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# 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.)
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