[Bug libc/6846] Full featured printf hooks

["rsa at us dot ibm dot com" <sourceware-bugzilla at sourceware dot org>]

------- Additional Comments From rsa at us dot ibm dot com  2008-11-25 22:31 -------
Here's a first pass at a design document:

http://sources.redhat.com/glibc/wiki/PrintfHooksDesign

It needs a lot of cleanup to solidify some of the remaining implementation
questions.

Some of the important bits:

- Since the `__const struct printf_info *__info' parameter of
printf_arginfo_function is "const" the arginfo callback functions can't set
directly into it.  If they want to set any special flags then they have to set
flags in __argstype in the range of 0xffff0000.

- With this in mind `struct printf_info' only needs the addition of one
additional word-sized user flag, rather than two.  Once an override is detected
for a particular length-modifier the printf internals can simply copy the flags
in 0xffff0000 to `struct printf_info::user'.

- I've not thought of how to allow support for 'flag characters' yet since none
of the C-Spec proposals (VSX, Altivec, AVX, or DFP) define any overrides.

- The section `Design Preclusions::Questionable' should probably be answered.  I
was able to come up with a design which didn't modify the existing `struct
printf_info' definition at all and I can present that design if you'd like.

- There's a lingering issue of `struct printf_info::spec' being defined as a
single wchar_t.  This doesn't really work if we want to support multi-byte
'conversion specifications'.  As far as I can tell none of the existing C-Spec
proposals define any multi-byte conv specs.

Thanks for the review.
Ryan

-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=6846

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