From: Kalle Olavi Niemitalo <kon@iki.fi>
Date: Thu, 20 Sep 2007 07:20:29 GMT
Subject: Re: [PATCH 1/5] strbuf API additions and enhancements.
Message-ID: <87lkb1iz0i.fsf@Astalo.kon.iki.fi>
In-Reply-To: <46F21097.5030901@eudaptics.com>

Johannes Sixt <j.sixt@eudaptics.com> writes:

> Edgar Toernig schrieb:
>> The second vsnprintf won't work as the first one consumed all args
>> from va_list ap.  You need to va_copy the ap.
>
> Your analysis is not correct. The second vsnprintf receives the same
> argument pointer as the first, and, hence, consumes the same set of
> arguments.

C99 7.9.16.2p2 has a footnote: "As the functions vfprintf,
vfscanf, vprintf, vscanf, vsnprintf, vsprintf, and vsscanf invoke
the va_arg macro, the value of arg after the return is
indeterminate."

Normative text in 7.15p3 confirms this: "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."

Therefore va_copy is needed here, at least in principle.