topic Re: using sizeof in Operating System - OpenVMS

using sizeof

Eric_369 — Tue, 13 Sep 2005 10:15:20 GMT

On the microsoft platform using Visual Studio it is valid to declare variables and use the sizeof() method on the variable to get the size. For instance I can declare the following variables (below) and use the
sizeof(variable name) and the correct size will return.

long x[10][10];
char y[20];

struct XXX
{
long x[10][10];
char y[20];
}strct;

sizeof(x) is 100;
sizeof(y) is 20;
sizeof(strct) is 120;

I have done some testing on the itanium and found it works! Is this just a fluke or is this a valid way to do this on the OpenVMS platform?

Thanks

Re: using sizeof

Bojan Nemec — Tue, 13 Sep 2005 10:25:56 GMT

Eric,

This works. I think that this is part of the ANSIC standard.

The documentation:

http://h71000.www7.hp.com/commercial/c/docs/6180profile_016.html#index_x_563

says that the parameter to sizeof can be an expression.

Bojan

Re: using sizeof

David Jones_21 — Tue, 13 Sep 2005 13:09:38 GMT

sizeof(x) should be 400 or 800, depending upon how big a 'long' is.

Re: using sizeof

Eric_369 — Tue, 13 Sep 2005 13:17:42 GMT

Bojan,
Thanks much!

Dave,
Thanks, I calculated wrong. It's 400
and 420 for 'strct.'
Eric

Re: using sizeof

John Gillings — Tue, 13 Sep 2005 17:44:06 GMT

Eric,

One thing to beware of... the sizes of objects might differ between your Windows and OpenVMS code. The OpenVMS compiler, by default, will align fields within a structure on their natural boundaries, inserting "padding" where necessary. This may result in a structure larger than the simple sum of the component fields. Usually this isn't a problem, but if you're expecting to share binary data files across platforms, you might need to check that the structures are compatible.

Re: using sizeof

Eric_369 — Wed, 14 Sep 2005 09:59:01 GMT

John,
Thanks! When we share data across platforms in the Client/Server environment we are using the #pragma pack(1) on our Windows clients and
#pragma nomember_alignment on the HP OpenVMS server aligning both structure members on byte boundries.
Eric

Re: using sizeof

Joseph Huber_1 — Wed, 14 Sep 2005 10:07:09 GMT

If this is an existing software, o.k. let it be so, but for a new project starting now, DON'T do it, You will regret later, e.g. when You have to port to a another architecture or just new "clients" programming in a different langauage.
Better use a portable data format like XDR or XML.