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From: Stephen Powell on 13 Apr 2010 18:20
I realize that this is not a C forum, per se, but this is a Debian-specific
C question. I am trying to add support to the parted utility for CMS-formatted
disks on the s390 architecture. The source code is written in C, of course.
But I am not a C programmer. I can spell C, but that's about it. It seems
that the people who know s390 don't know C and the people who know C don't
know s390, as a general rule. There are very few people who know both, and
they are busier than a one-armed wallpaper hanger in the Spring. Perhaps this
question belongs on a developer forum, but I'm embarrassed to ask such a
stupid question on a developer forum. They'd laugh me clear into next week.
Anyway...

What I need to do is to have two structures overlay each other; so that they
occupy the same storage. To be specific, here is a structure which describes
the volume label for an OS-formatted disk:

-----

struct __attribute__ ((packed)) volume_label {
char volkey[4]; /* volume key = volume label */
char vollbl[4]; /* volume label */
char volid[6]; /* volume identifier */
u_int8_t security; /* security byte */
cchhb_t vtoc; /* VTOC address */
char res1[5]; /* reserved */
char cisize[4]; /* CI-size for FBA,... */
/* ...blanks for CKD */
char blkperci[4]; /* no of blocks per CI (FBA), blanks for CKD */
char labperci[4]; /* no of labels per CI (FBA), blanks for CKD */
char res2[4]; /* reserved */
char lvtoc[14]; /* owner code for LVTOC */
char res3[29]; /* reserved */
};

-----

And here is a structure which describes the volume label for a CMS-formatted
disk:

-----

struct __attribute__ ((packed)) cms_label {
char label_id[4]; /* Label identifier */
char vol_id[6]; /* Volume identifier */
char version_id[2]; /* Version identifier */
u_int32_t block_size; /* Disk block size */
u_int32_t origin_ptr; /* Disk origin pointer */
u_int32_t usable_count; /* Number of usable cylinders/blocks */
u_int32_t formatted_count; /* Max # of formatted cylinders/blocks */
u_int32_t block_count; /* Disk size in CMS blocks */
u_int32_t used_count; /* Number of CMS blocks in use */
u_int32_t block_count; /* Disk size in CMS blocks */
u_int32_t used_count; /* Number of CMS blocks in use */
u_int32_t fst_size; /* File Status Table (FST) size */
u_int32_t fst_count; /* Number of FSTs per CMS block */
char format_date[6]; /* Disk FORMAT date (YYMMDDhhmmss) */
char reserved1[2]; /* Reserved fields. Note: the high-order */
/* bit of the first byte is now a century */
/* flag. 0 = 1900s, 1 = 2000s. It is used */
/* in conjunction with "format_date". */
u_int32_t disk_offset; /* Offset in blocks to the start of the */
/* reserved file when the disk is reserved */
u_int32_t map_block; /* Allocation map block with next hole */
u_int32_t hblk_disp; /* Displacement into HBLK data of next hole */
u_int32_t user_disp; /* Disp into user part of allocation map */
u_int32_t open_files; /* Count of SFS open files for this ADT. */
/* Note: open_files is not really part of */
/* the volume label. It is not used for */
/* minidisks. */
char segment_name[8]; /* Name of the shared segment. */
/* Note: segment_name is not really part of */
/* the volume label. It is not stored on */
/* disk. */
};

-----

Note that both structures have as their first member a character variable
of length 4. In the case of the "volume_label" structure it is "volkey"
and in the case of the "cms_label" structure it is "label_id". If the
value of this variable is "VOL1" (in EBCDIC) then it is the first structure
which maps the storage. If the value of this variable is "CMS1" (in EBCDIC)
then it is the second structure which maps the storage. The volume_label
structure is apparently a based structure, as references to the volkey
variable look something like this:

disk_specific->anchor->vlabel->volkey

How do I get these structures to overlap each other? I eventually want to
make a reference to the disk_offset variable in the cms_label structure,
something like

disk_specific->anchor->vlabel->disk_offset

but I get compile errors such as

dasd.c:327: error: 'volume_label_t' has no member named 'disk_offset'

"volume_label_t" is defined like this:

typedef struct volume_label volume_label_t;

I freely admit that I don't know what I'm doing, and I ought to have my
head examined for trying to enhance a program written in a language that
I don't know, but I'm the one who will benefit if I can get it to work.
The people who know how don't care, and the people who care (in this case
me) don't know how!

