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From: Hugo Vanwoerkom on 14 Apr 2010 12:30
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
> 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?
>

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

Hugo


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From: Ron Johnson on 14 Apr 2010 13:40
On 2010-04-14 09:31, Paul E Condon wrote:
[snip]
>
> 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.

http://www1.us.ioccc.org/main.html I guess they got bored looking
at normal production C code...

There was also an Obfuscated Perl Contest, but that only ran for 5
years due to the "Perl" and "obfuscation" being redundant.

> But the union construct is now there in the language in order to allow
> the professional practitioner to make his code slightly less

Unions have been in C for at least 20 years; probably since the
Early Days.

--
Dissent is patriotic, remember?


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From: John Hasler on 14 Apr 2010 14:20
Ron Johnson writes:
> Unions have been in C for at least 20 years; probably since the Early
> Days.

Unions are in the first edition of K&R.
--
John Hasler


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From: Paul E Condon on 14 Apr 2010 17:10
On 20100414_123342, Ron Johnson wrote:
> On 2010-04-14 09:31, Paul E Condon wrote:
> [snip]
> >
> >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.
>
> http://www1.us.ioccc.org/main.html I guess they got bored looking at
> normal production C code...
>
> There was also an Obfuscated Perl Contest, but that only ran for 5
> years due to the "Perl" and "obfuscation" being redundant.
>
> >But the union construct is now there in the language in order to allow
> >the professional practitioner to make his code slightly less
>
> Unions have been in C for at least 20 years; probably since the Early
> Days.

I checked. Union is described in the Ritchie, Johnson, Lesk, and
Kernighan, The C Programming Language, in Bell System Technical
Journal of JULY-AUGUST 1978, p. 1991. At that time the Bell System had
C compilers running on PDP-11, Honeywell 6000, IBM System/370, and
Interdata 8/32. You're right. >20years.

--
Paul E Condon
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From: Boyd Stephen Smith Jr. on 14 Apr 2010 23:10
On Tuesday 13 April 2010 17:16:03 Stephen Powell wrote:
> 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 */
[...]
> };
>
> 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 */
[...]
> };
>

union any_label {
struct volume_label vl;
struct cms_label cl;
};

> 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:

union any_label *label = /* Initialize somehow */;
struct volume_label *maybe_vl = &label->vl;
struct cms_label *maybe_cl = &label->cl;

if (strncmp(maybe_vl->volkey, "VOL1", 4) == 0) {
maybe_cl = NULL;
/* Use maybe_vl all you want, e.g. */
maybe_vl->security = 0xFF;
} else if (strncmp(maybe_cl->label_id, "CMS1", 4) == 0) {
maybe_vl = NULL;
/* Use maybe_cl all you want, e.g. */
printf("%lu\n", (unsigned long) maybe_cl->disk_offset);
} else {
assert(("Unrecognized disk!", 0));
}

Hope that helps.
--
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ICQ: 514984 YM/AIM: DaTwinkDaddy `-'(. .)`-'
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