基于flex/bison对SysY语⾔进行语法分析 代码

下面是一个基于Flex/Bison对SysY语言进行词法分析和语法分析的示例代码：

Flex文件（sysy.l）：

%{
#include "sysy.tab.h"
%}

%%

"int"           { return INT; }
"if"            { return IF; }
"else"          { return ELSE; }
"while"         { return WHILE; }
"return"        { return RETURN; }
"or"            { return OR_OP; }
"and"           { return AND_OP; }

"=="            { return EQ_OP; }
"!="            { return NE_OP; }
"<="            { return LE_OP; }
">="            { return GE_OP; }

"+"             { return '+'; }
"-"             { return '-'; }
"*"             { return '*'; }
"/"             { return '/'; }
"%"             { return '%'; }
"!"             { return '!'; }
"="             { return '='; }
"<"             { return '<'; }
">"             { return '>';}
";"             { return ';'; }
","             { return ','; }
"("             { return '('; }
")"             { return ')'; }
"{"             { return '{'; }
"}"             { return '}'; }

[0-9]+          { yylval.num = atoi(yytext); return NUM; }
[a-zA-Z][a-zA-Z0-9]* { yylval.id = strdup(yytext); return ID; }

[ \t\n]         { /* ignore whitespace */ }

.               { fprintf(stderr, "Unknown character: %s\n", yytext); }

%%

int yywrap() {
    return 1;
}

Bison文件（sysy.y）：

%{
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define YYSTYPE_IS_DECLARED
#define YYSTYPE int
#include "sysy.tab.h"

extern int yylex();
extern int yylineno;
extern char* yytext;

void yyerror(const char *msg);

%}

%token INT ID IF ELSE WHILE RETURN
%token <num> NUM

%left OR_OP
%left AND_OP
%left EQ_OP NE_OP
%left '<' '>' LE_OP GE_OP
%left '+' '-'
%left '*' '/' '%'
%right '!'
%right '='
%left UMINUS

%%

program:
    function_list
    ;

function_list:
    function
    | function_list function
    ;

function:
    type_specifier ID '(' parameter_list ')' compound_stmt
    ;

type_specifier:
    INT
    ;

parameter_list:
    parameter_decl
    | parameter_list ',' parameter_decl
    ;

parameter_decl:
    type_specifier ID
    ;

compound_stmt:
    '{' local_declarations statement_list '}'
    ;

local_declarations:
    /* empty */
    | local_declarations var_declaration
    ;

var_declaration:
    type_specifier ID ';'
    ;

statement_list:
    /* empty */
    | statement_list statement
    ;

statement:
    expression_stmt
    | compound_stmt
    | selection_stmt
    | iteration_stmt
    | return_stmt
    ;

expression_stmt:
    expression ';'
    | ';'
    ;

selection_stmt:
    IF '(' expression ')' statement
    | IF '(' expression ')' statement ELSE statement
    ;

iteration_stmt:
    WHILE '(' expression ')' statement
    ;

return_stmt:
    RETURN ';'
    | RETURN expression ';'
    ;

expression:
    assignment_expression
    ;

assignment_expression:
    logical_or_expression
    | unary_expression '=' assignment_expression
    ;

logical_or_expression:
    logical_and_expression
    | logical_or_expression OR_OP logical_and_expression
    ;

logical_and_expression:
    equality_expression
    | logical_and_expression AND_OP equality_expression
    ;

equality_expression:
    relational_expression
    | equality_expression EQ_OP relational_expression
    | equality_expression NE_OP relational_expression
    ;

relational_expression:
    additive_expression
    | relational_expression '<' additive_expression
    | relational_expression '>' additive_expression
    | relational_expression LE_OP additive_expression
    | relational_expression GE_OP additive_expression
    ;

additive_expression:
    multiplicative_expression
    | additive_expression '+' multiplicative_expression
    | additive_expression '-' multiplicative_expression
    ;

multiplicative_expression:
    unary_expression
    | multiplicative_expression '*' unary_expression
    | multiplicative_expression '/' unary_expression
    | multiplicative_expression '%' unary_expression
    ;

unary_expression:
    postfix_expression
    | '-' unary_expression %prec UMINUS
    | '!' unary_expression
    ;

postfix_expression:
    primary_expression
    | ID '(' argument_list ')'
    ;

primary_expression:
    ID
    | NUM
    | '(' expression ')'
    ;

argument_list:
    /* empty */
    | expression
    | argument_list ',' expression
    ;

%%

int main(int argc, char **argv) {
    yyparse();
    return 0;
}

void yyerror(const char *msg) {
    fprintf(stderr, "Syntax error at line %d: %s\n", yylineno, msg);
    exit(1);
}

在main函数中调用yyparse函数，就可以启动语法分析器进行语法分析。当语法分析器遇到错误时，会通过yyerror函数输出错误信息并退出程序。