Are packed structs portable?

Solution 1:

Considering the mentioned platforms, yes, packed structs are completely fine to use. x86 and x86_64 always supported unaligned access, and contrary to the common belief, unaligned access on these platforms has (almost) the same speed as aligned access for a long time (there's no such thing that unaligned access is much slower). The only drawback is that the access may not be atomic, but I don't think it matters in this case. And there is an agreement between compilers, packed structs will use the same layout.

GCC/clang supports packed structs with the syntax you mentioned. MSVC has #pragma pack, which can be used like this:

#pragma pack(push, 1)
struct Sensor1Telemetry {
    int16_t temperature;
    uint32_t timestamp;
    uint16_t voltageMv;
    // etc...
};
#pragma pack(pop)

Two issues can arise:

1. Endianness must be the same across platforms (your MCU must be using little-endian)
2. If you assign a pointer to a packed struct member, and you're on an architecture which doesn't support unaligned access (or use instructions which have alignment requirements, like movaps or ldrd), then you may get a crash using that pointer (gcc doesn't warn you about this, but clang does).

Here's the doc from GCC:

The packed attribute specifies that a variable or structure field should have the smallest possible alignment—one byte for a variable

So GCC guarantees that no padding will be used.

MSVC:

To pack a class is to place its members directly after each other in memory

So MSVC guarantees that no padding will be used.

The only "dangerous" area I've found, is the usage of bitfields. Then the layout may differ between GCC and MSVC. But, there's an option in GCC, which makes them compatible: -mms-bitfields

Tip: even, if this solution works now, and it is highly unlikely that it will stop working, I recommend you keep dependency of your code on this solution low.

Note: I've considered only GCC, clang and MSVC in this answer. There are compilers maybe, for which these things are not true.

Solution 2:

If

• endianness is not an issue
• both compilers handle packing correctly
• the type definitions on both C implementations are accurate (Standard compliant).

then yes, "packed structures" are portable.

For my taste too many "if"s, do not do this. It's not worth the hassle to arise.

Solution 3:

You could do that, or use a more reliable alternative.

For the hard core amongst the serialisation fanatics out there, there's CapnProto. This gives you a native structure to deal with, and undertakes to ensure that when it's transferred across a network and lightly worked on, it'll still make sense the other end. To call it a serialisation is nearly inaccurate; it aims to do a little as possible to the in-memmory representation of a structure. Might be amenable to porting to an M4

There's Google Protocol Buffers, that's binary. More bloaty, but pretty good. There's the accompanying nanopb (more suited to microcontrollers), but it doesn't do the whole of GPB (I don't think it does oneof). Many people use it successfully though.

Some of the C asn1 runtimes are small enough for use on micro controllers. I know this one fits on M0.