yacc

2021-09-28

yacc introduction

Section 1 describes the preparation of grammar rules,
Section 2 the preparation of the user supplied actions associated with these rules, and
Section 3 the preparation of lexical analyzers.
Section 4 describes the operation of the parser

参考yacc

section1: Basic Specifications

every specification file consists of three sections: the declarations, (grammar) rules, and programs. The sections are separated by double percent %% marks. (The percent %' is generally used in Yacc specifications as an escape character.)

Names representing tokens must be declared; this is most simply done by writing

1	%token name1 name2

The parser is designed to recognize the start symbol;

1	%start symbol

The rules section is made up of one or more grammar rules. A grammar rule has the form:

1	A : BODY ;

A represents a nonterminal name, and BODY represents a sequence of zero or more names and literals. The colon and the semicolon are Yacc punctuation.

1	A : '(' B ')'

‘(‘就是一个literal

section2: Actions

With each grammar rule, the user may associate actions to be performed each time the rule is recognized in the input process.

1 2	A : '(' B ')' { hello( 1, "abc" ); }

1
2
3

XXX :  YYY  ZZZ
       {       printf("a message\n");
               flag = 25;  }

yacc采用$$$$符号来表示返回值，用$1, $2, $3...等符号来引用被同一个产生式中前面的action返回的结果。

1	exp: '(' expr ')' { $$ = $2 ; }