core: forward declare msg_t in thread.h

[kaspar030]

Contribution description

Alternative to #16458.
This forward-declares msg_t in thread.h, and adapts thread.h, sched.h and msg.h accordingly.

Testing procedure

Issues/PRs references

[miri64]

I think if you use @ref struct msg here, doxygen will correctly render it. All in all, it would be nice to have a link to the actual struct, since this is in a completely different header.

[miri64]

Suggested change

	#include "thread.h"
	#include "thread.h" /* include forward declaration of `msg_t`. */

?

[maribu]

With this, thread.h and sched.h still depend on each other. As a result, it will remain impossible to use helpers like thread_getpid() or thread_getactive() from within sched.h.

The problem with the cross header forward declaration is that they mask the underlying problem of circular dependencies, rather than solving it.

[kaspar030]

The problem with the cross header forward declaration is that they mask the underlying problem of circular dependencies, rather than solving it.

Maybe, but I don't think that's the issue here. thread and sched are not really distinct modules. This shows in e.g., thread_getpid() and thread_getactive() returning sched internal variables. Maybe it is time to just merge sched into thread.

[miri64]

Maybe it is time to just merge sched into thread.

Shouldn't it rather be the other way around? A kernel can't have threads without a scheduler, but the other way around it works ;-).

[kaspar030]

Shouldn't it rather be the other way around? A kernel can't have threads without a scheduler, but the other way around it works ;-).

well, maybe. most parts of the system and most apps will use the thread API, though. not much from sched other than defines.

[miri64]

Then I think there should be a thread API, that is implemented by the sched module ;-).

[stale]

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