Re: smaller ramsize for newlib-nano builds

[Sebastian Huber <sebastian dot huber at embedded-brains dot de>]

On 02/02/18 11:23, Corinna Vinschen wrote:
> On Feb  2 07:04, Sebastian Huber wrote:
> > On 01/02/18 20:42, Orlando Arias wrote:
> > > On 02/01/2018 07:57 AM, Corinna Vinschen wrote:
> > > > On Jan 31 17:50, Jaap de Wolff wrote:
> > > > > I am working with embedded systems with have very limited ram resources.
> > > > >
> > > > > As known current version of the newlib-nano build uses 364 bytes of ram for
> > > > > __global_locale
> > > > This build option should be available via a configure option, ideally.
> > > > Also, the change itself basically changes __global_locale to read only,
> > > > so I wonder if the name of the option shouldn't reflect that, kind of
> > > > like
> > > >
> > > >     ./configure --enable-readonly-locale
> > > >
> > > > or
> > > >
> > > >     ./configure --enable-const-locale
> > > >
> > > > > I realize that MB_CAPABLE must be off for this modification, but are there
> > > > > other implications?
> > > > I don't think so, but a potential patch submission should make sure that
> > > > under normal circumstances no write access occurs with this option.
> > > Greetings,
> > >
> > > For a while I have been contemplating making a patch set that provides
> > > an option to utilize only the C locale, removing all locale stuff from
> > > newlib at build time. Moving the __global_locale to ROM may be a good
> > > temporary solution, however, it will not work well for resource limited
> > > devices. For example, the MSP430F2013 provides only 2 kB of ROM and the
> > > MSP430F2003 has only 1 kB. This is about 1/6 and 1/3 of the total
> > > program capacity of the device, respectively ( further exacerbating the
> > > problem is that these particular devices only provide 128 B of RAM).
> > >
> > > Unfortunately, I do not have much time to actually work on this
> > > proposal, but I am willing to give it a shot if I can find some
> > > documentation on how current locale stuff is implemented in newlib
> > > (other than ``the code is the documentation''). Any pointers on this
> > > would be greatly appreciated.
> > A long term solution (I am not sure if it is feasible in Newlib) would be to
> > use C11 thread-local storage instead of the custom thread-local storage via
> > the struct _reent. With C11 thread-local storage the linker would do all the
> > dirty work.
> This wouldn't work with Cygwin which has its own thread-local
> implementation.  I'm also not quite sure how this helps here.

I would replace

struct _reent
{
  int _errno;            /* local copy of errno */

  /* FILE is a big struct and may change over time.  To try to achieve 
binary
     compatibility with future versions, put stdin,stdout,stderr here.
     These are pointers into member __sf defined below.  */
  __FILE *_stdin, *_stdout, *_stderr;
[...]

with something like this:

thread_local int _errno;
thread_local __FILE *_stdin;
thread_local __FILE *_stdout;
thread_local __FILE *_stderr;

So, if you only use errno, then you don't get the rest of struct _reent.

--
Sebastian Huber, embedded brains GmbH

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