For any given input file, the file name suffix determines what kind of compilation is done. The following kinds of input file names are supported:
file.dD source files.
file.ddDdoc source files.
file.diD interface files.
You can specify more than one input file on the gdc command line,
each being compiled separately in the compilation process. If you specify a
-o file option, all the input files are compiled together,
producing a single output file, named file. This is allowed even
when using -S or -c.
A D interface file contains only what an import of the module needs,
rather than the whole implementation of that module. They can be created
by gdc from a D source file by using the -H option.
When the compiler resolves an import declaration, it searches for matching
.di files first, then for .d.
A Ddoc source file contains code in the D macro processor language. It is
primarily designed for use in producing user documentation from embedded
comments, with a slight affinity towards HTML generation. If a .d
source file starts with the string Ddoc then it is treated as general
purpose documentation, not as a D source file.