Arduino compile error

In file included from /home/marcel/dev/Arduino/encoders/main.ino:1:0:
/home/marcel/dev/Arduino/encoders/./Encoders.c: In static member function ‘static void Encoders::switchPressedExt0()’:
/home/marcel/dev/Arduino/encoders/./Encoders.c:7:33: error: ‘NULL’ was not declared in this scope
   if(Encoders::instances [0] != NULL){
                                 ^
/home/marcel/dev/Arduino/encoders/./Encoders.c: In static member function ‘static void Encoders::switchPressedExt1()’:
/home/marcel/dev/Arduino/encoders/./Encoders.c:13:34: error: ‘NULL’ was not declared in this scope
   if (Encoders::instances [1] != NULL){
                                  ^
/home/marcel/dev/Arduino/encoders/./Encoders.c: In member function ‘void Encoders::begin(int)’:
/home/marcel/dev/Arduino/encoders/./Encoders.c:21:23: error: ‘INPUT_PULLUP’ was not declared in this scope
    pinMode (whichPin, INPUT_PULLUP);
                       ^
/home/marcel/dev/Arduino/encoders/./Encoders.c:21:35: error: ‘pinMode’ was not declared in this scope
    pinMode (whichPin, INPUT_PULLUP);
                                   ^
/home/marcel/dev/Arduino/encoders/./Encoders.c:24:44: error: ‘CHANGE’ was not declared in this scope
      attachInterrupt(0, switchPressedExt0, CHANGE);
                                            ^
/home/marcel/dev/Arduino/encoders/./Encoders.c:24:50: error: ‘attachInterrupt’ was not declared in this scope
      attachInterrupt(0, switchPressedExt0, CHANGE);