[ECOS] Re: vfnprintf and floating point.

Bart Veer bartv@cygnus.co.uk
Wed Oct 6 04:44:00 GMT 1999


>>>>> "Sergei" == Sergei Organov <osv@Javad.RU> writes:

    >> > If you interested, I just changed first argument of 'cvt' from
    >> > 'double' to 'va_list*', and moved double argument extraction to this
    >> > routine. I also provided 'void skip_double(va_list*)' that is used to
    >> > skip double argument when floating point support is disabled in
    >> > 'vfnprintf'.
    >> 
    >> Since you've already done it, if you send me the patch (use `cvs diff -u5')
    >> then I'll put it in. You are working from the anonymous CVS version,
    >> yes?

    Sergei> Yes, I use anonymous CVS version. But I just found that my
    Sergei> initial solution while works for i386 causes troubles for
    Sergei> PowerPC because of different definition of 'va_list'. If
    Sergei> and when I find solution, I'll send patch to you.

I have just had a look at my copy of the ANSI C standard. It contains
the following statement (where ap is a va_list):

"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."

I think this means that passing a va_list to another function such as
cvt() which performs partial processing, and then continuing to use
the va_list in the current printf() function, is not allowed. The
description of e.g. vfprintf() appears to confirm this. It may work
with some architectures, but there are no guarantees. It is possible
that the gcc team would be willing to provide stronger guarantees than
provided by the standard, but since there already appear to be
problems with the PowerPC this seems unlikely.

Bart Veer // eCos net maintainer


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