构建编译器：详解Lex与Yacc理论与实践
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更新于2024-07-23
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本教程是关于Yacc的全面指南，由Tom Niemann编写，旨在帮助读者了解和掌握如何使用Lex和Yacc构建编译器。Lex和Yacc是两个关键的工具，它们分别负责词法分析和语法分析，是构造高级语言编译器的核心组成部分。
首先，"Lex & YACC Tutorial" 的前言部分介绍了教程的目的，假设读者具备基本的C编程能力，并对数据结构有所理解，如链表和树。它强调了编译器的基本构建块以及Lex和Yacc之间的协同工作原理。
在介绍部分，着重于编译器的基础概念，包括词法分析（Lex）和句法分析（Yacc）的作用。词法分析负责识别输入源代码中的单词和符号，而句法分析则解析这些元素以构造抽象语法树（AST），这是编译过程的关键步骤。
接下来的两大部分深入剖析了Lex和Yacc的理论与实践。在Lex部分，读者将学习到如何设计和实现一个词法分析器，处理常规的算术运算符和控制结构，如if-else和while。通过实例，如Calculator程序，作者演示了如何编写输入规则和处理输出。
Yacc理论部分详细阐述了如何创建解析器，包括如何定义语法规则、属性和动作。实践部分分为两部分，第一部分引导读者编写基础的Parser，第二部分则探讨如何扩展功能，比如处理字符串和保留关键字。同时，调试这两个工具的方法也得到了介绍，以便在开发过程中发现和解决问题。
遇到复杂情况时，教程涵盖了递归、if-else语句的歧义处理，以及错误消息的生成。此外，还介绍了如何利用继承属性和嵌入式动作来优化解析器的设计。最后，针对可能遇到的问题，教程提供了调试Lex和Yacc的策略。
整个教程以一个计算器项目作为实践案例，逐步引导读者构建一个完整的编译器系统。通过这个过程，读者不仅能掌握Yacc的基本用法，还能深入理解编译器设计的核心思想和技术。因此，这是一份适合有一定编程基础的学习者参考的实用教程。
Two op
erators allowed in a character class are the hyphen (“-”) and circumflex (“^”). When used
between two characters the hyphen represents a range of characters. The circumflex, when used
as the first character, negates the expression. If two patterns match the same string, the longest
match wins. In case both matches are the same length, then the first pattern listed is used.
... definitions ...
%%
... rules ...
%%
... subroutines ...
Input to Lex is divided into three sections with %% dividing the sections. This is best illustrated by
example. The first example is the shortest possible lex file:
%%
Input is copied to output one character at a time. The first %% is always required, as there must
always be a rules section. However if we don’t specify any rules then the default action is to
match everything and copy it to output. Defaults for input and output are stdin and stdout,
respectively. Here is the same example with defaults explicitly coded:
%%
/* match everything except newline */
. ECHO;
/* match newline */
\n ECHO;
%%
int yywrap(void) {
return 1;
}
int main(void) {
yylex();
return 0;
}
Two patterns have been specified in the rules section. Each pattern must begin in column one.
This is followed by whitespace (space, tab or newline) and an optional action associated with the
pattern. The action may be a single C statement, or multiple C statements, enclosed in braces.
Anything not starting in column one is copied verbatim to the generated C file. We may take
advantage of this behavior to specify comments in our lex file. In this example there are two
patterns, “.” and “\n”, with an ECHO action associated for each pattern. Several macros and
variables are predefined by lex. ECHO is a macro that writes code matched by the pattern. This is
the default action for any unmatched strings. Typically, ECHO is defined as:
#define ECHO fwrite(yytext, yyleng, 1, yyout)
Variable yytext is a pointer to the matched string (NULL-terminated) and yyleng is the length of
the matched string. Variable yyout is the output file and defaults to stdout. Function yywrap is
called by lex when input is exhausted. Return 1 if you are done or 0 if more processing is
required. Every C program requires a main function. In this case we simply call yylex that is the
main entry-point for lex. Some implementations of lex include copies of main and yywrap in a
library thus eliminating the need to code them explicitly. This is why our first example, the shortest
lex program, functioned properly.
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