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

[Len Maxwell <len@xxxxxxxx>]

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 \
>Â Â_build/main.native.o \
>Â Â-L/opt/ml/opam/4.02.1/lib/io-page -lio_page_xen_stubs \
>Â Â$(pkg-config --static --libs mirage-xen) \
>Â Â/usr/lib/gcc/x86_64-linux-gnu/4.9/libgcc.a \
>Â Â-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. Â

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`.

--

Len

Â

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

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