Question about include statements and details in Arduino.h file from core library
Does #include &lt;avr/somelibrary.h&gt; mean that this is a standard library from either avr-g++ or avr-gcc? As opposed to a standard g++ or gcc compiler?


No. It simply means that the compiler should check both its built-in include paths as well as any passed via the -I command line option (or its local equivalent) for a file called "avr/somelibrary.h". That AVR GCC knows where AVR Libc is located is incidental.


  One last question, how can there be an Arduino.h without an Arduino.c?


There is no obligation to match each header file with a .c/.cc/.cpp file, or vice versa.