On Sat, 29 Jan 2011 11:12:28 -0500, Magnus Lie Hetland <[email protected]>
wrote:
Hi!
Just read Andrei Alexandrescu's new book, and I'm starting to experiment
with using D in my algorithms research. Loved the book, and I'm loving
the language so far :D
I just hit a snag, though ... I was doing something simple, for which my
prototype code (in Python) was
d, u = max((D(u,v), v) for v in V)
I first started writing it explicitly with loops, but it got a bit too
verbose for my taste. Thought I'd use map and reduce, perhaps (although
I'm still not sure if that's practical, as I'm reducing with max, but
I'd like the argmax as well...).
Anyway -- while using attempting to use map, I suddenly got a segfault.
As I hadn't really done any crazy stuff with pointers, or circumvented
the bounds checks or the like, I was a bit surprised. I have now boiled
things down to the following little program:
import std.algorithm;
void f() {
auto x = 0;
double g(int z) { // Alt. 1: return int
auto y = x; // Alt. 2: remove this
return 0;
}
auto seq = [1, 2, 3];
auto res = map!(g)(seq);
}
void main() {
f();
}
When I compile and run this (dmd 2.051, OS X 10.5.8), I get a
segmentation fault.
Oddly enough, if I *either* change the return type to int *or* remove
the "y = x" line, things work just fine.
Am I correct in assuming this is a bug?
Yes, it's Issue 5073.
(http://d.puremagic.com/issues/show_bug.cgi?id=5073). I've tested your
test case using the listed patch + DMD 2.051 and it works. The issue used
to be a bad compile time error in earlier compiler versions but in DMD
2.051 it turned into a runtime error. The underlying error in DMD has to
do with an alias of a delegate having a bad hidden pointer. The reason
commenting out 'auto y = x;' works is that the incorrect hidden pointer is
never called, thus never causing a segfault (IIRC). Issue 5073's patch
works by passing delegates by value instead of by alias. Looking over
Issue 5064, Don is probably right in it being the root cause in DMD, but
if you just want map to work correctly, you might want to try the patch
from 5073.
