AIX I/O failures

[Mark Mitchell]

I now know at least one reason why I/O is not working on AIX.

The basic problem, of course, is that AIX does not do the SVR4-ish
depth first ordering of shared library initializers.  Instead, they
are run in a "random" order, i.e., one that is not useful to us. :-)

The ios::Init::Init trick is supposed to help with this -- every file
that includes I/O facilities gets one of these objects, and the first
one to get constructed sets things up.

But, in contrast to the placement-new patch that I first proposed, the
current sources contain explicit initializations of `cin' and friends
in `ios.cc':

 istream cin(NULL);

This is wrong, because these initializers run *after* the initializers
for the main program -- the ones that did placement new on these
objects to make sure that they were all set up nicely.  So, we
essentially blow away the work we did.

That's why my original patch used the trick:

  char cin[sizeof(istream)]
     __attribute__ ((__align (__alignof__ (istream))));

(or something similar) to make sure that the storage was allocated,
but that it was never actually initialized.  This is the only reliable
I know of to make this work, given the AIX constraints.

We have to design with the following basic constraint in mind:

  - We cannot assume initializers for libstdc++ will run before 
    initializers for other libraries.

Therefore, *any* use of static initializers in libstdc++ constitutes a
bug -- other than the ios::Init::Init trick where you call a function
that can safely be called more than once, and is guaranteed to be
called by any object that makes use of that functionality.

That means that all global objects that need what looks like static
initialization need to use the array of chars trick (or something else
I haven't thought of yet!) and placement new; that way they *look*
like global objects of the right type, but they're initialized when
needed, and only once.

Phil, if there are coding guidelines for V3 somewhere, would you mind
adding this idea in somewhere so that we continue to bear it in mind?
It's a fundamental design principle required to make the library
robust on AIX, and perhaps other systems as well.

Thanks,

--
Mark Mitchell                   mark@codesourcery.com
CodeSourcery, LLC               http://www.codesourcery.com