variable length record using struct/union - help

[Panchal V]

I want to access a variable length record in C, the format is as
follows :

+---+---+-----------+
| A | L | D A T A |
+---+---+-----------+
A - Some Data (1 BYTE)
L - Length the Data that follows (1 BYTE)
then actual data

I want to access and represent this record using struct/union in c,
how can I do it...
Data is variable length, L tell the length of data (in bytes) that
follows...
So that I can access it in this manner :
pData->A = 56;
pData->data[pData->L - 1] = '\0';



Now I want to represent
+---+---+-----------+---+
| A | L | D A T A | B |
+---+---+-----------+---+

where B - is some data ( 1 byte)
I want to access it like this
pData->B
(Remember it is after variable length data)

How can I do it....????

-Neo

[Mark A. Odell]

Re: variable length record using struct/union - help

vks_vikas@redif fmail.com (Panchal V) wrote in
news:13fbeb79.0 402020600.1afa8 655@posting.goo gle.com:
[color=blue]
> Now I want to represent
> +---+---+-----------+---+
>| A | L | D A T A | B |
> +---+---+-----------+---+
>
> where B - is some data ( 1 byte)
> I want to access it like this
> pData->B
> (Remember it is after variable length data)
>
> How can I do it....????[/color]

You can't.

--
- Mark ->
--

[nrk]

Re: variable length record using struct/union - help

Panchal V wrote:
[color=blue]
> I want to access a variable length record in C, the format is as
> follows :
>
> +---+---+-----------+
> | A | L | D A T A |
> +---+---+-----------+
> A - Some Data (1 BYTE)
> L - Length the Data that follows (1 BYTE)
> then actual data
>
> I want to access and represent this record using struct/union in c,
> how can I do it...
> Data is variable length, L tell the length of data (in bytes) that
> follows...
> So that I can access it in this manner :
> pData->A = 56;
> pData->data[pData->L - 1] = '\0';
>
>
>
> Now I want to represent
> +---+---+-----------+---+
> | A | L | D A T A | B |
> +---+---+-----------+---+
>
> where B - is some data ( 1 byte)
> I want to access it like this
> pData->B
> (Remember it is after variable length data)
>
> How can I do it....????
>[/color]

You can't really do what you want. But a reasonable alternative might be to
define something like:

struct Record {
unsigned char A;
unsigned char L;
unsigned char *data;
};

#define GETB(rptr) ((rptr)->data[(rptr)->L])
#define SETB(rptr, val) ((rptr)->data[(rptr)->L] = val)

and use it like this:
struct Record *rptr;
...
unsigned char b = GETB(rptr);
...
SETB(rptr, b);

-nrk.
[color=blue]
> -Neo[/color]

--
Remove devnull for email

[xarax]

Re: variable length record using struct/union - help

"Panchal V" <vks_vikas@redi ffmail.com> wrote in message
news:13fbeb79.0 402020600.1afa8 655@posting.goo gle.com...[color=blue]
> I want to access a variable length record in C, the format is as
> follows :
>
> +---+---+-----------+
> | A | L | D A T A |
> +---+---+-----------+
> A - Some Data (1 BYTE)
> L - Length the Data that follows (1 BYTE)
> then actual data
>
> I want to access and represent this record using struct/union in c,
> how can I do it...
> Data is variable length, L tell the length of data (in bytes) that
> follows...
> So that I can access it in this manner :
> pData->A = 56;
> pData->data[pData->L - 1] = '\0';
>
>
>
> Now I want to represent
> +---+---+-----------+---+
> | A | L | D A T A | B |
> +---+---+-----------+---+
>
> where B - is some data ( 1 byte)
> I want to access it like this
> pData->B
> (Remember it is after variable length data)
>
> How can I do it....????[/color]

=============== =============== =
/* Use the "struct hack" approach. */
typedef struct header
{
unsigned char a; /* something */
unsigned char len; /* number of "data" elements */
unsigned char data[1]; /* element count is a lie */
} Header, * Header_P;


/*
* get the value of "unsigned char B" that follows
* the variable-length "data" array.
*/
unsigned char getB(Header_P header_p)
{
return *(header_p->len + header_p->data);
}

/*
* set the value of "unsigned char B" that
* follows the "data" array.
*/
void setB(Header_P header_p, unsigned char b)
{
*(header_p->len + header_p->data) = b;
}
=============== =============== =


--
----------------------------
Jeffrey D. Smith
Farsight Systems Corporation
24 BURLINGTON DRIVE
LONGMONT, CO 80501-6906

http://www.farsight-systems.com

z/Debug debugs your Systems/C programs running on IBM z/OS!
Are ISV upgrade fees too high? Check our custom product development!

