> From: j...@wxcvbn.org (Jeremie Courreges-Anglas)
> Date: Mon, 01 Aug 2016 20:30:33 +0200
>
> Stefan Kempf <sisnk...@gmail.com> writes:
>
> > The constructor and destructor tables are declared as arrays with one
> > non-NULL element. Walking those until a NULL element is reached looks
> > like out-of-bound accesses to newer compilers, and they turn the code
> > into infinite loops (e.g. clang 3.8), because it is undefined behavior.
> >
> > Use constructor/destructor calling code that should be legal in both the
> > gcc and clang C dialect, to hopefully be immune from undefined behavior
> > optimizations in the future.
> >
> > While there, clean up crtbegin.c and crtbegin.S a little and make them
> > more similar.
> >
> > ok?
> >
> > Index: lib/csu/crtbegin.c
> > ===================================================================
> > RCS file: /cvs/src/lib/csu/crtbegin.c,v
> > retrieving revision 1.20
> > diff -u -p -r1.20 crtbegin.c
> > --- lib/csu/crtbegin.c 10 Nov 2015 04:14:03 -0000 1.20
> > +++ lib/csu/crtbegin.c 1 Aug 2016 16:56:31 -0000
> > @@ -85,36 +85,37 @@ long __guard_local __dso_hidden __attrib
> >
> >
> > static const init_f __CTOR_LIST__[1]
> > - __attribute__((section(".ctors"))) = { (void *)-1 }; /* XXX */
> > + __used __attribute__((section(".ctors"))) = { (void *)-1 }; /* XXX
> > */
> > static const init_f __DTOR_LIST__[1]
> > - __attribute__((section(".dtors"))) = { (void *)-1 }; /* XXX */
> > + __used __attribute__((section(".dtors"))) = { (void *)-1 }; /* XXX
> > */
> > +
> > +extern const init_f ctor_list[] asm(".ctors");
> > +extern const init_f dtor_list[] asm(".dtors");
> >
> > static void __dtors(void) __used;
> > static void __ctors(void) __used;
> >
> > static void
> > -__ctors()
> > +__ctors(void)
> > {
> > - unsigned long i = (unsigned long) __CTOR_LIST__[0];
> > - const init_f *p;
> > + int i;
> > +
> > + for (i = 0; ctor_list[i + 1] != NULL; i++)
> > + continue;
> >
> > - if (i == -1) {
> > - for (i = 1; __CTOR_LIST__[i] != NULL; i++)
> > - ;
> > + while (i > 0) {
> > + ctor_list[i]();
>
> The existing code tries to retrieve the number of valid ctors entries
> from __CTOR_LIST__[0], only when that number is -1 it tries to find
> the actual value by walking the array.
>
> The ld(1) info page states:
>
> The symbol `__CTOR_LIST__' marks the start of the global
> constructors, and the symbol `__CTOR_END__' marks the end.
> Similarly, `__DTOR_LIST__' and `__DTOR_END__' mark the start and
> end of the global destructors. The first word in the list is the
> number of entries, followed by the address of each constructor or
> destructor, followed by a zero word. The compiler must arrange to
> actually run the code. For these object file formats GNU C++
> normally calls constructors from a subroutine `__main'; a call to
> `__main' is automatically inserted into the startup code for
> `main'. GNU C++ normally runs destructors either by using
> `atexit', or directly from the function `exit'.
>
> If that is correct your code should behave the same. But what if...?
Right, the code is not equivalent. We'd have to look carefully at gcc
and ld to see if that matters.
Also, aren't ctor_list and dtor_list polluting the namespace?
