Question

When I attempt to compile the output of this trivial lex program:

# lex.l
integer   printf("found keyword INT");

using:

$ gcc lex.yy.c

I get:

Undefined symbols:
  "_yywrap", referenced from:
      _yylex in ccMsRtp7.o
      _input in ccMsRtp7.o
  "_main", referenced from:
      start in crt1.10.6.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

lex –version tells me I’m actually using ‘flex 2.5.35’ although ls -fla `which lex` isn’t a symlink. Any ideas why the output won’t compile?

Answer 1

From the Flex manual:

I get an error about undefined yywrap().

You must supply a yywrap() function of your own, or link to libfl.a (which provides one), or use

%option noyywrap

in your source to say you don’t want a yywrap() function.

Also:

When the scanner receives an
end-of-file indication from
YY_INPUT, it then checks the yywrap() function. If
yywrap() returns false (zero), then it is assumed that the
function has gone ahead and set up yyin to point to
another input file, and scanning continues. If it returns
true (non-zero), then the scanner terminates, returning 0
to its caller. Note that in either case, the start condi-
tion remains unchanged; it does not revert to INITIAL.