Lex/Yacc入门教程：构建C语言编译器基石

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"《LEX&YACC入门教程》由Tom Niemann编写，是一份详尽的指南，旨在教授如何使用LEX和YACC这两个工具来构建编译器。LEX负责词法分析，而YACC则处理语法解析。本教程假定读者已经具备一定的C编程能力，并对数据结构有所了解，如链表和树。教程首先通过简介介绍了编译器的基本构建块以及LEX和YACC之间的协作原理。接着，深入讲解了LEX的工作原理和实践，包括理论概念（如识别特定模式、生成token）以及如何编写和调试LEX输入文件，如识别算术运算符和字符串。 YACC部分同样分为理论和实践两部分，涵盖了语法规则的定义、解析树的构建以及如何处理诸如if-else和while这类控制结构，甚至探讨了可能遇到的语法歧义问题。嵌入式动作和错误处理也是YACC的重要内容，它们允许在解析过程中执行自定义操作，并提供有效的错误消息机制。随着理论知识的积累，作者带领读者实现了一个计算器的例子，展示实际应用中LEX和YACC的协同工作。进一步，教程还讨论了如何利用继承属性和调试工具优化解析过程。《LEX&YACC入门教程》不仅适合初学者入门，也适合有一定经验的开发者深化理解这两个工具，它提供了一个从基础到高级的完整学习路径，帮助读者构建功能丰富的语言解析器。通过跟随教程，读者将掌握如何构建一个能够处理基本算术和控制流程的编译器，并学会解决常见编译器构造中的问题。"

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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