[flatfinger]

What is meant by "portable code"? Should it refer only to code that should theoretically be usable on all imaginable implementations, or should it be expanded to include code which may not be accepted by all implementations, but which would have an unambiguous meaning on all implementations that accept it?

Historically, if there was some action or construct that different implementations would process in different ways that were well suited to their target platforms and purposes, but were incompatible with each other, the Standard would simply regard such an action as invoking Undefined Behavior, so as to avoid requiring that any implementations change in a way that would break existing code. This worked fine in an era where people were used to examining upon precedent to know how implementations intended for certain kinds of platforms and purposes should be expected to process certain constructs. Such an approach is becoming increasingly untenable, however.

If instead the Standard were to specify directives and say that if a program starts with directive X, implementations may either process integer overflow with precise wrapping semantics or refuse to process it altogether, if it starts with directive Y, implementations may either process it treating "long" as a 32-bit type or refuse to process it altogether, etc. this would make it much more practical to write portable programs. Not all programs would run on all implementations, but if many users of an implementation that targets a 64-bit platform need to use code that was designed around traditional microcomputer integer types, a directive demanding that "long" be 32 bits would provide a clear path for the implementation to meet its customers' needs.

[apotheon]

> What is meant by "portable code"? Should it refer only to code that should theoretically be usable on all imaginable implementations, or should it be expanded to include code which may not be accepted by all implementations, but which would have an unambiguous meaning on all implementations that accept it?

That's a good question. I'm not sure I know. I could hazard a guess at what would be "best", but I'm not particularly confident in my thoughts on the matter at this time. As long as how that is handled is thoughtful, practical, consistent, and well-established, though, I think we're much more than halfway to the right answer.

> Historically, if there was some action or construct that different implementations would process in different ways that were well suited to their target platforms and purposes, but were incompatible with each other, the Standard would simply regard such an action as invoking Undefined Behavior, so as to avoid requiring that any implementations change in a way that would break existing code.

If I understand correctly, that would actually be "implementation-defined", not "undefined".

> a directive demanding that "long" be 32 bits would provide a clear path for the implementation to meet its customers' needs

There are size-specific integer types specified in the C99 standard (e.g. `uint32_t`). I use those, except in the most trivial cases (e.g. `int main()`), and limit myself to those size-specific integer types that are "guaranteed" by the standard.

[flatfinger]

> If I understand correctly, that would actually be "implementation-defined", not "undefined".

That is an extremely common myth. From the point of view of the Standard, the difference between Implementation Defined behavior and Undefined Behavior is that implementations are supposed to document some kind of behavioral guarantee with regard to the former, even in cases where it would be impractical for a particular implementation to guarantee anything at all, and nothing that implementation could guarantee in those cases would be useful.

The published Rationale makes explicit an intention that Undefined Behavior, among other things, "identifies areas of conforming language extension".

> There are size-specific integer types specified in the C99 standard (e.g. `uint32_t`). I use those, except in the most trivial cases (e.g. `int main()`), and limit myself to those size-specific integer types that are "guaranteed" by the standard.

A major problem with the fixed-sized types is that their semantics are required to vary among implementations. For example, given

    int test(uint16_t a, uint16_t b, uint16_t c) { return a-b > c }

some implementations would be required to process test(1,2,3); so as to return 1, and some would be required to process it so as to return 0.

Further, if one has a piece of code which is written for a machine with particular integer types, and a compiler which targets a newer architecture but can be configured to support the old set of types, all one would need to do to port the code to the new platform would be to add a directive specifying the required integer types, with no need to rework the code to use the "fixed-sized" types whose semantics vary among implementations anyway.