basics of Bison have been discussed, it is time to move on to a more advanced problem. The above calculators provided only five functions, `+', `-', `*', `/' and `^'. It would be nice to have a calculator that provides other mathematical functions such as `sin', `cos', etc. It is easy to add new operators to the infix calculator as long as they are only single-character literals. The lexical analyzer `yylex' passes back all non-number characters as tokens, so new grammar rules suffice for adding a new operator. But we want something more flexible: built-in functions whose syntax has this form: FUNCTION_NAME (ARGUMENT) At the same time, we will add memory to the calculator, by allowing you to create named variables, store values in them, and use them later. Here is a sample session with the multi-function calculator: % mfcalc pi = 3.141592653589 3.1415926536 sin(pi) 0.0000000000 alpha = beta1 = 2.3 2.3000000000 alpha 2.3000000000 ln(alpha) 0.8329091229 exp(ln(beta1)) 2.3000000000 % Note that multiple assignment and nested function calls are permitted. * Menu: * Decl: Mfcalc Decl. Bison declarations for multi-function calculator. * Rules: Mfcalc Rules. Grammar rules for the calculator. * Symtab: Mfcalc Symtab. Symbol table management subroutines.  File: bison.info, Node: Mfcalc Decl, Next: Mfcalc Rules, Up: Multi-function Calc Declarations for `mfcalc' ------------------------- Here are the C and Bison declarations for the multi-function calculator. %{ #include /* For math functions, cos(), sin(), etc. */ #include "calc.h" /* Contains definition of `symrec' */ %} %union { double val; /* For returning numbers. */ symrec *tptr; /* For returning symbol-table pointers */ } %token NUM /* Simple double precision number */ %token VAR FNCT /* Variable and Function */ %type exp %right '=' %left '-' '+' %left '*' '/' %left NEG /* Negation--unary minus */ %right '^' /* Exponentiation */ /* Grammar follows */ %% The above grammar introduces only two new features of the Bison language. These features allow semantic values to have various data types (*note More Than One Value Type: Multiple Types.). The `%union' declaration specifies the entire list of possible types; this is instead of defining `YYSTYPE'. The allowable types are now double-floats (for `exp' and `NUM') and pointers to entries in the symbol table. *Note The Collection of Value Types: Union Decl. Since values can now have various types, it is necessary to associate a type with each grammar symbol whose semantic value is used. These symbols are `NUM', `VAR', `FNCT', and `exp'. Their declarations are augmented with information about their data type (placed between angle brackets). The Bison construct `%type' is used for declaring nonterminal symbols, just as `%token' is used for declaring token types. We have not used `%type' before because nonterminal symbols are normally declared implicitly by the rules that define them. But `exp' must be declared explicitly so we can specify its value type. *Note Nonterminal Symbols: Type Decl.  File: bison.info, Node: Mfcalc Rules, Next: Mfcalc Symtab, Prev: Mfcalc Decl, Up: Multi-function Calc Grammar Rules for `mfcalc' -------------------------- Here are the grammar rules for the multi-function calculator. Most of them are copied directly from `calc'; three rules, those which mention `VAR' or `FNCT', are new. input: /* empty */ | input line ; line: '\n' | exp '\n' { printf ("\t%.10g\n", $1); } | error '\n' { yyerrok; } ; exp: NUM { $$ = $1; } | VAR { $$ = $1->value.var; } | VAR '=' exp { $$ = $3; $1->value.var = $3; } | FNCT '(' exp ')' { $$ = (*($1->value.fnctptr))($3); } | exp '+' exp { $$ = $1 + $3; } | exp '-' exp { $$ = $1 - $3; } | exp '*' exp { $$ = $1 * $3; } | exp '/' exp { $$ = $1 / $3; } | '-' exp %prec NEG { $$ = -$2; } | exp '^' exp { $$ = pow ($1, $3); } | '(' exp ')' { $$ = $2; } ; /* End of grammar */ %%  File: bison.info, Node: Mfcalc Symtab, Prev: Mfcalc Rules, Up: Multi-function Calc The `mfcalc' Symbol Table ------------------------- The multi-function calculator requires a symbol table to keep track of the names and meanings of variables and functions. This doesn't affect the grammar rules (except for the actions) or the Bison declarations, but it requires some additional C functions for support. The symbol table itself consists of a linked list of records. Its definition, which is kept in the header `calc.h', is as follows. It provides for either functions or variables to be placed in the table. /* Data type for links in the chain of symbols. */ struct symrec { char *name; /* name of symbol */ int type; /* type of symbol: either VAR or FNCT */ union { double var; /* value of a VAR */ double (*fnctptr)(); /* value of a FNCT */ } value; struct symrec *next; /* link field */ }; typedef struct symrec symrec; /* The symbol table: a chain of `struct symrec'. */ extern symrec *sym_table; symrec *putsym (); symrec *getsym (); The new version of `main' includes a call to `init_table', a function that initializes the symbol table. Here it is, and `init_table' as well: #include main () { init_table (); yyparse (); } yyerror (s) /* Called by yyparse on error */ char *s; { printf ("%s\n", s); } struct init { char *fname; double (*fnct)(); }; struct init arith_fncts[] = { "sin", sin, "cos", cos, "atan", atan, "ln", log, "exp", exp, "sqrt", sqrt, 0, 0 }; /* The symbol table: