Should I use #include in headers?

Question

Is it necessary to #include some file, if inside a header (*.h), types defined in this file are used?

For instance, if I use GLib and wish to use the

Answer 1

During compilation preprocessor just replaces #include directive by specified file content. To prevent endless loop it should use

#ifndef SOMEIDENTIFIER
#define SOMEIDENTIFIER
....header file body........
#endif

If some header was included into another header which was included to your file than it is not necessary to explicitly include it again, because it will be included into the file recursively