Re: nano printf + powerpc gcc

[Eric Blake <eblake at redhat dot com>]

On 12/25/2017 07:39 AM, Alexander Fedotov wrote:
> Hi
> 
> I'm experienced a strange printf() for float variables behaviour with
> Nano version while regular is fine. It just prins out a garbage (even
> with enabled -u _printf_float). After some digging I've managed to
> track down issue until this line:
> 
> n = _printf_float (data, &prt_data, fp, pfunc, &ap);
> 
> 
> Moreover there are number of warnings also in build log:
> 
> src_newlib/newlib/libc/stdio/nano-vfprintf.c -o lib_a-nano-svfprintf.o
> ../../../../../../src_newlib/newlib/libc/stdio/nano-vfprintf.c: In
> function '_svfprintf_r':
> ../../../../../../src_newlib/newlib/libc/stdio/nano-vfprintf.c:645:55:
> warning: passing argument 5 of '_printf_float' from incompatible
> pointer type
>         n = _printf_float (data, &prt_data, fp, pfunc, &ap);
>                                                                        ^

One possibility for a workaround that I've seen used in other projects
is to do a va_copy() and pass a pointer to the copy rather than the
original ap passed in as a parameter.  Here's a thread from the qemu list:

https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg00605.html
https://lists.gnu.org/archive/html/qemu-devel/2017-10/msg00171.html

and where the va_copy() code is used in practice:

https://git.qemu.org/?p=qemu.git;a=blob;f=tests/libqtest.c;h=0ec8af292;hb=2077fef9#l472

in order to call a function taking va_list *.


 472 void qmp_fd_sendv(int fd, const char *fmt, va_list ap)
 473 {
 474     va_list ap_copy;
...
 484
 485     /* Going through qobject ensures we escape strings properly.
 486      * This seemingly unnecessary copy is required in case va_list
 487      * is an array type.
 488      */
 489     va_copy(ap_copy, ap);
 490     qobj = qobject_from_jsonv(fmt, &ap_copy, &error_abort);
 491     va_end(ap_copy);
 492

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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