[Leor Zolman]

Re: variable length record using struct/union - help

On 2 Feb 2004 06:00:01 -0800, vks_vikas@redif fmail.com (Panchal V)
wrote:
[color=blue]
>I want to access a variable length record in C, the format is as
>follows :
>
>+---+---+-----------+
>| A | L | D A T A |
>+---+---+-----------+
>A - Some Data (1 BYTE)
>L - Length the Data that follows (1 BYTE)
>then actual data
>
>I want to access and represent this record using struct/union in c,
>how can I do it...
>Data is variable length, L tell the length of data (in bytes) that
>follows...
>So that I can access it in this manner :
>pData->A = 56;
>pData->data[pData->L - 1] = '\0';
>
>
>
>Now I want to represent
>+---+---+-----------+---+
>| A | L | D A T A | B |
>+---+---+-----------+---+
>
>where B - is some data ( 1 byte)
>I want to access it like this
>pData->B
>(Remember it is after variable length data)
>
>How can I do it....????[/color]

If, in your structure, you represent your "data" with a _pointer_ and
allocate storage for it dynamically on-the-fly as you are reading your
data in, then you'll be able to use your desired syntax (since
subscripting works for pointers "as if" they were arrays).
-leor
[color=blue]
>
>-Neo[/color]

Leor Zolman
BD Software
leor@bdsoft.com
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

[Kevin D. Quitt]

Re: variable length record using struct/union - help

On 2 Feb 2004 06:00:01 -0800, vks_vikas@redif fmail.com (Panchal V) wrote:

[color=blue]
>I want to access a variable length record in C, the format is as
>follows :
>
>+---+---+-----------+
>| A | L | D A T A |
>+---+---+-----------+
>A - Some Data (1 BYTE)
>L - Length the Data that follows (1 BYTE)
>then actual data
>
>I want to access and represent this record using struct/union in c,
>how can I do it...
>Data is variable length, L tell the length of data (in bytes) that
>follows...
>So that I can access it in this manner :
>pData->A = 56;
>pData->data[pData->L - 1] = '\0';[/color]

struct
{
char A;
char L;
char data[];
} fred;

struct fred *pData;

[color=blue]
>Now I want to represent
>+---+---+-----------+---+
>| A | L | D A T A | B |
>+---+---+-----------+---+
>
>where B - is some data ( 1 byte)
>I want to access it like this
>pData->B
>(Remember it is after variable length data)
>
>How can I do it....????[/color]

You can't by name. The struct hack^W^W flexible array member has to be
the last member of the structure.

pData->data[ pData->L ] points to B.


--
#include <standard.discl aimer>
_
Kevin D Quitt USA 91387-4454 96.37% of all statistics are made up
Per the FCA, this address may not be added to any commercial mail list

[Old Wolf]

Re: variable length record using struct/union - help

> Now I want to represent[color=blue]
> +---+---+-----------+---+
> | A | L | D A T A | B |
> +---+---+-----------+---+
>
> where B - is some data ( 1 byte)
> I want to access it like this
> pData->B
> (Remember it is after variable length data)
>
> How can I do it....????
>
> -Neo[/color]

typedef unsigned char *NEO_PTR;
#define NEO_A(np) ((np)[0])
#define NEO_L(np) ((np)[1])
#define NEO_DATA(np) ((np) + 2)
#define NEO_DATA_END(np ) (NEO_DATA(np) + NEO_L(np))
#define NEO_B(np) (NEO_DATA_END(n p)[0])

You can't access with the pointer syntax as you described.
Also, remember that these are macros so don't go "++" in them, etc.

NEO_PTR np = somewhere;
printf("B=%d, L=%d, A=%d, data=%s",
NEO_B(np), NEO_L(np), NEO_A(np), NEO_DATA(np));
NEO_B(np) = 15;

[Jack Klein]

Re: variable length record using struct/union - help

On Mon, 02 Feb 2004 14:37:14 GMT, "xarax" <xarax@email.co m> wrote in
comp.lang.c:
[color=blue]
> "Panchal V" <vks_vikas@redi ffmail.com> wrote in message
> news:13fbeb79.0 402020600.1afa8 655@posting.goo gle.com...[color=green]
> > I want to access a variable length record in C, the format is as
> > follows :
> >
> > +---+---+-----------+
> > | A | L | D A T A |
> > +---+---+-----------+
> > A - Some Data (1 BYTE)
> > L - Length the Data that follows (1 BYTE)
> > then actual data
> >
> > I want to access and represent this record using struct/union in c,
> > how can I do it...
> > Data is variable length, L tell the length of data (in bytes) that
> > follows...
> > So that I can access it in this manner :
> > pData->A = 56;
> > pData->data[pData->L - 1] = '\0';
> >
> >
> >
> > Now I want to represent
> > +---+---+-----------+---+
> > | A | L | D A T A | B |
> > +---+---+-----------+---+
> >
> > where B - is some data ( 1 byte)
> > I want to access it like this
> > pData->B
> > (Remember it is after variable length data)
> >
> > How can I do it....????[/color]
>
> =============== =============== =
> /* Use the "struct hack" approach. */[/color]

