Can you go a little deeper with your question? mpiexec executes an MPI program with more processes passed as argument via -np option, we just know this, as I have already stated.

In Edit Scheme -> Environment Variables, enter VECTOR_SIZE as Name and 10 as Value. In Edit Scheme -> Arguments Passed On Launch, enter -np 4 "$(BUILT_PRODUCTS_DIR)/$(EXECUTABLE_NAME)" "$(VECTOR_SIZE)" In this case, the environment variable "works" because Xcode can read the 10 value as I can read it in my C program. Can I ask you to be more syntactically clear?

I did really not understand how I could use the "Wait for the executable to be launched". Anyway, at the same time I was using another environment variabile which, for the sake of simplicity, I have omitted from this post. It worked as I expect, for instance:

Minutes after, App Store proposed the Xcode version 16, now working.