just an idea: the header qtbwave.h can be modified the following way, to get compatibility with gcc using pure C calls.
Additionally: do you also have a C wrapper for ellipticity calculation?
I'm asking only because I don't want to reinvent the wheel (

Ciao and thanks.
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# ifndef QTBWAVE_H
# define QTBWAVE_H
#ifdef __cplusplus
extern "C" {
#endif
void dispersion_curve_init_();
void dispersion_curve_rayleigh_( int * nLayers, double * h, double * vp, double * vs, double * rho,
int * nSamples, double * omega,
int * nModes, double * slowness );
void dispersion_curve_love_( int * nLayers, double * h, double * vp, double * vs, double * rho,
int * nSamples, double * omega,
int * nModes, double * slowness );
#ifdef __cplusplus
}
#endif
# endif // QTBWAVE_H