I know how to do this in PL/I, but despite having spent the last two hours
paging through a C language reference manual, I couldn't find any examples
of overlaying two structures. I did find reference to something called
a union, but I don't have enough knowledge to know what to do. Does
anyone know how to do this?

--
.''`. Stephen Powell <zlinuxman(a)wowway.com>
: :' :
`. `'`
`-


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From: Ron Johnson on 13 Apr 2010 18:40
On 2010-04-13 17:16, Stephen Powell wrote:
> I realize that this is not a C forum, per se, but this is a Debian-specific
> C question. I am trying to add support to the parted utility for CMS-formatted
[snip]
>
> I know how to do this in PL/I, but despite having spent the last two hours
> paging through a C language reference manual, I couldn't find any examples
> of overlaying two structures. I did find reference to something called
> a union, but I don't have enough knowledge to know what to do. Does
> anyone know how to do this?
>

union is exactly what you need.

"I'm feeling lucky" sample c code union

http://www.wellho.net/resources/ex.php4?item=c209/union.c

--
Dissent is patriotic, remember?


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From: Mike Viau on 13 Apr 2010 19:10

> Tue, 13 Apr 2010 17:29:51 -0500 <ron.l.johnson(a)cox.net> wrote:
>
> On 2010-04-13 17:16, Stephen Powell wrote:
> > I realize that this is not a C forum, per se, but this is a Debian-specific
> > C question. I am trying to add support to the parted utility for CMS-formatted
> [snip]
> >
> > I know how to do this in PL/I, but despite having spent the last two hours
> > paging through a C language reference manual, I couldn't find any examples
> > of overlaying two structures. I did find reference to something called
> > a union, but I don't have enough knowledge to know what to do. Does
> > anyone know how to do this?
> >
>
> union is exactly what you need.
>

Yes thats absolutely right. Remember since C is a powerful, low level, and untyped programming language untagged unions are consider unsafe.

Good luck though!

> "I'm feeling lucky" sample c code union
>
> http://www.wellho.net/resources/ex.php4?item=c209/union.c
>
> --
> Dissent is patriotic, remember?
>
>
> --
> To UNSUBSCRIBE, email to debian-user-REQUEST(a)lists.debian.org
> with a subject of "unsubscribe". Trouble? Contact listmaster(a)lists.debian..org
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>

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From: Robert Baron on 13 Apr 2010 19:40
What a great little learning project.

My suggestion is to work out a simpler version of what you are trying to do:

typedef struct {
unsigned short rec_type;
long data;
} type1;

typedef struct {
unsigned short rec_type;
char data[4];
} type2;

......
some_type *chunk_of_mem;

long a = ((type1 *) chunk_of_mem)->data;
char *b = ((type2 *) chunk_of_mem)->data;
......

and either work out how to get the union syntax or casting. Both
should work equally well. One may be easier to work though with you
compiler.

Rob.


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From: Paul E Condon on 14 Apr 2010 10:40
On 20100413_193540, Robert Baron wrote:
> What a great little learning project.
>
> My suggestion is to work out a simpler version of what you are trying to do:
>
> typedef struct {
> unsigned short rec_type;
> long data;
> } type1;
>
> typedef struct {
> unsigned short rec_type;
> char data[4];
> } type2;
>
> .....
> some_type *chunk_of_mem;
>
> long a = ((type1 *) chunk_of_mem)->data;
> char *b = ((type2 *) chunk_of_mem)->data;
> .....
>
> and either work out how to get the union syntax or casting. Both
> should work equally well. One may be easier to work though with you
> compiler.
>
> Rob.

C is a remarkably flexible, close to the hardware, language. For many
years the C-way was, to the eyes of the non-expert, distinctly
non-obvious --- to the point that there was an open contest with
prizes awarded for the most obfuscated example of C code. The prizes
went to the code for which the judges were most surprised on seeing
it run after they read the code and tried to figure out what would
happen when it was run.

But the union construct is now there in the language in order to allow
the professional practitioner to make his code slightly less
obfuscated. I think you should use it. It will stick out in the
displayed text as a warning to future readers of your code that
something dangerous is being done. And it will not ever become an
out-of-date comment that can be safely ignored by that future reader.

You may be that future reader of your code who will appreciate what
you have done by using it.

--
Paul E Condon
pecondon(a)mesanetworks.net


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