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.

>>30
sub factor{my@f,$n=pop;map{$n/=$_,push@f,$_ until$n%$_}2..$n/2;@f?@f:($n)}
print "prime factors of $_: ",join',',factor$_ for@ARGV