[9fans] C question on struct Biobuf in bio.h

[9fans] C question on struct Biobuf in bio.h

[Carsten Kunze]

Hello,

in bio.h there is a

struct	Biobuf
{
	Biobufhdr;
	uchar	b[Bungetsize+Bsize];
};

where Biobufhdr is declared as

typedef	struct	Biobufhdr	Biobufhdr;

To make it compile with gcc under UNIX I changed the struct to

struct	Biobuf
{
	Biobufhdr Biobufhdr;
	uchar	b[Bungetsize+Bsize];
};

but is that what is meant by the original description above?

   Carsten

Re: [9fans] C question on struct Biobuf in bio.h

[Bence Fábián]

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hi,

no, it is an anonym field.  and it is used in a way that is not part
of ansi c.  there is an extension in newer versions of gcc which
supports it, but if you wan't to port plan 9 c to unix you can use the
libs from plan9port.  that's far more easier.

bence

2014-10-07 17:24 GMT+02:00 Carsten Kunze <carsten.kunze@arcor.de>:

> Hello,
>
> in bio.h there is a
>
> struct  Biobuf
> {
>         Biobufhdr;
>         uchar   b[Bungetsize+Bsize];
> };
>
> where Biobufhdr is declared as
>
> typedef struct  Biobufhdr       Biobufhdr;
>
> To make it compile with gcc under UNIX I changed the struct to
>
> struct  Biobuf
> {
>         Biobufhdr Biobufhdr;
>         uchar   b[Bungetsize+Bsize];
> };
>
> but is that what is meant by the original description above?
>
>    Carsten
>
>

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Re: [9fans] C question on struct Biobuf in bio.h

[David du Colombier]

> in bio.h there is a
>
> struct  Biobuf
> {
>         Biobufhdr;
>         uchar   b[Bungetsize+Bsize];
> };
>
> where Biobufhdr is declared as
>
> typedef struct  Biobufhdr       Biobufhdr;

This is an unnamed structure. Recent versions of GCC
should be able to handle them when setting the
-fplan9-extension flag.

Otherwise, your change is fine, but "hdr" would probably be
a better name than "Biobufhdr". Also, don't forget to update
your code to use b->hdr.fid instead of b->fid, and so on.

--
David du Colombier

Re: [9fans] C question on struct Biobuf in bio.h

[Carsten Kunze]

> Otherwise, your change is fine, but "hdr" would probably be
> a better name than "Biobufhdr". Also, don't forget to update
> your code to use b->hdr.fid instead of b->fid, and so on.

Thanks for all answers!

(They also helped to find a short documentation in /sys/doc/compiler)

   Carsten

Re: [9fans] C question on struct Biobuf in bio.h

[Álvaro Jurado]

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I'm using gcc to compile plan9 code from some time (4.7/4.8) and that
option are masking real behaviour. Do not warns or put out an error about
"anonymous structs", but you will have a conflict if to structs in the same
source are including a, for example, "Lock;" element, and ld could not
assign right the values if declarations are not specified ansi style
(variable initialization it's made by ld, not by the compiler): you will
get some dragons in some cases.
Look at LP49 source code and you will see that authors used your second way.

Álvaro Jurado Cuevas
colmenar.biz.tm

2014-10-07 17:58 GMT+02:00 Carsten Kunze <carsten.kunze@arcor.de>:

> > Otherwise, your change is fine, but "hdr" would probably be
> > a better name than "Biobufhdr". Also, don't forget to update
> > your code to use b->hdr.fid instead of b->fid, and so on.
>
> Thanks for all answers!
>
> (They also helped to find a short documentation in /sys/doc/compiler)
>
>    Carsten
>
>

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Re: [9fans] C question on struct Biobuf in bio.h

[Charles Forsyth]

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On 7 October 2014 18:27, Álvaro Jurado <elbingmiss@gmail.com> wrote:

>
> , and ld could not assign right the values if declarations are not
> specified ansi style (variable initialization it's made by ld, not by the
> compiler): you will get some dragons in some cases.


Initialisations of externals is done by the compiler, which lays things out
as .long etc. ld doesn't do anything special; it doesn't even know about
them.

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