a chain of `struct symrec'. */ symrec *sym_table = (symrec *)0; init_table () /* puts arithmetic functions in table. */ { int i; symrec *ptr; for (i = 0; arith_fncts[i].fname != 0; i++) { ptr = putsym (arith_fncts[i].fname, FNCT); ptr->value.fnctptr = arith_fncts[i].fnct; } } By simply editing the initialization list and adding the necessary include files, you can add additional functions to the calculator. Two important functions allow look-up and installation of symbols in the symbol table. The function `putsym' is passed a name and the type (`VAR' or `FNCT') of the object to be installed. The object is linked to the front of the list, and a pointer to the object is returned. The function `getsym' is passed the name of the symbol to look up. If found, a pointer to that symbol is returned; otherwise zero is returned. symrec * putsym (sym_name,sym_type) char *sym_name; int sym_type; { symrec *ptr; ptr = (symrec *) malloc (sizeof (symrec)); ptr->name = (char *) malloc (strlen (sym_name) + 1); strcpy (ptr->name,sym_name); ptr->type = sym_type; ptr->value.var = 0; /* set value to 0 even if fctn. */ ptr->next = (struct symrec *)sym_table; sym_table = ptr; return ptr; } symrec * getsym (sym_name) char *sym_name; { symrec *ptr; for (ptr = sym_table; ptr != (symrec *) 0; ptr = (symrec *)ptr->next) if (strcmp (ptr->name,sym_name) == 0) return ptr; return 0; } The function `yylex' must now recognize variables, numeric values, and the single-character arithmetic operators. Strings of alphanumeric characters with a leading nondigit are recognized as either variables or functions depending on what the symbol table says about them. The string is passed to `getsym' for look up in the symbol table. If the name appears in the table, a pointer to its location and its type (`VAR' or `FNCT') is returned to `yyparse'. If it is not already in the table, then it is installed as a `VAR' using `putsym'. Again, a pointer and its type (which must be `VAR') is returned to `yyparse'. No change is needed in the handling of numeric values and arithmetic operators in `yylex'. #include yylex () { int c; /* Ignore whitespace, get first nonwhite character. */ while ((c = getchar ()) == ' ' || c == '\t'); if (c == EOF) return 0; /* Char starts a number => parse the number. */ if (c == '.' || isdigit (c)) { ungetc (c, stdin); scanf ("%lf", &yylval.val); return NUM; } /* Char starts an identifier => read the name. */ if (isalpha (c)) { symrec *s; static char *symbuf = 0; static int length = 0; int i; /* Initially make the buffer long enough for a 40-character symbol name. */ if (length == 0) length = 40, symbuf = (char *)malloc (length + 1); i = 0; do { /* If buffer is full, make it bigger. */ if (i == length) { length *= 2; symbuf = (char *)realloc (symbuf, length + 1); } /* Add this character to the buffer. */ symbuf[i++] = c; /* Get another character. */ c = getchar (); } while (c != EOF && isalnum (c)); ungetc (c, stdin); symbuf[i] = '\0'; s = getsym (symbuf); if (s == 0) s = putsym (symbuf, VAR); yylval.tptr = s; return s->type; } /* Any other character is a token by itself. */ return c; } This program is both powerful and flexible. You may easily add new functions, and it is a simple job to modify this code to install predefined variables such as `pi' or `e' as well.  File: bison.info, Node: Exercises, Prev: Multi-function Calc, Up: Examples Exercises ========= 1. Add some new functions from `math.h' to the initialization list. 2. Add another array that contains constants and their values. Then modify `init_table' to add these constants to the symbol table. It will be easiest to give the constants type `VAR'. 3. Make the program report an error if the user refers to an uninitialized variable in any way except to store a value in it.  File: bison.info, Node: Grammar File, Next: Interface, Prev: Examples, Up: Top Bison Grammar Files ******************* Bison takes as input a context-free grammar specification and produces a C-language function that recognizes correct instances of the grammar. The Bison grammar input file conventionally has a name ending in `.y'. * Menu: * Grammar Outline:: Overall layout of the grammar file. * Symbols:: Terminal and nonterminal symbols. * Rules:: How to write grammar rules. * Recursion:: Writing recursive rules. * Semantics:: Semantic values and actions. * Declarations:: All kinds of Bison declarations are described here. * Multiple Parsers:: Putting more than one Bison parser in one program.  File: bison.info, Node: Grammar Outline, Next: Symbols, Up: Grammar File Outline of a Bison Grammar ========================== A Bison grammar file has four main sections, shown here with the appropriate delimiters: %{ C DECLARATIONS %} BISON DECLARATIONS %% GRAMMAR RULES %% ADDITIONAL C CODE Comments enclosed in `/* ... */' may appear in any of the sections. * Menu: * C Declarations:: Syntax and usage of the C declarations section. * Bison Declarations:: Syntax and usage of the Bison declarations section. * Grammar Rules:: Syntax and usage of the grammar rules section. * C Code:: Syntax and usage of the additional C code section.  