在Visual Studio 2010中编译为C代码

I am writing some C code to interface with a piece of hardware. This hardware has built-in API functions, which are defined in a header file, enclosed in an extern "C" block. For some reason, I cannot get the program to compile when I select "compile as C code" in the property sheets. Even stranger, I can get it to give no errors when I "compile as C++", don't change any code, then change it back to "compile as C". What is going on? The only two files I have are main.c, header_file.h, and a defines_file.h (which is I am writing some C code to interface with a pi