bsdtar core dumps
harti at freebsd.org
Tue Aug 24 12:03:08 PDT 2004
On Tue, 24 Aug 2004, Sean McNeil wrote:
SM>On Tue, 2004-08-24 at 11:08, Harti Brandt wrote:
SM>> On Tue, 24 Aug 2004, Sean McNeil wrote:
SM>> Sorry to jump in.
SM>> You cannot use a va_list twice. As soon as someone call va_arg() on the
SM>> ap all the aps in the calling functions get invalid. The only thing that
SM>> can and must be done is that the function that did the va_start() must
SM>> call va_end.
SM>> If you need it twice you must make a copy as in the patch below.
SM>> But the function call va_copy must also call va_end() on that copy
SM>> (this seems missing in the patch).
SM>I took the lead from /usr/src/lib/libc/stdio/vfprintf.c
SM>It happens to do a va_copy(orgap,ap) but never does a va_end(orgap).
SM>Either vfprintf.c is incorrect or it is not necessary to call va_end().
vfprintf.c is wrong. Paragraph 1 of 7.15.1 of C99 states: "Each invocation
of the va_start and va_copy macros shall be matched by a corresponding
invocation of the va_end macro in the same function."
SM>One point I would like to make clear - you _can_ use a va_list twice on
SM>architectures that will pass the va_list argument by value (i.e. i386).
SM>This makes it easy to produce faulty code for those architectures that
SM>pass by reference and thus have a side-effect. Since most people here
SM>are developing and testing on x86 machines I would think this problem to
SM>have happened more often than it has.
This depends on what you call _can_. Again citing C99 (paragraph 3 of
7.15): "The object ap may be passed as an argument to another function;
if that function invokes the va_arg macro with parameter ap, the value of
ap in the calling function is indeterminate and shall be passed to the
va_end macro prior to any further reference to ap." (ap is a va_list
That is rather clear. I wouldn't call the fact that it incidentally
happens to work on that concrete compiler/machine implementation an
argument for 'can be used twice'. Especially in the case if stdarg.h it is
very helpful to closly follow the standard.
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