When I compile my C files and output the resulting object files into their own directory, I need to make sure this directory exists.
I have seen two approaches for this:

1. Always create the directory of the current target:
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   <code>obj/%.o: src/%.c

   @mkdir -p $(@D)

   $(CC) ...

   </code>

   <code>obj/%.o: src/%.c @mkdir -p $(@D) $(CC) ... </code>

   obj/%.o: src/%.c
       @mkdir -p $(@D)
       $(CC) ...
   

2. Add the obj/ directory as a prerequisite
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   <code>obj/%.o: src/%.c | obj/

   $(CC) ...

   obj/:

   mkdir -p $@

   </code>

   <code>obj/%.o: src/%.c | obj/ $(CC) ... obj/: mkdir -p $@ </code>

   obj/%.o: src/%.c | obj/
       $(CC) ...
   
   obj/:
       mkdir -p $@
   

From 1. I like that it uses $(@D) to avoid repetition and to make it more robust, from 2. I like that mkdir is called only once and I feel like it makes more sense conceptually.

When I try to combine both like this

obj/%.o: src/%.c | $(@D)
    $(CC) ...

obj/:
    mkdir -p $@

then the compiler will complain that the obj/ directory does not exist.