🚀 go-pugleaf

In <3610F74A.A7A6BECD@mail.cern.ch> Marko Asplund <aspa@mail.cern.ch> writes:

>i have an application where i need to send C structs, with BSD socket
>interface's read() and write() routines, between heterogenous machine
>architectures. the structs are quite simple containing only char arrays
>and integers e.g.
>
>	struct data {
>		char somestring[9];
>		int status;
>		int foo;
>		char otherstring[5];
>	};
>
>
>so, my question is how do i accomplish sending this struct over the
>network, to a machine with different architecture? when i do a write()
>does the system send everything in the struct, including padding fields,
>over the network?

Yes.

>i assume i can rely on the C compiler to preserve the
>order of the struct members.

Yes.

>but, i probably can't rely on int member
>fields being of certain length (at least not on all platforms).

Correct.

>can i assume something about the paddings between members?

No.

>for example, can i
>assume that if the char arrays lengths are multiples of 4, there will be
>no padding fields in most architecture?

Probably, but there is no standard guaranteeing that.  A 64-bit
platform might prefer to align everything on 8-byte boundaries.

If you're dealing with negative integer values you have the additional 
problem that there is no unique representation for them required by the
C language.

You have to choose between restricting your application to platforms
satisfying a certain number of assumptions and biting the bullet and
writing code that doesn't rely on any assumption (other than a common
character set, which you need anyway for passing the strings).

If your strings don't contain any spaces, you can easily convert the
whole structure to a single string and send that string over the
network:

    sprintf(buff, "%s %d %d %s", x.somestring, x.status, x.foo, 
            x.otherstring);

If your strings can contain spaces, then you have to use another character
as field separator.

At the receiving end, you read the string and convert it back to a struct,
with a similar sscanf() call.  All you have to assume is ASCII everywhere,
which is a fair assumption for machines with a BSD socket interface.

Dan
--
Dan Pop
CERN, IT Division
Email: Dan.Pop@cern.ch 
Mail:  CERN - EP, Bat. 31 1-014, CH-1211 Geneve 23, Switzerland