command line ordering within a single rule when parallel building

[Nick.Thompson]

Hi,

I have a basic question about command line execution when using -jN to build a 
target. How are the command lines of a single make rule scheduled for execution 
when running parallel builds? It seems clear that they will be started in the 
order given, but does make wait for one to complete before starting the next? 
(By wait, I expect it can, and will, get on with other rules' command lines in 
parallel, if possible.) We have command sequences within single rules which 
have to follow step wise (so are implicitly interdependent) and we rely only on 
the ordering of the command lines within a rule to ensure this. Of course we 
could use && constructs to construct single command lines that the shell would 
then sequence for us, but this is not how the majority of our makefiles are 
written. Is it safe to turn on parallel building in this case or do we need to 
rewrite our rules?

As an example, is the following safe in a parallel build when each command 
needs to be complete before the next is run?:

target.x: input1.dat input2.dat
        extract_data input1.dat >output.dat
        extract_data input2.dat >>output.dat
            target_create output.dat >target.x
        rm output.dat

Thanks,
Ratbert.