Time stamp resolution

[Jason Pearce]

I am having problems with a build system based on Make. It is quite a 
complex issue but I have traced it to what I believe to be a time stamp 
issue. I would really appreciate some help.

I think the heart of my question is:

Is it safe to assume that two files, generated in sequential order, will 
always have increasing time stamps?
For example:
> touch file1; touch file2
Can I then rely on (time stamp of file1) < (time stamp of file2)?



The background details are as follows.

I am compiling VHDL. I have a script which generates the dependencies 
for each VHDL compile, generating a fragment file vhdldep.mak with the 
rules in it. But some of my sources are generated, so the compilation 
gets done in 2 steps.

> make sources
> make top_entity

The target top_entity comes from the makefile fragment, it is the top 
level test bench which is the result of compiling one of the generated 
sources. (A simplified Makefile is at the bottom of this mail).

The first call to make generates an incomplete vhdldep.mak file because 
the generated sources have not been generated yet! As that make session 
runs, the sources get generated and on the next call of make, the 
freshly generated source files (should) trigger vhdldep.mak to be 
regenerated, now in a complete form.
(Aside - I don't know how to do this in one step, I think Make will only 
create its dependency tree once per run).

Now this all works perfectly well under Cygwin (and I am completely 
confident it the generated dependencies are correct). But on a HP box 
and a Linux machine the rebuild of vhdldep.mak is not getting triggered. 
It seems to be caused by "identical" time stamps on my generated sources 
and vhdldep.mak. Touching any source or uncommenting the sleep command 
within the rules for vhdldep.mak makes the problem disappear. I get the 
same behavior on network and local disks.

So I am now unsure whether I should be checking the time/file system 
configuration of my Linux and HP boxes, or if I am simply relying on 
something I can not - that sequentially generated files will always have 
a monotonically increasing time stamps.


Regards,
Jason


VHDL_SOURCES    := $(wildcard *.vhd)

vhdldep.mak : ${VHDL_SOURCES}
   perl vhdldep.pl -t mak ${VHDL_SOURCES} > vhdldep.makfail
#    sleep 1
   /bin/mv vhdldep.makfail vhdldep.mak

include vhdldep.mak


sources:: generated1.vhd

generated1.vhd : generated1.vhd.m4
   m4 generated1.vhd.m4 > generated1.vhd.fail
   /bin/mv generated1.vhd.fail generated1.vhd
  
sources:: generated2.vhd

generated2.vhd : generated1.vhd.m4
   m4 generated2.vhd.m4 > generated2.vhd.fail
   /bin/mv generated2.vhd.fail generated2.vhd