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]

vk*******@redif fmail.com (Panchal V) wrote in
news:13******** *************** ***@posting.goo gle.com:

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....????

You can't.

--
- Mark ->
--

[nrk]

Panchal V wrote:

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....????

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.
-Neo

--
Remove devnull for email

[xarax]

"Panchal V" <vk*******@redi ffmail.com> wrote in message
news:13******** *************** ***@posting.goo gle.com...

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....????

=============== =============== =
/* 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]

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

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....????
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

-Neo

Leor Zolman
BD Software
le**@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]

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

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

struct fred *pData;

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....????

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]

> 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

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]

On Mon, 02 Feb 2004 14:37:14 GMT, "xarax" <xa***@email.co m> wrote in
comp.lang.c:

"Panchal V" <vk*******@redi ffmail.com> wrote in message
news:13******** *************** ***@posting.goo gle.com...

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....????

=============== =============== =
/* Use the "struct hack" approach. */

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/~a...FAQ-acllc.html

[Panchal V]

ol*****@inspire .net.nz (Old Wolf) wrote in message news:<84******* *************** ****@posting.go ogle.com>...

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

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;

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]

Jack Klein <ja*******@spam cop.net> wrote in message news:<eu******* *************** **********@4ax. com>...

On Mon, 02 Feb 2004 14:37:14 GMT, "xarax" <xa***@email.co m> wrote in
comp.lang.c:

"Panchal V" <vk*******@redi ffmail.com> wrote in message
news:13******** *************** ***@posting.goo gle.com...

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....????

=============== =============== =
/* Use the "struct hack" approach. */

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

What's undefined?

-Neo