Commands Reference, Volume 6

yacc Command

Purpose

Generates an LALR(1) parsing program from input consisting of a context-free grammar specification.

Syntax

yacc [ -b Prefix ] [ -C ] [ -d ] [ -l ] [ -NnNumber ] [ -NmNumber ] [ -NrNumber ] [ -p Prefix ] [ -s ] [ -t ] [ -v ] [ -y Path ] Grammar

Description

The yacc command converts a context-free grammar specification into a set of tables for a simple automaton that executes an LALR(1) parsing algorithm. The grammar can be ambiguous; specified precedence rules are used to break ambiguities.

You must compile the output file, y.tab.c, with a C language compiler to produce a yyparse function. This function must be loaded with the yylex lexical analyzer, as well as with the main subroutine and the yyerror error-handling subroutine (you must provide these subroutines). The lex command is useful for creating lexical analyzers usable by the yyparse subroutine. Simple versions of main and yyerror subroutines are available through the yacc library, liby.a. Also, yacc can be used to generate C++ output.

You can compile the yacc-generated C file (y.tab.c) with the -DYACC_MSG option to include code necessary to use the Message Facility. When you use this option during compilation, error messages generated by the yyparse subroutine and the YYBACKUP macro are extracted from the yacc_user.cat catalog.

This allows you to receive error messages in languages other than English in non-English locales. If the catalog cannot be found or opened, the yyparse and YYBACKUP subroutines display the default English messages.

The yacc command is affected by the LANG, LC_ALL, LC_CTYPE, and LC_MESSAGES environment variables.

Flags

-b Prefix	Use Prefix instead of y as the prefix for all output file names. The code file y.tab.c, the header file y.tab.h (created when -d is specified), and the description file y.output (created when -v is specified) are changed to Prefix.tab.c, Prefix.tab.h, and Prefix.output, respectively.
-C	Produces the y.tab.C file instead of the y.tab.c file for use with a C++ compiler. To use the I/O Stream Library for input and output, define the macro, _CPP_IOSTREAMS.
-d	Produces the file y.tab.h. This contains the #define statements that associate the yacc-assigned token codes with your token names. This allows source files other than y.tab.c to access the token codes by including this header file.
-l	Does not include any #line constructs in y.tab.c. Use this only after the grammar and associated actions are fully debugged.
-NnNumber	Changes the size of the token and nonterminal names array to Number. The default value is 8000. Valid values are only those greater than 8000.
-NmNumber	Changes the size of the memory states array to Number. Default value is 40000. Valid values are only those greater than 40000.
-NrNumber	Changes the internal buffer sizes to handle large grammars. The default value is 2000. Valid values are only those greater than 2000.
-p Prefix	Use Prefix instead of yy as the prefix for all external names created by the yacc command. External names affected include: yychar, yylval, yydebug, yyparse( ), yylex( ), and yyerror( ). (Previously, -p was used to specify an alternate parser; now, -yPath can be used to specify an alternate parser.)
-s	Breaks the yyparse function into several smaller functions. Since its size is somewhat proportional to that of the grammar, it is possible for the yyparse function to become too large to compile, optimize, or execute efficiently.
-t	Compiles run-time debugging code. By default, this code is not included when y.tab.c is compiled. However, the run-time debugging code is under the control of the preprocessor macro, YYDEBUG. If YYDEBUG has a nonzero value, the C compiler (cc) includes the debugging code, regardless of whether the -t flag is used. YYDEBUG should have a value of 0 if you don't want the debugging code included by the compiler. Without compiling this code, the yyparse subroutine will have a faster operating speed. The -t flag causes compilation of the debugging code, but it does not actually turn on the debug mode. To get debug output, the yydebug variable must be set either by adding the C language declaration, `int yydebug=1` to the declaration section of the yacc grammar file or by setting yydebug through dbx.
-v	Prepares the file y.output. It contains a readable description of the parsing tables and a report on conflicts generated by grammar ambiguities.
-y Path	Uses the parser prototype specified by Path instead of the default /usr/lib/yaccpar file. (Previously, -p was used to specify an alternate parser.)

Exit Status

This command returns the following exit values:

0	Successful completion.
>0	An error occurred.

Examples

The following command:
```
yacc grammar.y
```
draws yacc rules from the grammar.y file, and places the output in y.tab.c.
The following command:
yacc -d grammar.y
functions the same as example 1, but it also produces the y.tab.h file which would contain C-style #define statements for each of the tokens defined in the grammar.y file.

Files

y.output	Contains a readable description of the parsing tables and a report on conflicts generated by grammar ambiguities.
y.tab.c	Contains an output file.
y.tab.h	Contains definitions for token names.
yacc.tmp	Temporary file.
yacc.debug	Temporary file.
yacc.acts	Temporary file.
/usr/ccs/lib/yaccpar	Contains parser prototype for C programs.
/usr/ccs/lib/liby.a	Contains a run-time library.

Related Information

The lex command.

Creating an Input Language with the lex and yacc Commands in AIX 5L Version 5.2 General Programming Concepts: Writing and Debugging Programs.

The Example program for the lex and yacc programs in AIX 5L Version 5.2 General Programming Concepts: Writing and Debugging Programs.