BYACC(1)

💖 BYACC(1) — the girlies' guide to the LALR(1) parser generator 💖

User Commands · BYACC(1)

okay bestie, pull up a chair because we're about to get so into it. this is the manual for byacc, and yes it's giving very much "compiler girlie" energy but honestly? iconic. let's go 💅

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✨ NAME ✨

byacc — an LALR(1) parser generator 🦋

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✨ SYNOPSIS ✨

byacc [ -BdghilLPrtvVy ] [ -b file_prefix ] [ -H defines_file ] [ -o
output_file ] [ -p symbol_prefix ] filename

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✨ DESCRIPTION ✨

so here's the tea, bestie 🫖 — byacc reads a grammar specification out of the file filename and generates an LALR(1) parser for it. like, a whole parser. for you. no cap.

the parsers it spits out are made of two lil pieces:

• a set of LALR(1) parsing tables, and
• a driver routine written in the C programming language 👑

and normally byacc writes both of those — the parse tables and the driver routine — into the file y.tab.c. yes, the same y.tab.c you've been seeing in your nightmares. it's fine. it's fine. 🙈

the following options are available:

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✨ OPTIONS ✨

• -b file_prefix — this one changes the prefix that gets prepended to the output file names, and it sets it to whatever string you passed as file_prefix. the default prefix? it's the character y. simple. classic. y. 💛
• -B — creates a backtracking parser. note that this is a compile-time configuration for btyacc. 🌀
• -d — makes the header file y.tab.h get written out. inside it? #define's for all the token identifiers. your tokens are getting named, bestie, they're getting their moment ✨
• -h — prints a usage message to the standard error. that's it. that's the whole vibe. a little help, a little stderr. 🤲
• -H defines_file — this one makes the #define's for the token identifiers get written to the defines_file you gave it, instead of the y.tab.h file that the -d option would normally use. same energy, different address 🏠
• -g — causes a graphical description of the generated LALR(1) parser to be written to the file y.dot, in graphviz format, all ready to be processed by dot(1). yes girl, visualise that parser. graph it. slay 📊
• -i — causes a supplementary header file y.tab.i to be written. it contains extern declarations and supplementary #define's as needed, so that the conventional yacc yy-prefixed names get mapped to whatever the -p option may specify. the code file — like, say, y.tab.c — gets modified to #include this file as well as the y.tab.h file, which enforces consistent usage of the symbols defined in those files. consistency queen 👑

and honestly? the supplementary header file makes it simpler to separate compilation of lex-files and yacc-files. you're welcome.

• -l — so if you don't specify -l, byacc will insert #line directives in the generated code. those #line directives let the C compiler relate errors in the generated code back to the user's original code — which is, like, extremely helpful of it 🥹. but if you do specify -l, byacc will not insert the #line directives. and heads up: #line directives that you specified yourself will be retained regardless. periodt.
• -L — enables position processing, e.g. %locations. (this is a compile-time configuration for btyacc.) 📍
• -o output_file — specify the filename for the parser file. if this option isn't given, the output filename is the file prefix concatenated with the file suffix, e.g. y.tab.c. and fun fact: this option overrides the -b option. it's giving main character energy, sorry -b 😭
• -p symbol_prefix — changes the prefix prepended to yacc-generated symbols, setting it to the string denoted by symbol_prefix. the default prefix is the string yy. you know. yy. the original. the blueprint. 💜
• -P — creates a reentrant parser, e.g. %pure-parser. 🧼
• -r — causes byacc to produce separate files for code and tables. the code file is named y.code.c, and the tables file is named y.tab.c. and the prefix y. can be overridden using the -b option. splitting things up? healthy. boundaries? beautiful. 🌸
• -s — suppresses the #define statements that get generated for string literals in a %token statement, so it more closely matches original yacc behavior.

normally, when byacc sees a line such as

%token OP_ADD "ADD"

it notices that the quoted ADD is a valid C identifier, and generates a #define not only for OP_ADD, but for ADD as well, e.g.

