Lex和Yacc符号表的生成和操作

Question

我正在尝试使用lex进行词法分析阶段并使用yacc进行语法分析来构建一个简单的C编译器。我正在用lex构建符号表，并用在词法分析中遇到的所有标识符（现在只是标识符的名称，行号和范围）填充它。符号表本身采用链接列表的形式，并且指针指向链接列表的开头。现在，我想能够在yacc中访问此符号表，以更新每个标识符的值和数据类型。如何访问yacc中的符号表？

我已经在lex中将头指针定义为extern，但这没有帮助。这是完整的代码供参考-

Lex（ngrammar.l）-

%{
    #include<stdio.h>
    #include<string.h>
    #include<stdlib.h>
    #define MAX 1000
    extern int yylval;
    int scope = 0;
    int lineno = 1;
    int paran = 0;
    typedef struct node{
        int index;
        int scope;
        char symbol[MAX];
        char lineno[MAX];
        char type[MAX];
        char value[MAX];
        struct node* next; 
    }node;
    extern node* head;
    head = NULL;
    node* InsertSymbol(node*,int,char*,int,int);
    void display(node* head);
    node* InsertSymbol(node* head, int scope, char* symbol, int lineno, int paran){
        if(paran>0){
            scope++;
        }
        else{;
        }
        if(head == NULL){
            node* temp = (node*)malloc(sizeof(node));
            temp->index = 1;
            temp->scope = scope;
            strcpy(temp->symbol,symbol);
            char str[4];
            sprintf(str, "%d", lineno);
            strcpy(temp->lineno,str);
            temp->next = NULL;
            head = temp;
        }
        else{
            node* cur= head;
            node* prev = NULL;
            int present = 0;
            while(cur != NULL){
                if((cur->scope == scope) && (strcmp(cur->symbol,symbol)==0)){
                    char str[4];
                    sprintf(str, ", %d", lineno);
                    strcat(cur->lineno,str);
                    present = 1;
                }
                prev = cur;
                cur = cur->next;
            }
            if(present == 0){
                node* temp = (node*)malloc(sizeof(node));
                temp->index = (prev->index)+1;
                temp->scope = scope;
                strcpy(temp->symbol,symbol);
                char str[4];
                sprintf(str, "%d", lineno);
                strcpy(temp->lineno,str);
                temp->next = NULL;
                prev->next = temp;
            }
        }
        return head;
    }
    void display(node* head){
        node* p = head;
        printf("\t\t\t SYMBOL TABLE \t\t\t\n\n");
        printf("\t Index \t\t Symbol \t Scope \t\t Line Number \n");
        if(p == NULL){
            printf("nothing");
            return;
        }
        else{
            while(p != NULL){
            //printf("enter");
                printf("\t %d \t\t %s \t\t %d \t\t %s \n",p->index,p->symbol,p->scope,p->lineno);
                p = p->next;
            }
        }
    }
%}

alpha [A-Za-z]
digit [0-9]
und [_]
space [ ]
tab [   ]
line [\n]
acc [^"*/"]
str [^\"]

%%
\/\/(.*)[\n]* {;}
\/\*({acc}*\n)*{acc}*\*\/[\n]* {;}
for {return FOR;}
if {return IF;}
char {return CHAR;}
float {return FLOAT;}
int {return INT;}
continue {return CONTINUE;}
return {return RETURN;}
bool {return BOOL;}
main {return MAIN;}
else {return ELSE;}
printf {return PRINT;}
break {return BREAK;}
TRUE {return BOOLTRUE;}
FALSE {return BOOLFALSE;}
\|\| {return OR;}
\%d|\%c|\%f {return FORMATSPEC;}
\( {return OPENBRACES;paran++;}
\) {return CLOSEBRACES;paran--;}
\{ {return OPENCURLYBRACES;scope++;}
\} {return CLOSECURLYBRACES;scope--;}
[.] {return DOT;}
= {return ASSIGNOP;}
\+\+|\-\- {return UNARYOP;}
\+|\-|\*|\/ {return ARITHOP;}
>|<|>=|<=|!=|== {return RELOP;}
&& {return AND;}
[,] {return COMMA;}
[;] {return SEMICOLON;}
['] {return SINGLEQUOTES;}
["] {return DOUBLEQUOTES;}
["]{str}*["] {return STRING;}
{alpha}({alpha}|{digit}|{und})* {return IDENTIFIER;
                                head = InsertSymbol(head,scope,yytext,lineno,paran);}
{digit}+ {return INTCONST;}
({digit}+)\.({digit}+) {return FLOATCONST;}
[\n] {lineno++;}
[\t] {;}
[ ] {;}
. {return yytext[0];}
%%

