Re: Linking Problem

[mearvk]

On Feb 7, 1:59 am, Paul Pluzhnikov <ppluzhnikov-...@gmail.com> wrote:
> mearvk <mea...@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.


Does this mean I should be wrapping the log(double) function in the
math library with

#ifdef __cplusplus
extern "C" {
#endif

or that I should be doing this in lip.c's functions that reference the
math library functions?


Thanks,

Max