It looks like you are using some old version of the library (from 2012).

The latest version, which does not contain the problematic "extern" block, can be found here: https://github.com/adafruit/DHT-sensor-library

(The default example works nicely with my Arduino Nano v3.0 and DHT22 sensor)

The Due is SAM architecture. avr/io.h is specific to the AVR architecture of the Uno's microcontroller. The compiler will not look in the avr tools folder for that file and even if it did it wouldn't help because that file is not compatible with the SAM architecture.

This error indicates that part of the code you're attempting to compile (probably a library) is AVR specific. You will need to either port that code to the SAM architecture or find an alternative that is already compatible with Due.

try installing avr-libc and gcc-avr:

sudo apt-get install avr-libc gcc-avr

Then, to compile code for the AVR you need start by compiling it to an executable using: gcc-avr and then link it using avr-objcopy to create a hex file for the device:

avr-gcc -mmcu=atmegaX yourpgm.c -o yourpgm.elf

avr-objcopy -j .text -O ihex yourpgm.elf yourpgm.hex

Then, the hex file generated is the one to be installed on the device. You can use avrdude to import it. There are a lot more options that you can use for the compiler and linker, I suggest reading the man page for these tools; they are very helpful.

At least in my case the issue was that I needed to install avr-libc as suggested by @"emacs drives me nuts" in the comments.

From the GNU make documentation:

Compiling C++ programs

n.o is made automatically from n.cc, n.cpp, or n.C with a recipe of the form ‘(CPPFLAGS) $(CXXFLAGS) -c’.

Try setting the variable CXX instead of CC.

I’m guessing you started with the toolchain Arduino put on your machine.

Try explicitly including the header files on the include path with the -I switch.

avr-gcc -mmcu=atmega328p -I /path/to/avr/include -o main.out main.c

Just search for the io.h file. It should be somewhere close by to where you found the compiler.

avr/io.h is a C/C++ header that uses the C preprocessor, hence use

#include <avr/io.h>

.include would use gas's preprocessor, which has different syntax and runs at a different time (CPP runs prior to assembler, while gas's is run by the assembler proper).

avr-gcc -mmcu=atmega128 -I/usr/lib/avr/include ...

I'd remove that include path: A sane toolchain will know where to find it's headers. If the system headers are not found without further ado, then you have a broken installation.

FYI, with -save-temps added, CPP will save the pre-processed assembly in main.s.

Some more problems:

• .org ... might not do what you expect. Write your code without .org. As you are using avr-gcc to drive the compilation, your code will be linked against the startup-code from AVR-LibC which includes the vector table and startup-code. You can just start you program in main.

• out DDRB,r16: Here DDRB is the RAM-address of the SFR, not its I/O address!. See notes __SFR_OFFSET at https://www.nongnu.org/avr-libc/user-manual/group__avr__sfr__notes.html