functions.h
========
#ifndef H_OT_FUNCTIONS
#define H_OT_FUNCTIONS

extern float lin_interp(float pleth, float x[], float z[], int istart, int npts);

#endif
========

functions.c
=======
#include "functions.h"

float lin_interp(float pleth, float x[], float z[], int istart, int npts)
{
...
return whatever;
]
=======