>>>>> Simon Urbanek <simon.urba...@r-project.org>
>>>>> on Mon, 1 Feb 2016 08:36:56 -0500 writes:
> On Feb 1, 2016, at 4:16 AM, Martin Maechler <maech...@stat.math.ethz.ch>
wrote:
[..............]
>> Back to R: I'm not familiar with that part of the code, neither
>> the configuration, nor the usage (in R/src/unix/system.c ).
>> However, that code seems to be using a a glibc "feature" widely
>> available which does help making R startup (a very tiny bit ??)
>> faster.
>>
> No, it's actually very crucial as it is used to detect stack overflows.
> Cheers,
> Simon
Well, I think you misunderstood what I meant to say (or then I'm
happy for clarification if I misunderstood you) :
The #ifdef ... #elseif ... #else ... # endif
branch which *uses* the __libc_stack_end "variable" would
hopefully be a speedup in comparison with the alternatives; from
system.c mentioned above:
#if defined(HAVE_LIBC_STACK_END)
R_CStackStart = (uintptr_t) __libc_stack_end;
#elif defined(HAVE_KERN_USRSTACK)
{
/* Borrowed from mzscheme/gc/os_dep.c */
int nm[2] = {CTL_KERN, KERN_USRSTACK};
void * base;
size_t len = sizeof(void *);
(void) sysctl(nm, 2, &base, &len, NULL, 0);
R_CStackStart = (uintptr_t) base;
}
#else
if(R_running_as_main_program) {
/* This is not the main program, but unless embedded it is
near the top, 5540 bytes away when checked. */
R_CStackStart = (uintptr_t) &i + (6000 * R_CStackDir);
}
#endif
if(R_CStackStart == -1) R_CStackLimit = -1; /* never set */
/* printf("stack limit %ld, start %lx dir %d \n", R_CStackLimit,
R_CStackStart, R_CStackDir); */
}
#endif
so I'd hope that typically R_CStackStart would be set usefully
also when the __libc_stack_end is not available.
If not, that would mean that for the 'musl' lovers, R would not
be able to detect stack overflows.... which would probably be
quite undesirable.
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