flex-lexer – 未定义参考到yywrap

/* Just like UNIX wc */
%{
int chars = 0;
int words = 0;
int lines = 0;
%}

%%
[a-zA-Z]+ { words++; chars += strlen(yytext); }
\n        { chars++; lines++; }
.         { chars++; }
%%

int main()
{
    yylex();
    printf("%8d%8d%8d\n",lines,words,chars);
}

我运行flex count.l，一切正常，没有错误或警告，然后当我尝试做一个cc lex.yy.c我得到这个错误：

ubuntu@eeepc:~/Desktop$ cc lex.yy.c
/tmp/ccwwkhvq.o: In function yylex':
lex.yy.c:(.text+0x402): undefined reference toyywrap’
/tmp/ccwwkhvq.o: In function input':
lex.yy.c:(.text+0xe25): undefined reference toyywrap’
collect2: ld returned 1 exit status

哪里不对？