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Date: Mon, 17 Nov 2014 03:08:44 +0100
From: Polytropon <freebsd@edvax.de>
To: Doug Hardie <bc979@lafn.org>
Subject: Re: vsnprintf and friends modify the va_list argument
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On Sun, 16 Nov 2014 14:15:10 -0800, Doug Hardie wrote:
> I suspect this needs to be in a WARNING section of the
> printf(3) man page.  The print functions that use variable
> arguments modify the argument list so that only one can
> be called.

In "man 3 printf" there is no mentioning that ap is
being modified, but "man 3 stdarg" where va_start(),
va_arg(), va_copy() and va_end() are being described,
states the following (see 3rd sentence):

     The va_arg() macro expands to an expression that has the type and value
     of the next argument in the call.  The parameter ap is the va_list ap
     initialized by va_start().  Each call to va_arg() modifies ap so that the
     next call returns the next argument.  The parameter type is a type name
     specified so that the type of a pointer to an object that has the speci-
     fied type can be obtained simply by adding a * to type.

It's important to understand that functions like vsnprintf(),
vprintf() and fprintf() rely on the va_* functions / macros
which _modify_ the va_list pointer.

An additional hint is provided by the vprintf() prototypes
where no "const" is mentioned in regards of the ap parameter:

	int vprintf(const char * restrict format, va_list ap);

Maybe adding stdarg(3) to printf(3)'s SEE ALSO section would
be a helpful choice here.



-- 
Polytropon
Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...