On Wed, Sep 20, 2006 at 03:17:29PM -0400, Ethan Tira-Thompson wrote:
> For a quick example, try figuring out why this example program is
> crashing... The idea is simple: set up an array of strings containing
> 'a' through 'z', build a string from 100 random selections, and then
> display the result.
>
> No fair figuring it out from looking at the code (it's a suitably
> subtle newbie type problem, though obfuscated enough it's not
> glaringly obvious) -- pretend it's a "real" problem in a piece of
> fairly complicated code that isn't just a dozen lines long, try using
> gdb to figure it out. (for the lazy/hurried, answer posted here,
> including comparison to what happens in cygwin: http://ethan.tira-
> thompson.com/stuff/gdb-answer.html )
Had to admit to pull up the test case in a debugger because Ethan
was right, it wasn't obvious ;). Anyways, my method of dealing with
the immediate segfault type situation is to pull it up in gdb and
hope for a violation that will give me a useful backtrace. If that
doesn't occur, I just do a general 'b main' and next until I step
over the function that causes it and then do the same thing within
the violating function. In this case it was easy - since I recognized:
37619 [main] whack 2976 _cygtls::handle_exceptions: Exception:
STATUS_ACCESS_VIOLATION
38212 [main] whack 2976 open_stackdumpfile: Dumping stack trace to
whack.exe.stackdump
as being a typical buffer walk off *and* the fact that it wasn't a
large amount of iterations (which is really what made things to be
easy - otherwise it's serious pain).
int initStrs(char* strs[]) {
// initialize a string for each letter^M
int i;
for(i=0; i<'z'-'a'; i++) {^M
strs[i]=malloc(sizeof(char[2]));^M
strs[i][0]='a'+i; // current letter^M
strs[i][1]='\0'; // null terminate for a c-style string^M
}
return 26;
}
Culprit:
(gdb) p 'z' - 'a'
$16 = 25
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