Re: (lack of) va_copy, and segfaults on x86_64

[Thomas Dickey]

On Tue, 2 Mar 2010, Wim Lewis wrote:

> Compiling ncurses-5.7 on macosx-x86_64, I find that it segfaults frequently 
> within _nc_printf_string() in printw(). The reason seems to be the way that 
> the va_list is used twice (once to compute the buffer's length, and once to 
> actually format the string). Like some other architectures with a register 
> passing convention, va_list on this machine is a (pointer to a) structure, 
> not a simple pointer to stack memory as it is on an i386 or vax, and so the 
> implicit copy made by a function call doesn't actually keep the caller's 
> argument pointer from being modified by the callee.
>
> The solution is pretty simple, to use va_copy() to explicitly make a copy of 
> the argument pointer. It looks like an (unrelated?) use of va_copy was added 
> and then reverted in 20061202/20061217, though, so I'm not sure what issue

The change then didn't actually work (portably for sure), and none of the
platforms that I could test on had a C runtime that could exercise the
change (and need it...).

By coincidence, I hit the same issue with vile this week (predictably, 
for glibc on amd64).  So I worked through the kinks in the configure check 
to make it usable, and can add that to ncurses.

> needs to be dealt with in order to use va_copy(). (Perhaps just an autoconf 
> test for its existence?)
>
> I'd guess that this problem affects other architectures too --- I've had to 
> make similar changes in the past to get stuff to run on linux-ppc, eg.
>
> This is the change I made to my local copy --- not intended as a patch 
> submission, just to show what I'm talking about. With this change ncurses 
> seems to work fine:
>
>  _nc_printf_string(const char *fmt, va_list ap)
>  {
>      char *result = 0;
>
>      if (fmt != 0) {
>  #if USE_SAFE_SPRINTF
> !       va_list ap_copy;
> !       va_copy(ap_copy, ap);
> !       int len = _nc_printf_length(fmt, ap_copy);
> !       va_end(ap_copy);
>
>         if ((int) my_length < len + 1) {
>             my_length = 2 * (len + 1);
>
>
>
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--
Thomas E. Dickey
http://invisible-island.net
ftp://invisible-island.net