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Re: The thread model in newlib
Leif Ekblad wrote:
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.
OK. It seems like this macro is defined in config.h if the __linux__ define
is active. I suppose I can change here to make it active for __RDOS__
Besides, I remember why I had to patch for this in 1.10. Without a
function call to get the current thread's reent structure, you must
modify the pointer with every thread-switch. This was pretty much
out of the question since RDOS have kernel threads.
This removes the need of the user-code to call _r routines with a
specific reentrant structure (the _REENT macro is redefined to call
Yes, is this a major improvement.
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.
Yes, this is how I patched 1.10 too. I let the thread-creation code
allocate a FS selector unique to each thread. Since a thread-switch
saves all registers, this works as it should.
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.
Isn't linuxthreads an application implementation of threads rather than
a kernel implementation?
It's the old glibc thread implementation prior to NPTL. I mentioned it
merely to be helpful.
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.
OK, I can see how this works.
I see that the Linux implementation has lots of code in newlib. Is this
the preferred way to support a new OS? I also see that some systems
use newlib/libc/sys while some others use libgloss/sys. What is the
difference and what exactly is libgloss?
Libgloss is the place for embedded board support package libraries
whereas newlib/libc/sys is for OSes (e.g. RTEMS, Linux, Cygwin). There
are few platforms like ARM that are in libc/sys but this is just
historical and they truly belong in libgloss. The libgloss library
allows an end-user to choose at compile/link time the actual target
board/simulator. Having ARM in newlib has proven a true nuisance so a
libgloss directory has been created and eventually we'll have gcc use it
exclusively and remove the libc/sys/arm directory altogether.
RTEMS and Cygwin actually use newlib as a base that they override and
enhance so the syscalls, OS-specific functions are actually found in
their own projects. OTOH, the linux support was started in newlib and
has grown to where it is today. It too overrides functionality from the
shared sections of newlib as appropriate for Linux.
-- Jeff J.