Structs and Alignment

[Derrick Coetzee]

struct foo
{
char bar;
long baz;
};

How can I know exactly how many bytes into struct foo baz is?

This is what the offsetof macro, defined in <stddef.h>, is for (ref C99
7.17.3). However, if your program's behaviour relies on this value, it
is fundamentally nonportable, even between different versions of the
same compiler. Don't use this for loading/saving, in particular.

 

[xarax]

This is what the offsetof macro, defined in <stddef.h>, is for (ref C99
7.17.3). However, if your program's behaviour relies on this value, it
is fundamentally nonportable, even between different versions of the
same compiler. Don't use this for loading/saving, in particular.

<confused>
I had to read that a few times. At first, I thought you
were saying that relying on offsetof() makes your program
non-portable. Now, I think you mean that hard-coding a
numeric value, say 4, and comparing it to the result of
offset() is non-portable.
</confused>

 

[Old Wolf]

Apart from the fact that the alternative won't work (&f.baz and &f are
of incompatible types), why do you need an alternative when the
language provides offsetof() for exactly this purpose?

Some compilers, although 99% compliant to ANSI C, don't
provide offsetof().

 

[Keith Thompson]

Some compilers, although 99% compliant to ANSI C, don't
provide offsetof().

Really? Do you know of one?

 

[Ben Pfaff]

Some compilers, although 99% compliant to ANSI C, don't
provide offsetof().

I'd prefer to work around the problem if faced by such a
situation:

#include <stddef.h>

#ifndef offsetof
/* From the C FAQ. Normally works. */
#define offsetof(type, mem) ((size_t) \
((char *)&((type *)0)->mem - (char *)(type *)0))
#endif

 

[Derrick Coetzee]

This is what the offsetof macro, defined in <stddef.h>, is for (ref C99
7.17.3). However, if your program's behaviour relies on this value, it
is fundamentally nonportable [ . . . ]

<confused>
I had to read that a few times. At first, I thought you
were saying that relying on offsetof() makes your program
non-portable. Now, I think you mean that hard-coding a
numeric value, say 4, and comparing it to the result of
offset() is non-portable.
</confused>

Sorry, my language was unclear. More specifically, the assumption that
it either has a particular value or that the value is the same on two
different platforms is what would be in error, since it's
implementation-defined. I suppose that goes without saying, but it's not
unlike the common error of fwrite'ing a struct to disk and fread'ing it
back in on another platform.

 

[Emmanuel Delahaye]

Old Wolf wrote on 05/01/05 :

Some compilers, although 99% compliant to ANSI C, don't
provide offsetof().

Look in <stddef.h>. A lot of C-programmers don't know this header...

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html

"Mal nommer les choses c'est ajouter du malheur au
monde." -- Albert Camus.