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c++copenvswitch

What meaning arguments before the function have?

While I am seeing the OVS source code, I found very strange codes i never seen before.

https://github.com/openvswitch/ovs/blob/master/lib/ovs-rcu.h

void ovsrcu_postpone__(void (*function)(void *aux), void *aux);
#define ovsrcu_postpone(FUNCTION, ARG)                          \
    ((void) sizeof((FUNCTION)(ARG), 1),                         \
     (void) sizeof(*(ARG)),                                     \
     ovsrcu_postpone__((void (*)(void *))(FUNCTION), ARG))

I got the meaning of multi argument sizeof by searching

Why call sizeof operator with two arguments? http://www.vxdev.com/docs/vx55man/diab5.0ppc/c-additi.htm#3001432

If return of FUNCTION is int and type of ARG is char, macro becomes this form.

((void) 4, (void) 1, ovsrcu_postpone__((void (*)(void *))(function), arg))

I cannot catch the roles of two arguments before ovsrcu_postpone__ method.

Solution

  • So let's look at the example that they give in the source code:

    ovsrcu_postpone(free, ovsrcu_get_protected(struct flow *, &flowp));

    This will be expanded into:

    (
 (void) sizeof((free)(ovsrcu_get_protected(struct flow *, &flowp)), 1),
 (void) sizeof(*(ovsrcu_get_protected(struct flow *, &flowp))),
 ovsrcu_postpone__((void (*)(void *))(free), ovsrcu_get_protected(struct flow *, &flowp))
)

    So what we have here, is some type safety, and then the expected call. Here's what I can tease out of the requirements:

    • The first argument is a function with one argument.
    • The second argument has at least one level of indirection (it's a pointer).

    We can also understand this:

     sizeof(free, 1);

    That's using the comma operator, so other than making sure the syntax is valid, it will have the same return value as sizeof(1).