Build error: Previous declaration with C++ linkage; arduino

I am using arduino uno R3 with v1.6.5

1. I have attached a code which has 4 .ino files & 6 headers.
Setup is the main file which has loop & setup function.
Here I have removed entire code, just put code which was giving error.

2. Now if I go to key.h & uncomment static function prototype there,compiler through error. however if I uncomment it error goes & code works.

<code>
setup.ino:16:13: error: previous declaration of 'void get_key_number_if_valid()' with 'C++' linkage
In file included from key.ino:5:0:
C:\Users\abc\AppData\Local\Temp\build2487483355140505388.tmp\key.h:33:41: error: conflicts with new declaration with 'C' linkage
static void get_key_number_if_valid(void);
^
previous declaration of 'void get_key_number_if_valid()' with 'C++' linkage
</code>


3.However in setup.ino, I have one static function do_nothiong(), which has function prototype in setup.h, but it don't give any error there.