Re: The thread model in newlib

[Jeff Johnston <jjohnstn at redhat dot com>]

Leif Ekblad wrote:
> I'm trying to use GCC + newlib for my OS (RDOS, 
> http://www.rdos.net/rdos). I did try to
> do a port 2002, but as I've studied my sources it turns out the patches 
> for thread / reentrance
> support were quite ugly.
>
> RDOS only runs on i386, so portability is not a concern. I already have 
> a working environment
> with BCC 4.52 + TD32, which works since a couple of years ago. However, 
> I like the open
> environment better, and Borland no longer develops this compiler. With 
> PE-executables, the
> thread issue has a rather elegant solution for i386. You simply load a 
> segment register (FS in
> the case of PE), and let each thread have it's own selector. This way it 
> is easy to implement
> TLS and provide each thread with it's own data. It doesn't seem like it 
> is easy to implement
> this in newlib, at least not in version 1.10. I would rather only 
> implement the non _r function-calls.
> There is no need to provide an extra parameter, especially since every 
> function call in RDOS
> is reentrant. However, it is essential that the run-time library also is 
> reentrant.
>
> Any suggestions?

Have you looked at more recent sources?  For example, there is now a 
__DYNAMIC_REENT__ flag which tells newlib you will be providing a 
routine __getreent() to select the current thread's reentrancy 
structure.  This removes the need of the user-code to call _r routines 
with a specific reentrant structure (the _REENT macro is redefined to 
call __getreent()).  In the internal code, for example, you just include 
errno.h and set errno directly.  The macros take care of everything so 
that the __getreent() function gets called under the covers and the 
errno value is placed in the correct thread context.  In your situation, 
you could write a __getreent() routine that referred to a special 
pointer in TLS that is initialized at thread creation.

Also note there is a implementation of linuxthreads in the 
libc/sys/linux /linuxthreads directory that builds for i386.  Newlib 
altered the linuxthreads code to add the initialization of the reentrant 
structure for new threads and placing the pointer in a control structure 
to be accessed at any time.  If you supply most or all of the syscalls 
that Linux does, you probably can use a great deal of the thread code. 
Regarding your request to not supply _r routines:  the Linux port 
specifies -DMISSING_SYSCALL_NAMES in configure.host as it directly 
supplies reentrant syscalls (i.e. no _r syscalls are required).  You 
would want to do this as well.

-- Jeff J.


> Regards,
> Leif Ekblad