# Calling the GeographicLib C++ library from C

The geodesic routines in GeographicLib have been implemented as a native
C library.  See

  https://geographiclib.sourceforge.io/C/doc/

It is also possible to call the C++ version of GeographicLib directly
from C and this directory contains a small example, which convert
heights above the geoid to heights above the ellipsoid.  More
information on calling C++ from C, see

  https://isocpp.org/wiki/faq/mixing-c-and-cpp

To build and install this interface, do
```bash
mkdir BUILD
cd BUILD
cmake ..
make
```
This assumes that you have installed GeographicLib somewhere that cmake
can find it.  If you want just to use the version of GeographicLib that
you have built in the top-level BUILD directory, include, e.g.,
```bash
-D GeographicLib_DIR=../../BUILD
```

in the invocation of cmake (the directory is relative to the source
directory, wrapper/C).  To convert 20m above the geoid at 42N 75W to a
height above the ellipsoid, use
```bash
$ echo 42 -75 20 | ./geoidtest
-10.672
```

Notes:

* The geoid data (`egm2008-1`) should be installed somewhere that
  GeographicLib knows about.

* This prescription applies to Linux machines.  Similar steps can be
  used on Windows and MacOSX machines.

* It is essential that the application be linked with the C++ compiler,
  so that the C++ runtime library is included.

* In this example, the main program is compiled with the C compiler.  In
  more complicated situations, it may be necessary to use the C++
  compiler.  This is necessary to get static initializations of C++
  classes performed.  (However, GeographicLib doesn't need any static
  initialization.)