Warning about clearing of uninitialized global variables on reset

[brycecherry75]

When I define global variables with no initial values, they are cleared after initiating a reset with power applied even though a value other than zero has been written to them by the sketch beforehand.
From what I see, this is caused by the Arduino bootloader which in my belief tests the RAM and/or clears its contents on reset.

[per1234]

When I define global variables with no initial values, they are cleared after initiating a reset

This is how C++ (and thus the Arduino programming language) is designed and required to work, so it's the correct and expected behavior with or without a bootloader:
https://en.cppreference.com/w/cpp/language/zero_initialization

Zero initialization is performed in the following situations:

1. For every named variable with static or thread-local (since C++11) storage duration that is not subject to constant initialization, before any other initialization.

[matthijskooijman]

FYI, it is libc (or maybe the gcc startup libraries), not the bootloader that overwrites these values. You can access the pre-initilization values by putting a function in a special section (e.g. init1) to let it run before various initialization steps (see e.g. https://www.nongnu.org/avr-libc/user-manual/mem_sections.html)

Alternatively (and easier), you can put your variable in the .noinit section, which prevents the variable from being initialized at all, e.g.:

uint8_t some_variable __attribute__((section(".noinit")));

Note that this is feature must be supported by the architecture (or really, the linker script). I know AVR-libc has it and Arduino_Core_STM32 also defines it in its linker scripts since a while, but I'm not sure about others.