libcontrol

[Noel Hunt]

I am trying to compile libcontrol under plan9port. Various

of the structures used in the code rely on the Plan9 compiler's

support for anonymous structure elements of an aggregate. This

can be straightforwardly dealt with by naming the structure, e.g., we have such declarations as

struct Group {

        Control;

        ,,,,

};

and

struct Box {

        Control;

        ....

};

where 'Control' is a previously defined aggregate, so to make

this legal for non-Plan9 compilers

struct Group {

        Control c;

        ...

};

Group *g;

....g->event      =>      g->c.event

would seem to be the correct procedure, except there are some

cases like this

void

_ctladdgroup(Control *c, Control *q){

        Group *g = (Group*)c;

        ...

where'_ctladdgroup' is called with a real Control* struct as

argument one, not a cast, and then tries to cast this to a

Group*, and then references elements of the Group struct:

        g->kids = realloc(...);

This just will not work, so can someone explain what is going on

with this code, and how it compiles, and runs, under Plan9?

[Dan Cross]

I haven't looked at this in detail, but at a glance.... The first
element in a Group is a Control. My guess is that this code "knows"
that the control it receives is embedded inside of a Group and
therefore that it can cast to the containing type.

- Dan C.

[Noel Hunt]

Yes, I see what you mean. I'll have to look more closely at

what is being passed around in that case.

Thanks.

[Jacob Vosmaer]

Isn't this something you can do with C structs in general? Cast a pointer the struct to a point to its first member. This casting thing sounds orthogonal to the Plan 9 C anonymous struct member feature.

Op di 18 feb 2025 om 03:06 schreef Noel Hunt <noel...@gmail.com>:

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