All Retained Variables change their address in memory when new ones are added to the end of the list

Firmware Device OS
,
1

Device: P1
Device OS: 2.0.1

On the first version of the my firmware, I had a list of retained variables that are simply declared. Not in the class and not in the structure.
In (Picture 1), shown the physical memory addresses of each retained variable that I have declared.

Picture 1

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On the second firmware version I added two retained variables to the end of the list, as described in the reference (bool hasFlowSensor and double amountPerRotation, Picture 2 and 3).

Picture 2 and 3

3 2

I updated the firmware and looked at their addresses (Picture 4).

Picture 4

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The addresses of all variables have shifted - the count starts from the last. As a result, the last variable that had the address 0x40024000 became the address 0x40024009. This resulted in an SOS error (5 blinks between sos blinks). I can explain why, but this is irrelevant to the question.
The question is: why does the official reference say that the retained variable should be added after all the others? (Picture 5)

Picture 5

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If I add retained variables to the beginning of the list, then everything will be fine, the addresses of the variables will go after the old ones (Picture 6).

Picture 6

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Is this a mistake in the reference or am I missing something?

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@Vladek9921, do you have test code to share? I wonder if wrapping the variables in a structure would not better guarantee order. @ScruffR, any thoughts?

1 Like
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Not sure how it’s handled with the latest compiler, but in some previous version the order in which the variables were stored was inverted compared to the order of declaration - however there is no rule in which order a compiler “has to store” global variables.
A struct is definitely the better option in any case.

Relying on the placement of independent variables is never a good idea.
Irrespective of using struct or not, having a magic number for any new firmware revision and a check before using any persisted data would be the minimum I’d add. A checksum would be the next.
Since one needs a way to validate the data after a cold start anyhow, it should be common practice in all cases.

3 Likes
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@peekay123, Here’s the code that works as expected - the address is added at the end of the previous one (I used Web IDE).
You should first flash the first option, then change the comments and flash the second:

Simple code without libs

SerialLogHandler traceLog(LOG_LEVEL_TRACE);

SYSTEM_MODE(SEMI_AUTOMATIC);
SYSTEM_THREAD(ENABLED);
STARTUP(System.enableFeature(FEATURE_RETAINED_MEMORY));
STARTUP(WiFi.selectAntenna(ANT_AUTO));
STARTUP(System.disableFeature(FEATURE_WIFI_POWERSAVE_CLOCK));

// It is necessary to uncomment correctly
#define FIRST_VARIANT
//#define SECOND_VARIANT

#ifdef FIRST_VARIANT
retained uint32_t val1;
retained uint32_t val2;
retained uint32_t val3;
retained double val4;
retained double val5;
retained double val6;
retained uint32_t val7;
retained uint32_t val8;
#endif

#ifdef SECOND_VARIANT
retained uint32_t val1;
retained uint32_t val2;
retained uint32_t val3;
retained double val4;
retained double val5;
retained double val6;
retained uint32_t val7;
retained uint32_t val8;
retained bool val9;
retained double val10;
#endif

bool isNeed = false;
unsigned int timeUpd = 0;

void setup()
{
Particle.connect();
Serial.begin(9600);
}

void loop()
{
if (timeUpd + 5000 < millis())
{
#ifdef FIRST_VARIANT
Log.info(“1 %p”, &val1);
Log.info(“2 %p”, &val2);
Log.info(“3 %p”, &val3);
Log.info(“4 %p”, &val4);
Log.info(“5 %p”, &val5);
Log.info(“6 %p”, &val6);
Log.info(“7 %p”, &val7);
Log.info(“8 %p”, &val8);
#endif
#ifdef SECOND_VARIANT
Log.info(“1 %p”, &val1);
Log.info(“2 %p”, &val2);
Log.info(“3 %p”, &val3);
Log.info(“4 %p”, &val4);
Log.info(“5 %p”, &val5);
Log.info(“6 %p”, &val6);
Log.info(“7 %p”, &val7);
Log.info(“8 %p”, &val8);
Log.info(“9 %p”, &val9);
Log.info(“10 %p”, &val10);
#endif
timeUpd = millis();
}
}

But in the VSCode project I made a library. And here addressing starts from the end. I give a link to the project and you can test it.

VSCode project with library

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111
https://drive.google.com/file/d/18EKEStpnG8WNUbyuJ-7sTisSiX9ktipO/view?usp=sharing

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@ScruffR Got it, thanks for the answer. But the description in the reference can be confusing for newbies. Could it be better to make a clarification?

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@Vladek9921, in your VSCode library, where in the .ino file do you `#include “Retained.h”? I assume it is off screen in you screen capture.

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@peekay123 Yes of course. Otherwise it wouldn’t compile. I have attached a link to the project under the screenshots.

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@Vladek9921 did you try using a structure?

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@peekay123 Yes, I’ve used it before. But once I had some kind of problem with it (don’t remember). After removing the structure, the problem disappeared and I decided to live without it. It was on 1.5.2. But I’ll experiment with the structure again, thanks for the advice.