Hi Kanika,

> On 22 Feb 2018, at 20:34, KANIKA SAINI <kanika16...@iiitd.ac.in> wrote:
> Issue suggested by Simon
> In Unikraft - whenever compiler flags are changed due to an addition of a 
> library or something else according to the build rules, the make command 
> doesn't force recompilation of those files affected by the flag change unless 
> the files are altered too. The only way out is to "make clean" and "make" 
> again all the files which can be redundantly time-consuming.
> Solution
> One way is to create flag files for objects and add those as their dependency 
> in the Makefile.

> Apart from this, I'm wondering if rules are passed on to the Makefile and 
> changes are depicted in the Makefile itself, can we add Makefile as the 
> dependency of objects to solve this issue?

I think the answer is in the documentation, which explains the UI (if you to 
call it that way) for the build system (see 

You can also usually figure out which parameters are passed to make via the 
command line by searching for the origin function, in particular for "command 
line" return values 
This will give you a sense of the values that can be passed to the build system 
from outside of a makefile.

Now, you do need to make sure that you don't change the UI interface, as this 
would break everyone who uses Unikraft.

Assuming that all the flags we care about are already encoded in a Makefile 
(and don't come through the command line or through any other means), then 
adding a dependency may be sufficient. There is the question how granular you 
want this to be: 
a) Is it OK for everything to be rebuilt when say you change a -Dfoo 
b) Do you want this to be more fine grained

Have a google for something like "rebuilding when makefile flags change" and 
you will find a few example patterns with different trade-offs to solve this 
problem in general

The second challenge is that most substantial build rules get created 
dynamically via [unikraft/unikraft.git] 
<http://xenbits.xen.org/gitweb/?p=unikraft/unikraft.git;a=tree;hb=HEAD> / 
 / build 
 / * 
Thus, adding a dependency on a makefile may not be as simple as it sounds

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