for a variable in makefile1 which is included by makefile2, could the makefile2 use the variable directly without importing in makefile2 and exporting in makefile1? Actually, it can be directly used! Give me an explicit example.

Sure, here's an example to illustrate how variables can be used directly in an included Makefile without the need for explicit importing/exporting:

Makefile1:

VAR1 = hello

all:
    @echo "Variable VAR1 in Makefile1: $(VAR1)"
    @$(MAKE) -f Makefile2

Makefile2:

all:
    @echo "Variable VAR1 in Makefile2: $(VAR1)"

In this example, Makefile2 is included in Makefile1 using the `$(MAKE)` function. The variable `VAR1` is defined in Makefile1 and is used directly in Makefile2 without any explicit importing/exporting. When Make is run, it will first process Makefile1 and execute its `all` target, which will print the value of `VAR1` and then invoke Makefile2's `all` target using `$(MAKE)`. When Makefile2's `all` target is executed, it will also print the value of `VAR1`.

So in summary, variables defined in a parent Makefile are automatically available to any included Makefiles without the need for explicit importing/exporting.