yacc | 易学教程

Shift Reduce Conflict for arithmetic expressions in yacc

阅读更多关于 Shift Reduce Conflict for arithmetic expressions in yacc

问题 This grammar has given me conflict despite specifying precedence of operators. Even in the Dragon book it's been resolved in such a way(the way implemented as first 7 lines below) but it still gets conflict! Below is the code implemented in yacc %right THEN_KW %right ELSE_KW %left XOR_KW OR_KW %right '=' %left AND_KW ALSO_KW %left EQ_KW LT_KW GT_KW LE_KW GE_KW %left PLUS_KW MINUS_KW %left MULT_KW DIV_KW MOD_KW %right NOT_KW arthlogicexpr -> operand | arthlogicexpr arthop arthlogicexpr arthop

yyllocp->first_line returns uninitialized value in second iteration of a reEntrant Bison parser

阅读更多关于 yyllocp->first_line returns uninitialized value in second iteration of a reEntrant Bison parser

问题 I have a reEntrant parser which takes input from a string and has a structure to maintain context. A function is called with different input strings to be parsed. Relevant code of that function is: void parseMyString(inputToBeParsed) { //LEXICAL COMPONENT - INITIATE LEX PROCESSING yyscan_t scanner; YY_BUFFER_STATE buffer; yylex_init_extra(&parseSupportStruct, &scanner ); //yylex_init(&scanner); buffer = yy_scan_buffer(inputToBeParsed, i+2, scanner); if (buffer == NULL) { strcpy(errorStrings,

Ply shift/reduce conflicts: dangling else and empty productions

阅读更多关于 Ply shift/reduce conflicts: dangling else and empty productions

问题 I had lots of conflicts, most of them were due to operators and relational operators which had different precedences. But I still face some conflicts that I don't really know how to tackle them. some of them are below. I suspect that maybe I should do epsilon elimination for stmtlist but to be honest I'm not sure about it. state 70: state 70 (27) block -> LCB varlist . stmtlist RCB (25) varlist -> varlist . vardec (28) stmtlist -> . stmt (29) stmtlist -> . stmtlist stmt (30) stmtlist -> . (15

How to read multiple lines of input in lex and yacc?

阅读更多关于 How to read multiple lines of input in lex and yacc?

问题 I want the output as: a=3 mov a,3 a=fs mov b,fs b=32 mov b,32 Program for 3 address intermediate code generation - the lex file written for lexical analysis reads the input from command line and passes tokens: %{ #include "y.tab.h" #include "string.h" #include <math.h> %} %% [a-zA-Z]+ { yylval.var=(char *)malloc(sizeof(char *)); strcpy(yylval.var,yytext); return ID;} "=" return EQUALS; [0-9]+ {yylval.num=atoi(yytext);return DIGIT;} %% The corresponding yacc file: %{ #include "stdio.h"

How can flex return multiple terminals at one time

阅读更多关于 How can flex return multiple terminals at one time

问题 In order to make my question easy to understand I want to use the following example: The following code is called nonblock do-loop in fortran language DO 20 I=1, N ! line 1 DO 20 J=1, N ! line 2 ! more codes 20 CONTINUE ! line 4 Pay attention that the label 20 at line 4 means the end of both the inner do-loop and the outer do-loop. I want my flex program to parse the feature correctly: when flex reads the label 20 , it will return ENDDO terminal twice. Firstly, because I also use bison, so

Syntax error using empty production rule in python PLY(lex/yacc)

阅读更多关于 Syntax error using empty production rule in python PLY(lex/yacc)

问题 The full example is given here: import ply.lex as lex import Property # List of token names. This is always required tokens = [ 'CheckupInformation', 'Introduction', 'Information', 'perfect', 'sick', 'LPAREN', 'RPAREN', 'CHAR', 'NUMBER' ] def t_CheckupInformation(t) : 'CheckupInformation' ; return t def t_Introduction(t) : 'Introduction' ; return t def t_Information(t) : 'Information' ; return t def t_perfect(t): 'perfect'; return t def t_sick(t) : 'sick'; return t t_LPAREN = r'\(' t_RPAREN =

