Re: Multiple rules for same target: what is the order of prerequisites in $^ and what does $< expand to?

[Mark Piffer]

Is there a case (common or rare) where the answer to this question has real 
world consequences? 

regards,
Mark

> Am 27.09.2023 um 12:17 schrieb Renaud Pacalet 
> <renaud.pacalet@telecom-paris.fr>:
> 
> Dear all,
> 
> I read the manual carefully (especially section 4.11 "Multiple Rules for One 
> Target") and searched the help-make archives but couldn't find a clear 
> description of how $^ and $< are computed when there are several rules for 
> the same target and only one has a recipe.
> 
> According the manual "All the prerequisites mentioned in all the rules are 
> merged into one list of prerequisites for the target". But when the recipe 
> gets expanded what is the order of prerequisites in $^ and, more important, 
> what is $<? Does it depend on the order of rules in the Makefile(s)? On the 
> type of rules (pattern, static pattern, explicit...)? On which rule has a 
> recipe? Is it deterministic? etc.
> 
> I did some tests and came up with the following algorithm for $^ and $< 
> computation:
> 
> 0. When the list of prerequisites of a rule is added to $^, the order is 
> preserved.
> 
> 1. Initialize $^ with the list of prerequisites of the rule with a recipe, 
> implicit or not.
> 
> 2. Consider all other applicable explicit or static pattern rules, in their 
> order of appearance in the Makefile(s), and for each of them append its list 
> of prerequisites to $^.
> 
> 3. Ignore all other implicit rules (which seems to contradict 4.11).
> 
> 4. Remove duplicates from $^, keeping only the first occurrence.
> 
> 5. Set $< to the first prerequisite in $^.
> 
> Can someone confirm?
> 
> Renaud.
>