No, don't, it's undefined behavior.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++

http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html

[Panchal V]

Re: variable length record using struct/union - help

oldwolf@inspire .net.nz (Old Wolf) wrote in message news:<843a4f78. 0402021446.1bd8 c394@posting.go ogle.com>...[color=blue][color=green]
> > Now I want to represent
> > +---+---+-----------+---+
> > | A | L | D A T A | B |
> > +---+---+-----------+---+
> >
> > where B - is some data ( 1 byte)
> > I want to access it like this
> > pData->B
> > (Remember it is after variable length data)
> >
> > How can I do it....????
> >
> > -Neo[/color]
>
> typedef unsigned char *NEO_PTR;
> #define NEO_A(np) ((np)[0])
> #define NEO_L(np) ((np)[1])
> #define NEO_DATA(np) ((np) + 2)
> #define NEO_DATA_END(np ) (NEO_DATA(np) + NEO_L(np))
> #define NEO_B(np) (NEO_DATA_END(n p)[0])
>
> You can't access with the pointer syntax as you described.
> Also, remember that these are macros so don't go "++" in them, etc.
>
> NEO_PTR np = somewhere;
> printf("B=%d, L=%d, A=%d, data=%s",
> NEO_B(np), NEO_L(np), NEO_A(np), NEO_DATA(np));
> NEO_B(np) = 15;[/color]

This approach seems to be good as compared to other hacks... if
side-effects are taken care of (sequence points) etc.

Thanks for your views...

- Neo

[Panchal V]

Re: variable length record using struct/union - help

Jack Klein <jackklein@spam cop.net> wrote in message news:<eu6u109k2 84mnj2jbcmtd9ls rklr8t2431@4ax. com>...[color=blue]
> On Mon, 02 Feb 2004 14:37:14 GMT, "xarax" <xarax@email.co m> wrote in
> comp.lang.c:
>[color=green]
> > "Panchal V" <vks_vikas@redi ffmail.com> wrote in message
> > news:13fbeb79.0 402020600.1afa8 655@posting.goo gle.com...[color=darkred]
> > > I want to access a variable length record in C, the format is as
> > > follows :
> > >
> > > +---+---+-----------+
> > > | A | L | D A T A |
> > > +---+---+-----------+
> > > A - Some Data (1 BYTE)
> > > L - Length the Data that follows (1 BYTE)
> > > then actual data
> > >
> > > I want to access and represent this record using struct/union in c,
> > > how can I do it...
> > > Data is variable length, L tell the length of data (in bytes) that
> > > follows...
> > > So that I can access it in this manner :
> > > pData->A = 56;
> > > pData->data[pData->L - 1] = '\0';
> > >
> > >
> > >
> > > Now I want to represent
> > > +---+---+-----------+---+
> > > | A | L | D A T A | B |
> > > +---+---+-----------+---+
> > >
> > > where B - is some data ( 1 byte)
> > > I want to access it like this
> > > pData->B
> > > (Remember it is after variable length data)
> > >
> > > How can I do it....????[/color]
> >
> > =============== =============== =
> > /* Use the "struct hack" approach. */[/color]
>
> No, don't, it's undefined behavior.[/color]

What's undefined?

-Neo

[Panchal V]

Re: variable length record using struct/union - help

Kevin D. Quitt <KQuittUNMUNG@I EEIncUNMUNG.com > wrote in message news:<ct0t109b9 fkpljocer8h59ms fgumkhpq2d@4ax. com>...[color=blue]
> On 2 Feb 2004 06:00:01 -0800, vks_vikas@redif fmail.com (Panchal V) wrote:
>
>[color=green]
> >I want to access a variable length record in C, the format is as
> >follows :
> >
> >+---+---+-----------+
> >| A | L | D A T A |
> >+---+---+-----------+
> >A - Some Data (1 BYTE)
> >L - Length the Data that follows (1 BYTE)
> >then actual data
> >
> >I want to access and represent this record using struct/union in c,
> >how can I do it...
> >Data is variable length, L tell the length of data (in bytes) that
> >follows...
> >So that I can access it in this manner :
> >pData->A = 56;
> >pData->data[pData->L - 1] = '\0';[/color]
>
> struct
> {
> char A;
> char L;
> char data[];
> } fred;
>
> struct fred *pData;
>
>[color=green]
> >Now I want to represent
> >+---+---+-----------+---+
> >| A | L | D A T A | B |
> >+---+---+-----------+---+
> >
> >where B - is some data ( 1 byte)
> >I want to access it like this
> >pData->B
> >(Remember it is after variable length data)
> >
> >How can I do it....????[/color]
>
> You can't by name. The struct hack^W^W flexible array member has to be
> the last member of the structure.
>
> pData->data[ pData->L ] points to B.[/color]

