For C programs, the piapix.h file provides access to all of the function prototypes and necessary defines to compile against the extended PI-API. You need not include the standard PI-API include file piapi.h; the extended PI-API include file does this for you.
The pistatus.h file contains symbolic names for PI-API error codes. You may wish to use these in your programs to make them more readable. The file can be used for both C and C++ application development.