Treating 'char' Structure Members as Contiguous Bytes

[Martin]

The following compiles cleanly on gcc3.0.4 using -W -Wall -ansi -
pedantic. Lint gives this warning:
_
p_member_09 = p_start + OFFSET_TO_MEMBE R_09;
t_offset.c 27 Warning 416: creation of out-of-bounds pointer (33
beyond end of data) by operator ptr+int' [Reference: file t_offset.c:
lines 26, 27]

The underscore Lint uses to indicate the point of warning is over the
'O' in OFFSET_TO_MEMBE R_09.

My question is this. Is the technique used here well-defined and safe
practice?


typedef unsigned char uchar;

struct byte_array {
uchar member_01;
uchar member_02[8];
uchar member_03[2];
uchar member_04[2];
uchar member_05[8];
uchar member_06[2];
uchar member_07[2];
uchar member_08[8];
uchar member_09[2];
uchar member_10[2];
};

struct byte_array byte_array;

#define OFFSET_TO_MEMBE R_06 21
#define OFFSET_TO_MEMBE R_09 33

int main( void )
{
uchar *p_start;
uchar *p_member_09;

p_start = &byte_array.mem ber_01;
p_member_09 = p_start + OFFSET_TO_MEMBE R_09;

return 0;
}

[Thad Smith]

Re: Treating 'char' Structure Members as Contiguous Bytes

Martin wrote:

The following compiles cleanly on gcc3.0.4 using -W -Wall -ansi -
pedantic. Lint gives this warning:
_
p_member_09 = p_start + OFFSET_TO_MEMBE R_09;
t_offset.c 27 Warning 416: creation of out-of-bounds pointer (33
beyond end of data) by operator ptr+int' [Reference: file t_offset.c:
lines 26, 27]
>
The underscore Lint uses to indicate the point of warning is over the
'O' in OFFSET_TO_MEMBE R_09.
>
My question is this. Is the technique used here well-defined and safe
practice?
>
>
typedef unsigned char uchar;
>
struct byte_array {
uchar member_01;
uchar member_02[8];
uchar member_03[2];
uchar member_04[2];
uchar member_05[8];
uchar member_06[2];
uchar member_07[2];
uchar member_08[8];
uchar member_09[2];
uchar member_10[2];
};
>
struct byte_array byte_array;
>
#define OFFSET_TO_MEMBE R_06 21
#define OFFSET_TO_MEMBE R_09 33
>
int main( void )
{
uchar *p_start;
uchar *p_member_09;
>
p_start = &byte_array.mem ber_01;
p_member_09 = p_start + OFFSET_TO_MEMBE R_09;
>
return 0;
}

It is not defined because the struct may have padding between members
(although it is unlikely when the members are arrays of unsigned char).

You can instead write
p_member_09 = byte_array.memb er_09;
or
p_member_09 = (uchar*)byte_ar ray + offsetof (struct byte_array, member_09);


--
Thad

[Richard Bos]

Re: Treating 'char' Structure Members as Contiguous Bytes

Martin <martin.o_brien @which.netwrote :

The following compiles cleanly on gcc3.0.4 using -W -Wall -ansi -
pedantic. Lint gives this warning:
_
p_member_09 = p_start + OFFSET_TO_MEMBE R_09;
t_offset.c 27 Warning 416: creation of out-of-bounds pointer (33
beyond end of data) by operator ptr+int' [Reference: file t_offset.c:
lines 26, 27]

My question is this. Is the technique used here well-defined and safe
practice?

I'll let others figure out the exact safety of the technique you use -
my first instincts say that it's undefined behaviour in theory, but very
unlikely to break - and suggest that regardless of the outcome, you
switch to a guaranteed safe technique: use offsetof(). You'll need to
#include <stddef.h>.

Richard

[Army1987]

Re: Treating 'char' Structure Members as Contiguous Bytes

Martin wrote:
[...]

typedef unsigned char uchar;
>
struct byte_array {
uchar member_01;
uchar member_02[8];

[...]

uchar member_10[2];
};
>
struct byte_array byte_array;
>
#define OFFSET_TO_MEMBE R_06 21
#define OFFSET_TO_MEMBE R_09 33

#include <stddef.h>

>
int main( void )
{
uchar *p_start;
uchar *p_member_09;
>
p_start = &byte_array.mem ber_01;
p_member_09 = p_start + OFFSET_TO_MEMBE R_09;

Why not p_start + offsetof(byte_a rray, member_09)?
[...]


--
Army1987 (Replace "NOSPAM" with "email")