I'm now really diving into my OS project, called ForestOS, but now I'm needing to dive more into some simple and basic things of C. As now I'm having many problems with the correct variable to use and functions.
I want resources that only talk about variables, functions and how to develop without headers(stdio.h, math.h and all the others).
Best starting place is probably the book The C Programming Language.
The book was central to the development and popularization of the C programming language and is still widely read and used today.
A guide to OS development suggests CProgramming.com as the best place to start. There's tutorials, links to further resources, and everything for free.
Building an OS is non-trivial, I suggest if you are "having many problems with the correct variable to use and functions" then you may be attempting to walk before you can run!
Quote:
how to develop without headers(stdio.h, math.h and all the others).
I assume that you actually mean that you want to code without using the standard library rather than "without headers". Header files are intrinsic to modularisation in C; if you did not use headers, your code would have to be one monolithic module. Don't confuse headers with libraries.
However, even then there is no need not to use the standard library when writing 'bare-metal' code. You simply need a library that does not have OS dependencies, and you write the low level glue code to make things like stdio and memory allocation work on your system. Such a library is Newlib for example. It will make your life a whole lot easier if you have standard library support.
You only need headers to provide declarations of functions and external variables.
It is possible to eliminate the header files and provide your declarations within the translation unit (a.k.a. source file). Although possible, this is not recommended.
Here is an example of a legal C program without header files:
/* Forward declaration of main(). */
int main(void);
/* Definition for main() function. */
int
main(void)
{
return 13; /* 42 is such an overrated number. */
}
Some reasons for using header files are: code / typing reduction and single point of maintenance. If two modules need the same structure declaration, placing it in a header file will reduce typing (you only have to #include it in both files instead of copying it into both files). Also, if you need to change any declaration, if it is copied, you'll have to hunt down all copies and change every instance vs. making one change in a header file.
As far as standard header files, such as math.h and stdio.h, if you don't need them, don't include them. An OS should not require stdio.h, but may use math.h. Most standard header files do not contribute to the code size; only to the compile time.
I highly suggest you focus on the correctness of your OS and don't worry about trivialities such as header files. After your OS is working correctly and robust, go ahead and trim the fat.