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

[Panchal V]

Kevin D. Quitt <KQ**********@I EEIncUNMUNG.com > wrote in message news:<ct******* *************** **********@4ax. com>...

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.

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]

On Tue, 03 Feb 2004 03:56:21 GMT, Jack Klein <ja*******@spam cop.net>
wrote:

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

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

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]

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

Kevin D. Quitt <KQ**********@I EEIncUNMUNG.com > wrote in message news:<ct******* *************** **********@4ax. com>...

You can't by name. The struct hack^W^W flexible array member has to be
the last member of the structure.
char data[];
This doesn't seem to work on All implementations (compilers)

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

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

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]

Kevin D. Quitt <KQ**********@I EEIncUNMUNG.com > wrote:

On Tue, 03 Feb 2004 03:56:21 GMT, Jack Klein <ja*******@spam cop.net>
wrote:

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

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

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

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]

On Tue, 03 Feb 2004 16:08:41 GMT, rl*@hoekstra-uitgeverij.nl (Richard Bos)
wrote:

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.

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

[Old Wolf]

> > struct

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

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

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

This is a very horrible compiler (probably you have no choice though?)
You may find you have to employ many non-standard hacks, to get
anything useful done.

Go
char data[1];
instead.

[Mark A. Odell]

ol*****@inspire .net.nz (Old Wolf) wrote in
news:84******** *************** ***@posting.goo gle.com:

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

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

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

This is a very horrible compiler (probably you have no choice though?)

It is a *very good* C51 compiler for the 8051. There are other choices but
this is one of the best. As for C you cannot use char data[];
You may find you have to employ many non-standard hacks, to get
anything useful done.

Go
char data[1];
instead.

You mean you must do char data[1] instead since C89 will not allow char
data[]. Keil good, undimensioned arrays - not permitted.

--
- Mark ->
--

[Old Wolf]

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

This is a very horrible compiler (probably you have no choice though?)

It is a *very good* C51 compiler for the 8051. There are other choices but
this is one of the best. As for C you cannot use char data[];

Perhaps I just have an old version then? (V4.01)
Or perhaps I just remember hours and hours of mucking around with
platform-specific issues and linker memory allocation bugs (eg.
putting concurrently-used variables at overlapping locations),
and am tarring the compiler with the same brush.

You may find you have to employ many non-standard hacks, to get
anything useful done.

Go
char data[1];
instead.

You mean you must do char data[1] instead since C89 will not allow char
data[]. Keil good, undimensioned arrays - not permitted.

I didn't mean to imply that Keil was bad for requiring "char data[1]"
(although I do consider accessing members past the end of an
array to be a 'nonstandard hack').

[Mark A. Odell]

ol*****@inspire .net.nz (Old Wolf) wrote in
news:84******** *************** ***@posting.goo gle.com:

>> I'm using c51 by KEIL, it reports unknown array size...
>
> This is a very horrible compiler (probably you have no choice
> though?)

It is a *very good* C51 compiler for the 8051. There are other choices
but this is one of the best. As for C you cannot use char data[];

Perhaps I just have an old version then? (V4.01)

Old would be an understatment. My god man, upgrade now!

:-)

--
- Mark ->
--