Re: Linking Problem

[Paul Pluzhnikov]

mearvk <mearvk@gmail.com> writes:

> "g++ -c lip.C" gives me lip.o.
> "g++ -c test.C" gives me test.o
> "g++ -o out test.o lip.o -lm" should give me 'out' as an executable
> but it doesn't, instead giving me:
>
> lip.o(.text+0xf88d): In function `zfread(_IO_FILE*, long**)':
> : undefined reference to `log(double)'

The problem is that lip.o references C++ mangled function
'log(double)', but it should be referencing 'extern "C"' function
'log'.

Here is an example showing the same problem:

    $ cat t.c
    double log(double); /* problem */
    int main() { double d = log(100); return 0; }

When above code is compiled in "C" mode, it works:

  $ gcc -fno-builtin t.c -lm

But when compiled as C++, it doesn't:

    $ g++ -fno-builtin t.c -lm
    /tmp/cc0o3mP7.o(.text+0x20): In function `main':
    : undefined reference to `log(double)'
    collect2: ld returned 1 exit status

The problem is that 'double log(double);' is correct for C, but it
is *not* correct for C++. It should be:

  #ifdef __cplusplus
  extern "C"
  #endif
  double log(double);

I am guessing that your 'lip.C' should actually be named 'lip.c',
and should be compiled with 'gcc' instead of 'g++'.

Cheers,
-- 
In order to understand recursion you must first understand recursion.
Remove /-nsp/ for email.