Ubuntu安装flex和bison

阅读更多关于 Ubuntu安装flex和bison

在学习虎书，第二章练习要用lex和yacc，然而作者提供的网址上放的是flex和bison，当然这样更好啦（flex和bison完全兼容lex和yacc）。考虑到在windows下用这两个玩意还要安装masm，所以最后决定上虚拟机（之前操作系统课就装过了，幸亏没卸载哈哈哈）。 Ubuntu下使用 sudo apt-get install flex bison 命令，不一会就装好了，如图：然后写了一个测试程序，就用Ubuntu自带的gedit写，保存为.l文件： %% [\t] + is | am | are | was | were { printf ( "%s: is a verb\n" ,yytext);} [a-zA-Z] + { printf ( "%s: is not a verb" ,yytext);} .|\ n %% int main (){ yylex (); } 使用命令 lex verb.l 再使用命令 gcc lex.yy.l -lfl 最后运行 ./a.out 这个程序会判断输入的单词是不是动词：程序成功运行也说明我们安装配置成功了。来源： oschina 链接： https://my.oschina.net/u/4518215/blog/4751194

Go实现自定义语言的基础

阅读更多关于 Go实现自定义语言的基础

0x01 自言自语一直就对解析文档，比较感兴趣，一直没深入研究，只停留在仅知道 Lex & yacc 和 antlr 的名词阶段，最近看了go-zero的api解析器，觉得甚好，是时候花时间学习一下了。简单看了go-zero发现是自己实现了词法分析、语法解析，这不符合我的一贯偷懒作风，所以并未其源码开始学习。既然用golang那么他自带的goyacc就是我学习的不二之选。当然你可能会听说Lex&yacc 已经很古老了，antlr更先进一点。但是既然goyacc能成为golang官方工具，那么肯定还是值得你学习的。 goyacc的文档非常的少，少到什么程度？少到你未来一定能搜到这篇。甚至连github上的使用例子也不多，大致就分两类：计算器、sql解析器，其中计算器目测是国外某大学的课程。所以研究goyacc我花了好几个通宵、掉了少许头发。不经让这篇文章有了一个营销文案：花了一夜时间，搞懂了外国的一堂编译原理课。个人对技术文章的理解是，文章可以有自己的观点、啰嗦、甚至幽默，但尽量不要放在学术部分，毕竟技术是严禁的。所以下面描述，我可尽能做一个无情的打字机，尽可能的按照文档风描述。 0x02 goyacc简易入门安装 goyacc golang 1.8 版本之前 yacc 直接再带与go tool 无需自行安装。鉴于使用的频率太少，遂在 golang 1.8 版本后

MySQL利用ODBC转义（{}）注入手法分析

阅读更多关于 MySQL利用ODBC转义（{}）注入手法分析

前言最近由于工作需要在看libinjection的源码，查到一位师傅发了一篇绕过libinjection的文章， https://www.o2oxy.cn/2772.html 由于URL编码和HEX编码的问题，感觉这位师傅写的不是很对，就发评论跟他讨论，当然他整个libinjection的流程分析还是很不错的。讨论了半天，这位师傅又抛出了一个payload,说libinjection也检测不出来 1 = { date ( if ( mid ( ( updatexml ( 1 , concat ( 0x7e , ( select user ( ) ) , 0x7e ) , 1 ) ) , 1 , 1 ) = '1' , 2 , 1 ) ) } 尝试了一下的确如此: 翻看MySQL语法分析的产生式很久之前看到过这种注入的手法，当时没有深究，这次打算好好研究一下。 mysql的SQL解析语法文件： https://github.com/mysql/mysql-server/blob/8.0/sql/sql_yacc.yy#L9899 （这个是yacc的产生式文件，有兴趣的师傅可以去找找lex&yacc的资料，顺便看看编译原理更佳~） PS：开头注释表明5.7之后语法分析就没有变过看这个文件懵逼了很久，最后直接搜索’{’（三个字符）找到了关键点： '{' 标志符表达式 '}'

Go实现自定义语言的基础

阅读更多关于 Go实现自定义语言的基础

订阅 yacc