File: bison.info, Node: C Declarations, Next: Bison Declarations, Up: Grammar Outline The C Declarations Section -------------------------- The C DECLARATIONS section contains macro definitions and declarations of functions and variables that are used in the actions in the grammar rules. These are copied to the beginning of the parser file so that they precede the definition of `yyparse'. You can use `#include' to get the declarations from a header file. If you don't need any C declarations, you may omit the `%{' and `%}' delimiters that bracket this section.  File: bison.info, Node: Bison Declarations, Next: Grammar Rules, Prev: C Declarations, Up: Grammar Outline The Bison Declarations Section ------------------------------ The BISON DECLARATIONS section contains declarations that define terminal and nonterminal symbols, specify precedence, and so on. In some simple grammars you may not need any declarations. *Note Bison Declarations: Declarations.  File: bison.info, Node: Grammar Rules, Next: C Code, Prev: Bison Declarations, Up: Grammar Outline The Grammar Rules Section ------------------------- The "grammar rules" section contains one or more Bison grammar rules, and nothing else. *Note Syntax of Grammar Rules: Rules. There must always be at least one grammar rule, and the first `%%' (which precedes the grammar rules) may never be omitted even if it is the first thing in the file.  File: bison.info, Node: C Code, Prev: Grammar Rules, Up: Grammar Outline The Additional C Code Section ----------------------------- The ADDITIONAL C CODE section is copied verbatim to the end of the parser file, just as the C DECLARATIONS section is copied to the beginning. This is the most convenient place to put anything that you want to have in the parser file but which need not come before the definition of `yyparse'. For example, the definitions of `yylex' and `yyerror' often go here. *Note Parser C-Language Interface: Interface. If the last section is empty, you may omit the `%%' that separates it from the grammar rules. The Bison parser itself contains many static variables whose names start with `yy' and many macros whose names start with `YY'. It is a good idea to avoid using any such names (except those documented in this manual) in the additional C code section of the grammar file.  File: bison.info, Node: Symbols, Next: Rules, Prev: Grammar Outline, Up: Grammar File Symbols, Terminal and Nonterminal ================================= "Symbols" in Bison grammars represent the grammatical classifications of the language. A "terminal symbol" (also known as a "token type") represents a class of syntactically equivalent tokens. You use the symbol in grammar rules to mean that a token in that class is allowed. The symbol is represented in the Bison parser by a numeric code, and the `yylex' function returns a token type code to indicate what kind of token has been read. You don't need to know what the code value is; you can use the symbol to stand for it. A "nonterminal symbol" stands for a class of syntactically equivalent groupings. The symbol name is used in writing grammar rules. By convention, it should be all lower case. Symbol names can contain letters, digits (not at the beginning), underscores and periods. Periods make sense only in nonterminals. There are three ways of writing terminal symbols in the grammar: * A "named token type" is written with an identifier, like an identifier in C. By convention, it should be all upper case. Each such name must be defined with a Bison declaration such as `%token'. *Note Token Type Names: Token Decl. * A "character token type" (or "literal character token") is written in the grammar using the same syntax used in C for character constants; for example, `'+'' is a character token type. A character token type doesn't need to be declared unless you need to specify its semantic value data type (*note Data Types of Semantic Values: Value Type.), associativity, or precedence (*note Operator Precedence: Precedence.). By convention, a character token type is used only to represent a token that consists of that particular character. Thus, the token type `'+'' is used to represent the character `+' as a token. Nothing enforces this convention, but if you depart from it, your program will confuse other readers. All the usual escape sequences used in character literals in C can be used in Bison as well, but you must not use the null character as a character literal because its ASCII code, zero, is the code `yylex' returns for end-of-input (*note Calling Convention for `yylex': Calling Convention.). * A "literal string token" is written like a C string constant; for example, `"<="' is a literal string token. A literal string token doesn't need to be declared unless you need to specify its semantic value data type (*note Value Type::), associativity, precedence (*note Precedence::). You can associate the literal string token with a symbolic name as an alias, using the `%token' declaration (*note Token Declarations: Token Decl.). If you don't do that, the lexical analyzer has to retrieve the token number for the literal string token from the `yytname' table (*note Calling Convention::). *WARNING*: literal string tokens do not work in Yacc. By convention, a literal string token is used only to represent a token that consists of that particular string. Thus, you should use the token type `"<="' to represent the string `<=' as a token. Bison does not enforces this convention, but if you depart from it, people who read your program will be confused. All the escape sequences used in string literals in C