Undefined Reference To yywrap

I have a simple "language" that I'm using Flex(Lexical Analyzer), it's like this:

/* 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);
}

The I run a flex count.l, all goes ok without errors or warnings, then when I try to do a cc lex.yy.c I got this errors:

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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

What is wrong?

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.

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;
}

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

int yywrap(){return(1);}

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

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