That's a good question. (One I hadn't thought about much before I have to admit.)

I think it would be easier for the programmer to use a language feature than a library; the resulting code would be easier to read. I'm thinking how regular expressions are easier to use in Perl than they are in Java because they're part of the language, or how maps are easier to use in scripting languages than say Map in Java because they're part of the language.

If you had to have some e.g. string or external file describing the syntax of the hardware-independent struct, and then some calls like "read_entity(void structure, char fieldname)", it would all get nasty - you're going to have strings in the code which can't be checked at compile-time, you're going to be doing type casting which can't be checked at compile-time, and so on.

But a code generation system could be an option - you define the structure in a file in a certain syntax, and then code is generated with the right types and attribute names being visible to the compiler.

But I still think being part of the language would just be simpler for the user, and I don't consider this to be some obscure feature which would dilute the purity of the language by its introduction; binary file formats and protocols are here to stay.

P.S. Yes Google Protocol Buffers could well be the thing I've been searching for since I first saw the C struct many years ago.

Yeah, protocol buffers are a bit heavyweight, a bit harder to read than a language feature could be, and add an extra dependency. That's why I don't necessarily disagree, though the reason I asked is it occurred to me that any time I have personally needed to deal with byte ordering has been code that can easily justify a heavyweight solution (unlike regex's which end up in tiny scripts that may only be run once) and they have also all been places where being able to easily interact with with the same data in other languages would have been super useful, as would being able to easily add to the format without a versioning headache.

... of course, now that I write that I realize a language feature could actually provide all of that, too.