Re: [Xenomai-help] What returns rt_task_self in relation to rt_task_create

[Jan Kiszka <jan.kiszka@domain.hid>]

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M. Koehrer wrote:
> Hi all,
> 
> I have a simple question concerning the Xenomai native API:
> According to the API documentation, rt_task_self returns the 
> address of the caller's task descriptor.
> I have now written the following (simple application):
> ---------- START ----------
> #include <stdio.h>
> #include <sys/mman.h>
> #include <native/task.h>
> 
> RT_TASK task_desc;
> 
> void mytask(void *cookie)
> {
>     RT_TASK *tsk = rt_task_self();
>     printf("rt_task_self  %p\n"
>            "task_desc     %p\n", tsk, &task_desc);
> }
> 
> int main(void)
> {
>     mlockall(MCL_CURRENT|MCL_FUTURE);
> 
>     rt_task_create(&task_desc, "mytaskname", 0, 80, T_JOINABLE);
>     rt_task_start(&task_desc, &mytask, NULL);
> 
>     rt_task_join(&task_desc);
> 
>     return 0;
> }
> ------------ END -----------
> 
> I expect now that rt_task_self() returns exactly the address of the task_desc.
> However, a different address is returned. 
> The output of the application from above is:
> rt_task_self  0x804a050
> task_desc     0x8049878
> 
> How are those addresses related - how can I find out the descriptor address 
> used for rt_task_create() at runtime?

The documentation is not precise enough here: what you obtain from
rt_task_self is /some/ task descriptor for the currently running task,
it is not a reference to the same piece of memory containing the task
descriptor. Check the library implementation for further insights.

> 
> Background of the question:
> I want to write an application that uses a couple of similar tasks.
> I have to store some task-specific information (internal states...).
> Now I am looking for a simple way to get this information at runtime from my task
> by calling rt_task_self() (or something similar) to use this address to reach my
> additional information.

If you are on NPTL glibc, set up a __thread variable for the per-thread
information. It will always contain the thread-related data that you
can, e.g., fill in during the thread routine's prologue.

Jan


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