#define OP_ADD 257
#define ADD 258

but the original yacc does not generate that second #define. so the -s option suppresses this #define. no sneaky extra defines. bye 👋

also worth knowing: POSIX (IEEE 1003.1 2004) documents only names and numbers for %token, though original yacc and bison also accept string literals. so like... the standards girlies are being strict, and byacc is being flexible. we love range 💅

• -t — changes the preprocessor directives generated by byacc so that debugging statements will be incorporated in the compiled code. debugging era 🔍

and a lil drama for you: byacc sends debugging output to the standard output (compatible with both the original yacc and btyacc), while btyacc writes debugging output to the standard error (like bison). same vibes, different stream. we don't make the rules 😭

• -v — causes a human-readable description of the generated parser to be written to the file y.output. for the girlies who like to read their parser. respect. 📖
• -V — prints the version number to the standard output. hi, what version are we on, bestie ✨
• -y — byacc ignores this option, which bison supports for ostensible POSIX compatibility. it's just there. vibing. doing nothing. honestly? relatable. 💀

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okay so about the filename parameter: it is not optional, bestie. it's required. no filename, no parser.

but 🫶 — byacc does accept a single - to read the grammar from the standard input. and a double -- marker denotes the end of options. a single filename parameter is expected after a -- marker. so if you're piping in from stdin, use -, and if you need to say "no more options, thanks", that's --. got it? got it. 💖

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✨ DIAGNOSTICS ✨

• if there are rules that are never reduced, the number of such rules is reported on standard error. 😬
• if there are any LALR(1) conflicts, the number of conflicts is reported on standard error. 💀

so like, if something's off, byacc will tell you. on standard error. it's not being mean, it's being honest. tough love. 🎀

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🌈 EXTENSIONS 🌈

okay so byacc is literally out here giving extra credit 😭💖 It accepts several long options which have equivalents in byacc, all for compatibility with bison and other implementations of yacc. The %destructor and %locations features are available only if byacc has been configured and compiled to support the back-tracking (btyacc) functionality. The remaining features are always available, no config required, periodt:

✨ %code keyword { code } ✨

Adds the indicated source code at a given point in the output file. The optional keyword tells byacc where to insert the code:

• top — just after the version-definition in the generated code-file.
• requires — just after the declaration of public parser variables. If the -d option is given, the code is inserted at the beginning of the defines-file.
• provides — just after the declaration of private parser variables. If the -d option is given, the code is inserted at the end of the defines-file.

If no keyword is given, the code is inserted at the beginning of the section of code copied verbatim from the source file. Multiple %code directives may be given; byacc inserts those into the corresponding code- or defines-file in the order that they appear in the source file. ✨ (order is sacred bestie, it's giving chronological)

🎀 %debug 🎀

This has the same effect as the -t command-line option. same energy, different font 💅

💖 %destructor { code } symbol+ 💖

defines code that is invoked when a symbol is automatically discarded during error recovery. This code can be used to reclaim dynamically allocated memory associated with the corresponding semantic value for cases where user actions cannot manage the memory explicitly. (translation: it's your little cleanup fairy 🧚)

On encountering a parse error, the generated parser discards symbols on the stack and input tokens until it reaches a state that will allow parsing to continue. This error recovery approach results in a memory leak if the YYSTYPE value is, or contains, pointers to dynamically allocated memory. 💀 so yeah, this one's load-bearing.

The bracketed code is invoked whenever the parser discards one of the symbols. Within code, $$ or $<tag>$ designates the semantic value associated with the discarded symbol, and @$ designates its location (see %locations directive).

A per-symbol destructor is defined by listing a grammar symbol in symbol+. A per-type destructor is defined by listing a semantic type tag (e.g., <some_tag>) in symbol+; in this case, the parser will invoke code whenever it discards any grammar symbol that has that semantic type tag, unless that symbol has its own per-symbol destructor.

Two categories of default destructor are supported that are invoked when discarding any grammar symbol that has no per-symbol and no per-type destructor:

• the code for <*> is used for grammar symbols that have an explicitly declared semantic type tag (via %type);
• the code for <> is used for grammar symbols that have no declared semantic type tag.

