Hi Ming-Chia,
>Is there any difference between declaring functions with prefix "extern" and without "extern" keyword in header files?
From a C++ vantage point...
Effectively (by and large), no there's no difference.
Philosophically, yes. Some consider (including myself) that supplying the (optional) <code>extern</code> keyword is good practice because it is self-documenting code.
Also, be aware that <code>extern "C"</code> has several important (critical!) implications regarding C++ code.
Also, in some areas, <code>extern</code> is REQUIRED to distinguish something that is "just a declaration" from being mistaken for a "definition" (without having the <code>extern</code>). See Stroustrup's C++ Programming Language (3rd or special edition) section 9.2.
I haven't done C work since 1990, so my comments may not be appropriate for C "good style / best practices".
HTH, --Eljay
