header files in C

Should you have to include a header file into its own library? I can't understand why I sometimes, but not always, get compile errors because it's not getting the header file prototypes before it compiles the libraries.

>>11
It's not that I don't want to include them, it's just that I was curious as to how it's supposed to work. I thought that since the headers are included in the main program file that including them again is redundant.