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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;
]
=======

Compile Error
=========
[nnutter@ebb nnutter]$ makecc src/offtrac.c src/read.c src/initialize.c src/iocdf.c src/par_IO.c src/tracadv.c src/sponge.c src/step.c src/paroff.c src/masks.c src/set_metrics.c -o offtrac -lm -lnetcdf -I/usr/local/netcdf/gcc/include -L/usr/local/netcdf/gcc/lib -ffast-math -O2 -march="athlon64" -pipe

/tmp/ccDiRwK4.o: In function `po4_obs_layer':
step.c:(.text+0x1007): undefined reference to `lin_interp'
collect2: ld returned 1 exit status
make: *** [offtrac] Error 1
=========