问编译lex和yacc文件时出现大量错误

EN

bas.y:34:16: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘{’ token
 int main(void) {
y.tab.c:499:2: error: expected declaration specifiers before ‘;’ token
 };

In file included from lex.yy.c:459:0:
/usr/include/unistd.h: In function ‘yyerror’:
/usr/include/unistd.h:258:22: error: storage class specified for parameter ‘useconds_t’
 typedef __useconds_t useconds_t;
                      ^~~~~~~~~~

%{
    #include <stdlib.h>
    #include "y.tab.h"
    void yyerror(char *)
%}

%%

    /* a is value of last expresion */
a   {
        yyval = *yytext - 'a';
        return LAST;
    }

    /* integers */
[0-9]+  {
        yyval = atoi(yytext);
        return INTEGER;
    }

    /* operators */
[-+()=/*\n] { return *yytext; }

    /* skip whitespace */
[ \t]       { ; }

    /* all else is error */
.   yyerror("invalid character");

%%

int yywrap(void) {
    return 1;
}

%token INTEGER LAST
%left '+' '-'
%left '*' '/'

%{
    void yyerror(char *)
    int yylex(void);
    int lastval;
%}

%%

program:
    program expr '\n'   { lastval = $2; }
    |
    ;

expr:
        INTEGER
    | LAST          { $$ = lastval; }
    | expr '+' expr     { $$ = $1 + $3; }
    | expr '-' expr     { $$ = $1 - $3; }
    | expr '*' expr     { $$ = $1 * $3; }
    | expr '/' expr     { $$ = $1 / $3; }
    | '(' expr ')'      { $$ = $2; }
    ;

%%

void yyerror(char *) {
    fprintf(stderr, "%s\n", s);
}

int main(void) {
    yyparse();
    return 0;
}