All variable names starting with an underscore are reserved for use by the pkgsrc infrastructure. They shall not be used by package Makefiles.

In .for loops you should use lowercase variable names for the iteration variables.

All list variables should have a ``plural'' name, e.g. PKG_OPTIONS or DISTFILES.

Whenever you use the value of a list, think about what happens to leading or trailing whitespace. If the list is a well-formed shell expression, you can apply the :M* modifier to strip leading and trailing whitespace from each word. The :M operator first splits its argument according to the rules of the shell, and then creates a new list consisting of all words that match the shell glob expression *, that is: all. One class of situations where this is needed is when adding a variable like CPPFLAGS to CONFIGURE_ARGS. If the configure script invokes other configure scripts, it strips the leading and trailing whitespace from the variable and then passes it to the other configure scripts. But these configure scripts expect the (child) CPPFLAGS variable to be the same as the parent CPPFLAGS. That's why we better pass the CPPFLAGS value properly trimmed. And here is how we do it:

    CPPFLAGS=               # empty
    CPPFLAGS+=              -Wundef -DPREFIX=\"${PREFIX:Q}\"
    CPPFLAGS+=              ${MY_CPPFLAGS}

    CONFIGURE_ARGS+=        CPPFLAGS=${CPPFLAGS:M*:Q}

    all:
            echo x${CPPFLAGS:Q}x            # leading and trailing whitespace
            echo x${CONFIGURE_ARGS}x        # properly trimmed

The example above contains one bug: The ${PREFIX} is a properly quoted shell expression, but there is the C compiler after it, which also expects a properly quoted string (this time in C syntax). The version above is therefore only correct if ${PREFIX} does not have embedded backslashes or double quotes. If you want to allow these, you have to add another layer of quoting to each variable that is used as a C string literal. You cannot use the :Q operator for it, as this operator only works for the shell.

Whenever a variable can be empty, the :Q operator can have surprising results. Here are two completely different cases which can be solved with the same trick.

    EMPTY=                  # empty
    empty_test:
            for i in a ${EMPTY:Q} c; do \
                    echo "$$i"; \
            done

    for_test:
    .for i in a:\ a:\test.txt
            echo ${i:Q}
            echo "foo"
    .endfor

The first example will only print two of the three lines we might have expected. This is because ${EMPTY:Q} expands to the empty string, which the shell cannot see. The workaround is to write ${EMPTY:Q}"". This pattern can be often found as ${TEST} -z ${VAR:Q} or as ${TEST} -f ${FNAME:Q} (both of these are wrong).

The second example will only print three lines instead of four. The first line looks like a:\ echo foo. This is because the backslash of the value a:\ is interpreted as a line-continuation by make(1), which makes the second line the arguments of the echo(1) command from the first line. To avoid this, write ${i:Q}"".