在Bison中使用$ n表示n

在Bison中使用$ n表示n <0(Using $n for n < 0 in Bison)

在Bison中有没有办法检查当前的令牌堆栈大小？

我想使用$ n和n作为负数来访问另一个规则的语义值，但前提是堆栈足够大。

谢谢。

Is there a way in Bison to check the current token stack size?

I'd like to use $n with n as a negative number to access a semantic value of another rule but only if the stack is large enough.

Thank you.

最满意答案

给出如下规则：

stmt: ID '=' DIGIT { $$ = $3; } ;

生成的代码片段是：

{ (yyval) = (yyvsp[(3) - (3)]); }

因此， yyvsp '数组'是答案的一部分。 进一步向上（常规 - 而不是GLR）生成的代码，你发现yyvsp实际上是一个指针，而不是一个数组。 例如：

yyvsp = yyvs + yysize - 1;

看起来yysize是你想要的价值; 不幸的是，它是一个块的本地变量，它在执行用户操作之前已经终止，因此它不能直接使用。 但是，您也可以找到以下代码：

yyvsp = yyvs;

看看代码，似乎yyvs是指向堆栈基础的指针（可以动态分配），而yyvsp是指向堆栈的指针。 这些变量在整个yyparse()函数中都是可见的yyparse()因此，特别是在用户操作中可见。并且您需要的答案是：

int nrules = yyvsp - yyvs;

点击这样的源代码并不是远程清理，但它确实能为您提供答案。

如果使用GLR语法，则必须检查Bison GLR语法是否具有相同含义的相同变量; 它可能不会。

Given a rule such as:

stmt: ID '=' DIGIT { $$ = $3; } ;

The generated code fragment is:

{ (yyval) = (yyvsp[(3) - (3)]); }

The yyvsp 'array', therefore, is part of the answer. Further up the (regular - not the GLR) generated code, you find that yyvsp is actually a pointer, not an array. For example:

yyvsp = yyvs + yysize - 1;

It looks like yysize is the value you want; unfortunately, though, it is a variable local to a block which has terminated before the user actions are executed, so it is not directly available. However, you can also find code with:

yyvsp = yyvs;

Looking at the code, it seems that yyvs is a pointer to the base of the stack (which can be dynamically allocated), and yyvsp is a pointer part way up the stack. Those variables are both visible throughout the yyparse() function (and therefore, in particular, are visible within user actions. And the answer you need is:

int nrules = yyvsp - yyvs;

It isn't remotely clean to poke at the source code like this, but it does get you an answer.

If you use a GLR grammar, you would have to examine whether a Bison GLR grammar has the same variables with the same meanings; it may not.