🌸 %empty 🌸

ignored by byacc. that's it. that's the whole feature. she's just there, vibing 🙈

⭐ %expect number ⭐

tells byacc the expected number of shift/reduce conflicts. That makes it only report the number if it differs. (so it stops yapping unless something's actually wrong, love that for us)

🌈 %expect-rr number 🌈

tell byacc the expected number of reduce/reduce conflicts. That makes it only report the number if it differs. This is (unlike bison) allowable in LALR parsers. 🔥

🦋 %locations 🦋

tells byacc to enable management of position information associated with each token, provided by the lexer in the global variable yylloc, similar to management of semantic value information provided in yylval.

As for semantic values, locations can be referenced within actions using @$ to refer to the location of the left hand side symbol, and @N (N an integer) to refer to the location of one of the right hand side symbols. Also as for semantic values, when a rule is matched, a default action is used the compute the location represented by @$ as the beginning of the first symbol and the end of the last symbol in the right hand side of the rule. This default computation can be overridden by explicit assignment to @$ in a rule action.

The type of yylloc is YYLTYPE, which is defined by default as:

typedef struct YYLTYPE {
    int first_line;
    int first_column;
    int last_line;
    int last_column;
} YYLTYPE;

YYLTYPE can be redefined by the user (YYLTYPE_IS_DEFINED must be defined, to inhibit the default) in the declarations section of the specification file. As in bison, the macro YYLLOC_DEFAULT is invoked each time a rule is matched to calculate a position for the left hand side of the rule, before the associated action is executed; this macro can be redefined by the user.

This directive adds a YYLTYPE parameter to yyerror(). If the %pure-parser directive is present, a YYLTYPE parameter is added to yylex() calls.

💅 %lex-param { argument-declaration } 💅

By default, the lexer accepts no parameters, e.g., yylex(). Use this directive to add parameter declarations for your customized lexer. feed her some args bestie 🫶

👑 %parse-param { argument-declaration } 👑

By default, the parser accepts no parameters, e.g., yyparse(). Use this directive to add parameter declarations for your customized parser.

🍓 %pure-parser 🍓

Most variables (other than yydebug and yynerrs) are allocated on the stack within yyparse, making the parser reasonably reentrant. main character energy honestly.

💜 %token-table 💜

Make the parser's names for tokens available in the yytname array. However, byacc does not predefine $end, $error or $undefined in this array. (she said "not my problem" 😭)

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🌍 PORTABILITY 🌍

According to Robert Corbett,

Berkeley Yacc is an LALR(1) parser generator. Berkeley Yacc has been made as compatible as possible with AT&T Yacc. Berkeley Yacc can accept any input specification that conforms to the AT&T Yacc documentation. Specifications that take advantage of undocumented features of AT&T Yacc will probably be rejected.

The rationale in

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/yacc.html

documents some features of AT&T yacc which are no longer required for POSIX compliance.

That said, you may be interested in reusing grammar files with some other implementation which is not strictly compatible with AT&T yacc. For instance, there is bison. Here are a few differences:

• Yacc accepts an equals mark preceding the left curly brace of an action (as in the original grammar file ftp.y):

    |   STAT CRLF
        = {
                statcmd();
        }

• Yacc and bison emit code in different order, and in particular bison makes forward reference to common functions such as yylex, yyparse and yyerror without providing prototypes.
• Bison's support for %expect is broken in more than one release. For best results using bison, delete that directive. 🚩 not me warning y'all about a bug 🚩
• Bison has no equivalent for some of byacc's command-line options, relying on directives embedded in the grammar file.
• Bison's -y option does not affect bison's lack of support for features of AT&T yacc which were deemed obsolescent.
• Yacc accepts multiple parameters with %lex-param and %parse-param in two forms

{type1 name1} {type2 name2} ...
{type1 name1,  type2 name2 ...}

Bison accepts the latter (though undocumented), but depending on the release may generate bad code.

• Like bison, byacc will add parameters specified via %parse-param to yyparse, yyerror and (if configured for back-tracking) to the destructor declared using %destructor. Bison puts the additional parameters first for yyparse and yyerror but last for destructors. Yacc matches this behavior.

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👀 SEE ALSO 👀

bison(1), btyacc(1), lex(1), flex(1), yacc(1)

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okay that's the whole page bestie, you ate that 💅✨ this one's from Berkeley Yacc, dated 2024-12-31, and it lives at BYACC(1) — go build a parser, you've got this 🫶🌈