Newlib reentrancy

[Sebastian Huber]

On 04/22/2013 12:09 PM, Chris Johns wrote:
> Sebastian Huber wrote:
>> Hello,
>>
>> Newlib stores all global state in a so called reentrancy structure, e.g.
>> errno, stdin, random number state, etc. In RTEMS every thread can have
>> its own reentrancy structure (a reference it is part of the thread
>> control block). In Newlib there are two mechanisms to access the
>> executing thread's reentrancy structure
>>
>> 1. via a global pointer _impure_ptr, or
>> 2. via a function __getreent().
>>
>> See also:
>>
>> http://sourceware.org/ml/newlib/2013/msg00031.html
>>
>> Currently RTEMS uses option 1. The global pointer is updated during a
>> thread dispatch in threaddispatch.c.
>>
>> The option 1. is unsuitable for SMP since here a global pointer cannot
>> be used to store thread specific data. I suggest to use option 2. for
>> RTEMS.
>
> I agree. What is needed to define this support when building newlib ?

RTEMS has to define __DYNAMIC_REENT__ in "newlib/libc/include/sys/config.h".

>
>> For the single-processor RTEMS this has the benefit that the
>> thread dispatch code is simplified, e.g. the code block
>>
>> /*
>> * Switch libc's task specific data.
>> */
>> if ( _Thread_libc_reent ) {
>> executing->libc_reent = *_Thread_libc_reent;
>> *_Thread_libc_reent = heir->libc_reent;
>> }
>>
>> can be removed. The execution time of _Thread_Dispatch() is critical for
>> the interrupt and thread switch latency.
>>
>> The implementation of __getreent() for RTEMS is easy:
>>
>> struct _reent *__getreent(void)
>> {
>> struct _reent *reent = _Thread_Executing->libc_reent;
>>
>> if ( reent == NULL ) {
>> reent = _global_impure_ptr;
>> }
>>
>> return reent;
>> }
>
> Does this code need locking ?

No, but I haven't tested anything yet.

-- 
Sebastian Huber, embedded brains GmbH

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