can be used in Bison as well. A literal string token must contain two or more characters; for a token containing just one character, use a character token (see above). How you choose to write a terminal symbol has no effect on its grammatical meaning. That depends only on where it appears in rules and on when the parser function returns that symbol. The value returned by `yylex' is always one of the terminal symbols (or 0 for end-of-input). Whichever way you write the token type in the grammar rules, you write it the same way in the definition of `yylex'. The numeric code for a character token type is simply the ASCII code for the character, so `yylex' can use the identical character constant to generate the requisite code. Each named token type becomes a C macro in the parser file, so `yylex' can use the name to stand for the code. (This is why periods don't make sense in terminal symbols.) *Note Calling Convention for `yylex': Calling Convention. If `yylex' is defined in a separate file, you need to arrange for the token-type macro definitions to be available there. 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. The symbol `error' is a terminal symbol reserved for error recovery (*note Error Recovery::); you shouldn't use it for any other purpose. In particular, `yylex' should never return this value.  File: bison.info, Node: Rules, Next: Recursion, Prev: Symbols, Up: Grammar File Syntax of Grammar Rules ======================= A Bison grammar rule has the following general form: RESULT: COMPONENTS... ; where RESULT is the nonterminal symbol that this rule describes and COMPONENTS are various terminal and nonterminal symbols that are put together by this rule (*note Symbols::). For example, exp: exp '+' exp ; says that two groupings of type `exp', with a `+' token in between, can be combined into a larger grouping of type `exp'. Whitespace in rules is significant only to separate symbols. You can add extra whitespace as you wish. Scattered among the components can be ACTIONS that determine the semantics of the rule. An action looks like this: {C STATEMENTS} Usually there is only one action and it follows the components. *Note Actions::. Multiple rules for the same RESULT can be written separately or can be joined with the vertical-bar character `|' as follows: RESULT: RULE1-COMPONENTS... | RULE2-COMPONENTS... ... ; They are still considered distinct rules even when joined in this way. If COMPONENTS in a rule is empty, it means that RESULT can match the empty string. For example, here is how to define a comma-separated sequence of zero or more `exp' groupings: expseq: /* empty */ | expseq1 ; expseq1: exp | expseq1 ',' exp ; It is customary to write a comment `/* empty */' in each rule with no components.  File: bison.info, Node: Recursion, Next: Semantics, Prev: Rules, Up: Grammar File Recursive Rules =============== A rule is called "recursive" when its RESULT nonterminal appears also on its right hand side. Nearly all Bison grammars need to use recursion, because that is the only way to define a sequence of any number of somethings. Consider this recursive definition of a comma-separated sequence of one or more expressions: expseq1: exp | expseq1 ',' exp ; Since the recursive use of `expseq1' is the leftmost symbol in the right hand side, we call this "left recursion". By contrast, here the same construct is defined using "right recursion": expseq1: exp | exp ',' expseq1 ; Any kind of sequence can be defined using either left recursion or right recursion, but you should always use left recursion, because it can parse a sequence of any number of elements with bounded stack space. Right recursion uses up space on the Bison stack in proportion to the number of elements in the sequence, because all the elements must be shifted onto the stack before the rule can be applied even once. *Note The Bison Parser Algorithm: Algorithm, for further explanation of this. "Indirect" or "mutual" recursion occurs when the result of the rule does not appear directly on its right hand side, but does appear in rules for other nonterminals which do appear on its right hand side. For example: expr: primary | primary '+' primary ; primary: constant | '(' expr ')' ; defines two mutually-recursive nonterminals, since each refers to the other.  File: bison.info, Node: Semantics, Next: Declarations, Prev: Recursion, Up: Grammar File Defining Language Semantics =========================== The grammar rules for a language determine only the syntax. The semantics are determined by the semantic values associated with various tokens and groupings, and by the actions taken when various groupings are recognized. For example, the calculator calculates properly because the value associated with each expression is the proper number; it adds properly because the action for the grouping `X + Y' is to add the numbers associated with X and Y. * Menu: * Value Type:: Specifying one data type for all semantic values. * Multiple Types:: Specifying several alternative data types. * Actions:: An action is the semantic definition of a grammar rule. * Action Types:: Specifying data types for actions to operate on. * Mid-Rule Actions:: Most actions go at the end of a rule. This says when, why and how to use the exceptional action in the middle of a rule.  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.