Newlib reentrancy

[Sebastian Huber]

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.  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;
}

-- 
Sebastian Huber, embedded brains GmbH

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