Yesterday, when I was debugging C program, I made this mistake, and my mentality was broken. But fortunately through their own a period of a period of program debugging, finally or to find the error. I will write it down, and I hope that the same small white encountered such mistakes, in time to find.
When the following error occurs:

There may be several reasons:
1. Library function spelling error. Such as: printf, Scanf, etc
2. The function name defined was misspelled during the call
3, when the execution of the function may function body itself
4. I haven’t noticed (maybe I haven’t come across yet) that other people say there are programs running.

That’s all I’ve found so far in writing C programs, and I hope it helps you