Re: [MirageOS-devel] Fix warnings in Xen C code

[Thomas Leonard <talex5@xxxxxxxxx>]

On 13 August 2015 at 17:48, Len Maxwell <len@xxxxxxxx> wrote:
>
>
> On Thu, Aug 13, 2015 at 9:02 AM, Thomas Leonard <talex5@xxxxxxxxx> wrote:
>>
>> On 13 August 2015 at 13:19, Len Maxwell <len@xxxxxxxx> wrote:
>> >
>> > On Thu, Aug 6, 2015 at 4:58 AM, Thomas Leonard <talex5@xxxxxxxxx> wrote:
>> >>
>> >> On 6 August 2015 at 00:26, Len Maxwell <len@xxxxxxxx> wrote:
>> >> >
>> >> > Hi,
>> >> >
>> >> > After working with minios-xen a bit, I thought I could help with this
>> >> > pioneer project.  There seem to be significantly fewer C warnings
>> >> > than
>> >> > the
>> >> > last time I looked (months ago).
>> >>
>> >> That would be very useful!
>> >>
>> >
>> > Hi,
>> >
>> > If I start from mirage-platform master (@ 94f01d), and make this change
>> > to
>> > xen-posix/include/stdio.h:
>> >
>> > -#define fprintf(x,rest...) printk(rest)
>> > +int fprintf(FILE *stream, const char *format, ...) __THROW
>> > __attribute__((__format__(__printf__,2,3)));
>> >
>> > ...it should eliminate a missing prototype warning for printk in
>> > xen-ocaml.
>> > The package builds OK, but produces link errors when I try to make a
>> > unikernel:
>> >
>> > $ make clean && make
>> > [ ... ]
>> > ld -d -static -nostdlib \
>> >   _build/main.native.o \
>> >   -L/opt/ml/opam/4.02.1/lib/io-page -lio_page_xen_stubs \
>> >   $(pkg-config --static --libs mirage-xen) \
>> >   /usr/lib/gcc/x86_64-linux-gnu/4.9/libgcc.a \
>> >   -o mir-console.xen
>> >
>> > /opt/ml/opam/4.02.1/lib/pkgconfig/../../lib/mirage-xen-ocaml/libxenasmrun.a(fail.o):
>> > In function `caml_array_bound_error':
>> > fail.c:(.text+0x2c8): undefined reference to `fwrite'
>> >
>> > /opt/ml/opam/4.02.1/lib/pkgconfig/../../lib/mirage-xen-ocaml/libxenasmrun.a(misc.o):
>> > In function `caml_fatal_error':
>> > misc.c:(.text+0x42): undefined reference to `fputs'
>> >
>> > /opt/ml/opam/4.02.1/lib/pkgconfig/../../lib/mirage-xen-ocaml/libxenasmrun.a(parsing.o):
>> > In function `caml_parse_engine':
>> > parsing.c:(.text+0x225): undefined reference to `fputs'
>> > parsing.c:(.text+0x258): undefined reference to `fputc'
>> > parsing.c:(.text+0x273): undefined reference to `fwrite'
>> > parsing.c:(.text+0x420): undefined reference to `fwrite'
>> > parsing.c:(.text+0x46b): undefined reference to `fwrite'
>> > Makefile:34: recipe for target 'build' failed
>> > make: *** [build] Error 1
>> >
>> >
>> > Each of these caml_ functions calls fprintf.  fprintf is implemented in
>> > xen-posix/src/mini_libc.c, but it does not use fwrite/fputs/fputc, it's
>> > just
>> > a wrapper for mini-os print().
>> >
>> > How do I find out where these undefined references are coming from?
>>
>> Most likely, the C compiler is optimising the calls to fprintf and
>> replacing them with calls to the other functions.
>>
>
> Adding `-fno-builtin` to the xen-ocaml build resolves this.
>
> Without this flag, gcc makes assumptions about the semantics/implementation
> of standard library functions.  In this case, as you suggest, it is inlining
> calls to fprintf using its own built-in implementation instead of
> mini_libc's.  Very interesting.
>
> Not sure how this affects other parts of xen-ocaml; it can be made more
> specific with `-fno-builtin-fprintf`.

Well, this isn't a bad optimisation (although it doesn't really matter
for Mirage, since we normally use our own console). We'd just have to
provide implementations for fputs, etc.


-- 
Dr Thomas Leonard        http://roscidus.com/blog/
GPG: DA98 25AE CAD0 8975 7CDA  BD8E 0713 3F96 CA74 D8BA

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