c++

Yes, header files are needed to provide the declarations for any functions defined outside of the current "compilation unit" (single a ".c" or ".cpp" file). The exception to that rule is if you include the declarations (that would normally go in a header file) in the compilation unit itself, but that isn't terrible useful/good in most cases.

There are tools to automatically create header files from .c files but it's been a while since I did C/C++ programming so I don't know what's available now, and I presume it would depend on your OS.

Thanks
- Mark

Yes, the compiler must be able to find the modules from the headers you are referencing. Those header files contain modules/functions that allow you to use the standard coding references. One way I used to like to do that is in an included file. All header files are listed in the included file. It always seemed nice and neat in the code w/o the huge header. Also you can make that file a standard part of your repertoire, bookshelfed. Just a consideration.

Is it necessary (ie, is it strictly required by the language definition)?

No. Anything that can be declared/defined inside a header file can be declared/defined inside the (.c) source file itself. You are not strictly required to use header files. (Why you would chose not to include the file when it would be clearly to your advantage to do so is an entirely different issue altogether).

Is it recommended/preferable? Yes. Nearly always. (But as with everything else, there are exceptions.)