filesystem timestamps and their usage

[deeptech71 at gmail.com]

Let's talk about file system timestamps. What kind of timestamps are there 
currently, and what are they used for? (or what new timestamps would be 
needed/beneficial, or whatever.)

Currently I can think of only two:
1. data_time: The last time the file was written to.
2. file_time: The last time the file was moved or written to. If a directory is 
moved, all files within are also time-updated.
It should be considered invalid, if data_time > file_time, but can happen if, 
for example, a file is copied from somewhere, like an FTP server, and ?therefore 
not be automatically corrected? (it does not imply filesystem corruption).

Some basic usage for compiling. Here's func.x86's dependency tree:

common.h }   (func.c includes func.h, which includes common.h)
func.h   }
func.c   } -> func.o }               (func.x86 is generated
                      } -> func.x86   from func.o and main.o)
common.h } -> main.o }
main.h   }
main.c   }   (main.c includes main.h, which includes common.h)

We have compiled everything already. Now let's do something.

As a requirement, the data_time of a derived file must be later in time, for it 
to be considered up to date. That is, if I edit func.h, it's timestamp will be 
higher than func.c's timestamp, meaning that I have to recompile func.c, and 
link the executable. Generally, a file's (like func.x86's) time must be higher 
than every other file it depends on (func.o, main.o).

Next, if I replace common.h with a different file (because I want to recompile 
with someone else's common.h), the data_time of common.h may be lower than 
main.h. So all the data_time and file_time of main.o and func.o must be higher 
than the ones of common.h. Generally both of the file's times should be higher 
than any of its dependencies' times.

If a bad time (data_time > file_time) is detected, and its file depends on 
something, it's remade. If it does not depend on anything, I don't know, maybe 
the compiler should warning or use the file_time only.

If I want to replace the .o (wtf for?), then I'll have to build, replace, then 
link only.

OK that would be the basic usage of these two timestamps in compiling. If I'm 
wrong, or something would be more practical maybe, please tell. And, what about 
other timestamps?

THX