Undefined Reference To yywrap

Question

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:

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?

Answer 1

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.

Answer 2

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; }