Why use object files in C?

Question

When I compile a C program, for ease I've been including the source file for a certain header at the end. So, if main.c includes util.h, util.h will have all the headers util.c will use, outlines types or structs, etc, then at the very end it include util.c. Then, when I compile I only have to use gcc main.c -o main, and the rest is all taken care of.

I've been looking up C coding standards, trying to figure out what the best way to do things is, and there are just so many, and so many conflicting opinions I don't know what to think. Why do so many places reccomend compiling object files individually instead of including all of them in a web? util never touches anything but util.c, so the two are perfectly independent, and in theory (my theory) it would be fine, but I'm probably wrong since this is computer science and people are wrong even when they're right, so if I'm already wrong I'm probably wrong.

Some people say header files should ONLY be prototypes, and the source file be the one that includes it, and it's necessary system headers. From purely as aesthetic point of view I much prefer having all the info (types, system headers used, prototypes) in the header (in this case util.h) and having ONLY function code in util.c (excluding one "#include "util.h"" at the very top).

I guess the point I'm getting at is, with all this stuff that works, selecting a method sounds arbitrary to someone who doesn't understand the background (me). Please tell me why and what.

Answer 1

While your program is small, this will work. At some point, however, your program will get large enough that recompiling the whole program every time you change one line is a pain in the rear.

This -- even more than avoiding editing huge files -- is the reason to split up your program. If main.c and util.c are seperately compiled into object files, changing one line in a function in main.c will no longer require you to recompile all the code in util.c.

By the time your program is made up of a few dozen files, this will be a big win.