未定义的对 yywra 的引用

我有一个简单的“语言”，我正在使用 Flex (Lexical Analyzer) ，它是这样的:

/* 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);
}

我运行一个 flex count.l，没有错误或警告，一切都很好，然后当我尝试做一个 cc lex.yy.c我得到了这个错误:

Ubuntu@eeepc: ~/Desktop $cc lex.yy.c
/tmp/ccwwkhvq.o: 在函数 < code > yylex’中: C: (. text + 0 x402) : 对 yywra’< br/> 的未定义引用 /tmp/ccwwkhvq.o: 在函数 < code > input’: C: (. text + 0 xe25) : 对 yywra’< br/> 的未定义引用 Collect2: ld 返回1退出状态

怎么了？

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最佳答案

The scanner calls this function on end of file, so you can point it to another file and continue scanning its contents. If you don't need this, use 

%option noyywrap

in the scanner specification.

Although disabling yywrap is certainly the best option, it may also be possible to link with -lfl to use the default yywrap() function in the library fl (i.e. libfl.a) provided by flex. Posix requires that library to be available with the linker flag -ll and the default OS X install only provides that name.

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I prefer to define my own yywrap(). I'm compiling with C++, but the point should be obvious. If someone calls the compiler with multiple source files, I store them in a list or array, and then yywrap() is called at the end of each file to give you a chance to continue with a new file.

int yywrap() {
// open next reference or source file and start scanning
if((yyin = compiler->getNextFile()) != NULL) {
line = 0; // reset line counter for next source file
return 0;
}
return 1;
}
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flex doesn't always install with its development libraries (which is odd, as it is a development tool). Install the libraries, and life is better.

On Redhat base systems:

yum -y install flex-devel
./configure && make

On Debian based systems

sudo apt-get install libfl-dev
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As a note for followers, flex 2.6.3 has a bug where libfl.a "typically would" define yywrap but then doesn't in certain instances, so check if that's your version of flex, might be related to your problem:

https://github.com/westes/flex/issues/154

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int yywrap(){return(1);}

use this code at the end of the program..Simple


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