Struct __ATTRIBUTE__ ((packed) results in "expected initializer before" error

Projects Programming
1

I am trying to ensure a data structure is packed (for transmission in a UDP packet). All the examples I have found use this exact syntax, yet I am encountering this error.

struct __ATTRIBUTE__ ((packed)) dttmMsg {
  bool dttm_valid;
};

void setup() {
}

void loop() {
}
sketch_jan21a:1:33: error: expected initializer before 'dttmMsg'
 struct __ATTRIBUTE__ ((packed)) dttmMsg {
                                 ^~~~~~~

I am compiling for a ESP32 board, and I am using the latest Arduino IDE. Removing the attribute/packed bit, and the compile is fine.

Any suggestions would be greatly appreciated.

Thanks,

Allan.

2

I believe that syntax is specific to GCC. If a different compiler is used for ESP32 then you may have to use that compiler's syntax.

__ATTRIBUTE__ is very likely a macro which is expanding to nothing leaving the compiler with this...

struct ((packed)) dttmMsg {

__attribute__ is the keyboard for GCC; try that.

3

That did it! Many thanks!

struct __attribute__ ((packed)) dttmMsg {
  bool     dttm_valid;
};
4

Bear in mind that sizeof(bool) is not guaranteed to be 1 which makes it a poor choice for on-wire formats.

5

I don’t think you can capitalize ATTRIBUTE, did you try lower case ?

Édit/ just realized I’m late to the question :slight_smile:

6

I also got confused by OP telling that they found many examples of it

7

Bear in mind that sizeof(bool) is not guaranteed to be 1 which makes it a poor choice for on-wire formats.

The actual struct I have is:

struct __attribute__ ((packed))dttm_msg_s {
  bool     dttm_valid;
  uint32_t dttm;
  uint32_t ntp_last_rcvd;
  uint8_t  tz;
};

I simplified the code in the original post to the bare minimum to show the compile issue. I can easily switch the bool to a uint_8 to be safer.

Thanks.

8

I also got confused by OP telling that they found many examples of it

So I go this morning to give an example of it in upper case, and every single example is in lower case! My mind must have been playing tricks on me.

9

Excellent choice.