Are packed structs portable?

Question

I have some code on a Cortex-M4 microcontroller and'd like to communicate with a PC using a binary protocol. Currently, I'm using packed structs using the GCC-specific packed attribute.

Here is a rough outline:

struct Sensor1Telemetry {
    int16_t temperature;
    uint32_t timestamp;
    uint16_t voltageMv;
    // etc...
} __attribute__((__packed__));

struct TelemetryPacket {
    Sensor1Telemetry tele1;
    Sensor2Telemetry tele2;
    // etc...
} __attribute__((__packed__));

My question is:

• Assuming that I use the exact same definition for the TelemetryPacket struct on the MCU and the client app, will the above code be portable accross multiple platforms? (I'm interested in x86 and x86_64, and need it to run on Windows, Linux and OS X.)
• Do other compilers support packed structs with the same memory layout? With what syntax?

EDIT:

• Yes, I know packed structs are non-standard, but they seem useful enough to consider using them.
• I'm interested in both C and C++, although I don't think GCC would handle them differently.
• These structs are not inherited and don't inherit anything.
• These structs only contain fixed-size integer fields, and other similar packed structs. (I've been burned by floats before...)

Answer 1

If you want something maximally portable, you can declare a buffer of uint8_t[TELEM1_SIZE] and memcpy() to and from offsets within it, performing endianness conversions such as htons() and htonl() (or little-endian equivalents such as the ones in glib). You could wrap this in a class with getter/setter methods in C++, or a struct with getter-setter functions in C.

Answer 2

Not always. When you send data to different architect processor, you need to consider about Endianness, primitive data type, etc. Better to use Thrift or Message Pack. If not, create yourself Serialize and DeSerialize methods instead.

Answer 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.