将C Flex lexer链接到C ++解析器

Question

I am using Flex to develop a lexer for a language of mine. I want to create the parser in C++ and I am using my own approach for it. Since the lexer is in C (Flex), I want to compile the lexer ( gcc -c lex.yy.c -lfl ) and my program ( g++ -c file1.cc file2.cc ... ) separately and then link them together to create the final executable.

In particular, after creating the lexer, I write the following code for my C++ program:

#include <iostream>

extern int yylex();
int main(int argc, char** argv);

int main (int argc, char** argv) {
  yylex();
  return 0;
}

But I get link error when I link stuff together like:

g++ main.cc lex.yy.o -lfl

But I always get:

/tmp/ccBrHObp.o: In function `main':
main.cc:(.text+0x10): undefined reference to `yylex()'
collect2: ld returned 1 exit status

Where is the problem? Thankyou

Answer 1

What happens here is that you compile the C code, which is something like this:

int yylex() {
  // ...
}

It generates a yylex symbol. Then you compile the C++ code. As C++ allows two different functions with the same name to exist, it can't just use the name of the function as the symbol name. When you write int yylex(); on C++ code, GCC looks for this symbol: _Z5yylexv . It doesn't find, link error. The solution is to say that this is a C function and we should use its name as symbol name:

#include <iostream>

extern "C"
int yylex();

int main (int argc, char** argv) {
  std::cout << yylex() << std::endl;
  return 0;
}