Using C files in Arduino

The .ino files of Arduino sketches are compiled as C++ after a little bit of preprocessing. If you want to use C functions in C++, you need to wrap their declarations in:

extern "C" {}
There are a couple ways you can do this:

In the sketch:

extern "C" {
#include "SomeCFunctions.h"
}
Or, in the .h file:

#ifdef __cplusplus
extern "C" {
#endif

// C function declarations here

#ifdef __cplusplus
}
#endif