野牛的减少没有按预期进行 [英] bison's reduction didn't work as expected

问题描述

我正在尝试编写EPL解析器，所以我正在学习flex和bison.我尝试将其与以下规则(SQL)结合使用:

I'm trying to write a EPL parser,so I'm learning flex and bison.I try using it with following rules(SQL):

SELECT { cout<<"SELECT detected"<<endl;return SELECT; }
FROM { cout<<"FROM detected"<<endl;return FROM;}
[a-zA-Z][0-9a-zA-Z]* { cout<<"IDENTIFIER detected"<<endl;yylval.c=yytext;
                         return IDENTIFIER; }
'$' { return DOL;}
[ \t] { cout<<"space founded:"<<int(yytext[0])<<endl; }
\n { return EOL;}
. {}

和野牛规则是:

sel_stmt : {cout<<"VOID"<<endl;}
         | SELECT identifier_expr FROM identifier_expr {    cout<<"select statement founded"<<endl; }
         ;

identifier_expr : DOL IDENTIFIER {
$$=$2;
cout<<"ident_expr:"<<$$<<endl;
}
;

所有令牌和非终结符的类型均为"char *"

all tokens and non-terminals have type "char*"

因为来自stdin的输入是从$ ddd中选择$ abc" 词法分析器在"identifier_expr"操作中返回令牌FROM时发生倒退，输出为 "ident_expr:abc来自" 为什么会这样?

as the input from stdin is "select $abc from $ddd" reudction happend when lexer returned token FROM,in "identifier_expr" action ,output is "ident_expr:abc from" why this happened?

推荐答案

如果要在flex操作之外使用令牌字符串(yytext)，则必须创建的副本. yytext指向的字符串是一个临时值，重新输入词法分析器后将立即对其进行修改.

You must create a copy of the token string (yytext) if you want to use it outside of the flex action. The string pointed to by yytext is a temporary value, and will be modified as soon as the lexer is re-entered.

请参见 bison FAQ ， flex手册或任何数量的SO问题(由于许多发问者误诊了该问题，因此较难搜索).

See the bison FAQ, the flex manual, or any number of SO questions (which are harder to search for because many of the questioners misdiagnose the problem).

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