char data[];
This doesn't seem to work on All implementations (compilers)

I'm using c51 by KEIL, it reports unknown array size...

And lets say we define one more variable after it, compiler starts
complaining, in GCC too...

-Neo

[Kevin D. Quitt]

Re: variable length record using struct/union - help

On Tue, 03 Feb 2004 03:56:21 GMT, Jack Klein <jackklein@spam cop.net>
wrote:[color=blue][color=green]
>> /* Use the "struct hack" approach. */[/color]
>
>No, don't, it's undefined behavior.[/color]

No, it isn't. It's part of C99, although they call it "flexible array
member".


--
#include <standard.discl aimer>
_
Kevin D Quitt USA 91387-4454 96.37% of all statistics are made up
Per the FCA, this address may not be added to any commercial mail list

[Kevin D. Quitt]

Re: variable length record using struct/union - help

On 3 Feb 2004 00:54:49 -0800, vks_vikas@redif fmail.com (Panchal V) wrote:

[color=blue]
>Kevin D. Quitt <KQuittUNMUNG@I EEIncUNMUNG.com > wrote in message news:<ct0t109b9 fkpljocer8h59ms fgumkhpq2d@4ax. com>...[color=green]
>> You can't by name. The struct hack^W^W flexible array member has to be
>> the last member of the structure.[/color]
>
> char data[];
>This doesn't seem to work on All implementations (compilers)
>
>I'm using c51 by KEIL, it reports unknown array size...[/color]

Then it's not a C99 compiler. Per the Standard, 6.7.2.1:

16 As a special case, the last element of a structure with more than one
named member may have an incomplete array type; this is called a flexible
array member. With two exceptions, the flexible array member is ignored.
First, the size of the structure shall be equal to the offset of the last
element of an otherwise identical structure that replaces the flexible
array member with an array of unspecified length.106) Second, when a . (or
->) operator has a left operand that is (a pointer to) a structure with a
flexible array member and the right operand names that member, it behaves
as if that member were replaced with the longest array (with the same
element type) that would not make the structure larger than the object
being accessed; the offset of the array shall remain that of the flexible
array member, even if this would differ from that of the replacement
array. If this array would have no elements, it behaves as if it had one
element but the behavior is undefined if any attempt is made to access
that element or to generate a pointer one past it.


[color=blue]
>And lets say we define one more variable after it, compiler starts
>complaining, in GCC too...[/color]

Of course, because the flexible array member must be the last member.


--
#include <standard.discl aimer>
_
Kevin D Quitt USA 91387-4454 96.37% of all statistics are made up
Per the FCA, this address may not be added to any commercial mail list

[Richard Bos]

Re: variable length record using struct/union - help

Kevin D. Quitt <KQuittUNMUNG@I EEIncUNMUNG.com > wrote:
[color=blue]
> On Tue, 03 Feb 2004 03:56:21 GMT, Jack Klein <jackklein@spam cop.net>
> wrote:[color=green][color=darkred]
> >> /* Use the "struct hack" approach. */[/color]
> >
> >No, don't, it's undefined behavior.[/color]
>
> No, it isn't. It's part of C99, although they call it "flexible array
> member".[/color]

But what xanax (to whom Jack Klein replied) demonstrated wasn't a C99
flexible array member, but a C89 mini-array-and-malloc struct hack. And
that _is_ undefined.

Richard

[Kevin D. Quitt]

Re: variable length record using struct/union - help

On Tue, 03 Feb 2004 16:08:41 GMT, rlb@hoekstra-uitgeverij.nl (Richard Bos)
wrote:[color=blue]
>But what xanax (to whom Jack Klein replied) demonstrated wasn't a C99
>flexible array member, but a C89 mini-array-and-malloc struct hack. And
>that _is_ undefined.[/color]

Indeed - my bad.

--
#include <standard.discl aimer>
_
Kevin D Quitt USA 91387-4454 96.37% of all statistics are made up
Per the FCA, this address may not be added to any commercial mail list