[vox-tech] cygwin - segfault on array allocation

[Peter Jay Salzman]

On Wed 08 Feb 06, 11:29 AM, Micah J. Cowan <micah at cowan.name> said:
> On Wed, Feb 08, 2006 at 12:12:19PM -0500, Peter Jay Salzman wrote:
> > i'm finding that cygwin segfaults on a very simple program:
> > 
> >    #include <iostream>
> >    using std::cout;
> >    using std::endl;
> >    // 50760 is the last
> > 
> >    int main( int argc, char *argv[] )
> >    {
> >       const long int N = strtol(argv[1], (char **)0, 0);
> >       double a[N], b[N], c[N], d[N], ans[N];
> > 
> >       return 0;
> >    }
> 
> First: note that the above is not portable code. C++ does not support
> variable-length arrays (that is, the bracketed expression must be a
> constant). As long as you're constraining yourself to g++, though, or
> other ones that support this extension, you're fine. (VLAs /are/
> supported in the current version of the ISO C Standard, BTW).
 
How does one gain this kind of knowledge at fingertips?

(snip)

> >    $ gdb arg.exe
> >    GNU gdb 6.3.50_2004-12-28-cvs (cygwin-special)
> >    (gdb) set args 50761
> >    (gdb) break 1
> >    Breakpoint 1 at 0x401050: file arg.cc, line 1.
> >    (gdb) run
> >    Starting program: /cygdrive/c/Documents and
> >    Settings/psalzman/home/tests/args/arg.exe 50761
> > 
> >    Breakpoint 1, main (argc=2, argv=0x4c2b90) at arg.cc:7
> >    7       {
> >    (gdb) n
> > 
> >    Program received signal SIGSEGV, Segmentation fault.
> >    0x610ae938 in pthread_key_create () from /usr/bin/cygwin1.dll
> >    (gdb)
> > 
> > which is odd; I don't believe this line number.
> 
> What line number? It didn't break at 7, it broke after you "stepped"
> after 7, which means you don't necessarily know where it broke. I
> usually let segfaulting programs dump core, and then use gdb to examine
> where it was when it got the signal. I don't think you'll get very
> useful information that way for this example, however.

Yeah, did that.  Didn't give any line number associated with the signal.  It
actually said there were corrupted frames on the stack frame.

Pete