c++ - Can't get past "undefined reference to `XXXX'"

Question

I'm currently writing a header-only wrapper library which is supposed to provide C++ access to some FORTRAN code. But I'm stuck at the 'undefined reference (...)'

I must say I'm a little bit surprised because I compiled my own functions in C++ using this FORTRAN routine, with no problems at all.

Maybe a few words about the structure. The library follows the singleton pattern, but without the possibility of instantiating the Library by the user. So the only entry point for the user is a static method MultiNestWrapper::Wrapper<T>::run(). MultiNestWrapper is a namespace, Wrapper<T> is a templated class (so that in future you can benchmark how choosing on which type to perform calculations affects results and performance). The FORTRAN routine is declared outside of this namespace, as

extern "C" {
  extern void __nested_MOD_nestrun(int *, int *, int *, double *, double *, int *, int *, int *, int *, int *, double *, const char *, int *, int *, int *, int *, void (*Loglike)(double *, int *, int *, double *), void (*dumper)(int *, int *, int *, double **, double **, double *, double *, double *), int *context);
}

and I call it like

__nested_MOD_nestrun(&_mmodal, &_ceff, &_nlive, &_tol, &_efr, &_ndims, &_nPar, &_nClsPar, &_maxModes, &_updInt, &_Ztol, _root, &_seed, _pWrap, &_fb, &_resume, internalLogLike, internalDumper, &_context);

And the types for arguments match.

When I try to compile it, I get the following error:

[dare2be@schroedinger multinest-interfejs]$ make all
g++ -c ExampleLibMnCpp.cpp -o ExampleLibMnCpp.o
gfortran -lstdc++ -llapack -lblas -lnest3 -L/usr/local/lib ExampleLibMnCpp.o -o ExampleLibMnCpp
ExampleLibMnCpp.o: In function `MultiNestWrapper::Wrapper<double>::run(MultiNestWrapper::MNParams<double>*, double (*)(double const*, int), bool, std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int, double, int, double, void (*)(int*, int*, int*, double**, double**, double*, double*, double*), int, double, bool)':
ExampleLibMnCpp.cpp:(.text._ZN16MultiNestWrapper7WrapperIdE3runEPNS_8MNParamsIdEEPFdPKdiEbSsididPFvPiS9_S9_PPdSB_SA_SA_SA_Eidb[MultiNestWrapper::Wrapper<double>::run(MultiNestWrapper::MNParams<double>*, double (*)(double const*, int), bool, std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int, double, int, double, void (*)(int*, int*, int*, double**, double**, double*, double*, double*), int, double, bool)]+0x585): undefined reference to `__nested_MOD_nestrun'
collect2: ld returned 1 exit status
make: *** [ExampleLibMnCpp] Error 1

But, note that

[dare2be@schroedinger multinest-interfejs]$ nm /usr/local/lib/libnest3.a | grep __nested_MOD_nestrun
000000000001e0f0 T __nested_MOD_nestrun

I've always dealt with those undefined references myself. But now I can't wrap my head around this. I specify -lnest3 -L/usr/local/lib explicite and /usr/local/lib/libnest3.a contains the routine the linker whines about... Please, help a brother out! :)

EDIT: fixed typos

Answer 1

The order in which you specify libraries in GCC's command-line matters. Read GCC man pages for details - in short you have to specify libraries (-lnest3 etc) after modules that use them. (I always specify libraries in the end of the command-line.)

In your case you have to write

gfortran ExampleLibMnCpp.o -o ExampleLibMnCpp -L/usr/local/lib -lstdc++ -llapack -lblas -lnest3