int yywrap()
{
    return 1;
}

Yacc（ngrammar.y）-

%{
    #include<stdio.h>
    #include<string.h>
    int valid=1;
%}

%token CHAR INT FLOAT BOOL MAIN FOR IF ELSE PRINT BREAK CONTINUE BOOLTRUE BOOLFALSE FORMATSPEC OPENBRACES CLOSEBRACES OPENCURLYBRACES CLOSECURLYBRACES UNARYOP ARITHOP RELOP ASSIGNOP COMMA SEMICOLON SINGLEQUOTES DOUBLEQUOTES UNDERSCORE RETURN AND OR DOT STRING IDENTIFIER INTCONST FLOATCONST

%%
start: INT MAIN OPENBRACES CLOSEBRACES OPENCURLYBRACES statement return_statement CLOSECURLYBRACES;
statement: compound_statement | expression_statement | jump_statement | print_statement | for_loop | if_else;
for_loop: FOR OPENBRACES declaration condition_statement SEMICOLON for_expression_statement CLOSEBRACES statement;
if_else: IF OPENBRACES condition_statement CLOSEBRACES statement ELSE statement;
return_statement: RETURN identifier SEMICOLON | RETURN intconstant SEMICOLON;

declaration: declaration_specifiers init_declarator_list SEMICOLON | init_declarator_list SEMICOLON;
declaration_specifiers: CHAR | INT | FLOAT | BOOL;
init_declarator_list: init_declarator | init_declarator COMMA init_declarator_list;
init_declarator: identifier | identifier ASSIGNOP initializer;
initializer: numconstant | identifier;

compound_statement: OPENCURLYBRACES CLOSECURLYBRACES | OPENCURLYBRACES block_item_list CLOSECURLYBRACES | block_item_list;
block_item_list: block_item block_item | block_item;
block_item: declaration | statement |;
expression_statement: identifier ASSIGNOP expression SEMICOLON | identifier UNARYOP SEMICOLON | UNARYOP identifier SEMICOLON;
expression: identifier ARITHOP identifier | identifier ARITHOP numconstant | numconstant ARITHOP numconstant;
for_expression_statement: identifier ASSIGNOP expression  | identifier UNARYOP | UNARYOP identifier;

condition_statement: identifier RELOP identifier | identifier RELOP numconstant | numconstant RELOP identifier | condition_statement AND condition_statement | condition_statement OR condition_statement | BOOLTRUE | BOOLFALSE;
jump_statement: BREAK SEMICOLON | CONTINUE SEMICOLON;

print_statement: PRINT OPENBRACES stringconstant CLOSEBRACES SEMICOLON | PRINT OPENBRACES DOUBLEQUOTES FORMATSPEC DOUBLEQUOTES COMMA identifier CLOSEBRACES SEMICOLON;

identifier: IDENTIFIER;
intconstant: INTCONST;
numconstant: INTCONST | FLOATCONST;
stringconstant: STRING;
%%
#include "lex.yy.c"
int yyerror()
{
    printf("\nDoes not satisfy the grammar\n");
    valid=0;
    return 0;
}
int main()
{
    yyin = fopen("in.c","r");
    yyparse();
    if(valid)
    {
        printf("\nSatisfies the grammar \n");
    }
    display(head);  
}

以下屏幕截图显示了警告和执行后的输出-

（（我尚未验证代码以更新值和数据类型。这只是为了以yacc显示符号表。）

Answer 1

C程序的顶层只能包含声明和定义。它不能包含语句。因此，在C程序的顶级中，以下内容不合法：

extern node* head;
head = NULL;

第二行被转换成一个声明，因为GCC仍然允许使用隐式类型的标准C之前的声明（类型默认为int）。它确实会警告您，但是警告不是很有帮助。

结果是上面的解释好像已经被解释了>>

extern node* head;
int head = NULL;
这显然是非法的，因为您不能给全局定义使用相同的名称。然后，GCC继续使用第二个定义，其余错误级联。

这与flex或bison无关，与您的符号表实现无关，我没有看过。但这很好地说明了为什么在编写解析器时应该考虑生成良好的错误消息。