Exceptions are not enabled correctly for Arduino-ESP8266

[maxgerhardt]

The documentation (https://docs.platformio.org/en/latest/platforms/espressif8266.html#exceptions) tells us that in order to activate exceptions, we must add

; Remove default exceptions disabled flag
build_unflags = -fno-exceptions

; Enable exceptions
build_flags = -fexceptions

to the platformio.ini.

This is wrong for Arduino-ESP8266, as documented in this topic: https://community.platformio.org/t/platformio-cannot-handle-exceptions-with-esp8266/16738

The problem is that when a user selects "Exceptions: Enabled" in the Arduino IDE, it links the firmware against a different C++ standard library which has exceptions enabled: libstdc++-exc.a instead of libstdc++.a. This can be seen in https://github.com/esp8266/Arduino/tree/master/tools/sdk/lib.

The framework's builder code links blindly only against the non-exception-enabled C++ library

https://github.com/esp8266/Arduino/blob/7f38e141c7502032af594fdd502c1b1f6049377c/tools/platformio-build.py#L124-L128

This should be adapted so that e.g a macro like PIO_FRAMEWORK_ARDUINO_EXCEPTIONS_ENABLED changes the LIBS correctly.

For testing, the code

#include <Arduino.h>
#include <iostream>
#include <exception>

using namespace std; 

void func(void) {
    throw runtime_error("exception thrown.");
}

void setup() { 
    Serial.begin(115200);
    delay(200);
  
    cout << "Program started." << endl;
    try {
        func();
    } catch (const exception& e) {
        cout << "exception caught: " << e.what() << endl;
    }
    Serial.println("Program continues");
}

void loop() {
    while(true) {
        yield();
    }
}

and platformio.ini

[env:esp8266]
platform = espressif8266
board = nodemcuv2
framework = arduino
build_unflags = -fno-exceptions
build_flags = -fexceptions

Without a modified LIBS value (replace stdc++ with stdc++-exc), the firmware does an immediate abort() when an exception is encountered, even though it is catched.

User exception (panic/abort/assert)
--------------- CUT HERE FOR EXCEPTION DECODER ---------------

Abort called

            stack>>>

ctx: cont
sp: 3ffffe60 end: 3fffffc0 offset: 0000
3ffffe60: 00000000 00000000 00000000 00000000
3ffffe70: 000000fe 00000000 00000000 00000000
3ffffe80: 00000000 00000000 00000000 b8b1aabc
3ffffe90: 3fff0230 3fff1aac 00000000 3ffeee40
3ffffea0: 3ffeedd8 3ffef348 3fff1adc 40201f12
3ffffeb0: 00000000 00000000 00000000 40201f24
3ffffec0: 00000000 3fffff5c 00000000 402018e9
3ffffed0: 3fffff54 00000000 00000000 00000000
3ffffee0: 00000000 00000000 00000000 40212d30
3ffffef0: 00000000 3ffeee40 3ffeedd8 40212d41
3fffff00: 3fff1adc 3fff1aac 3fff1adc 40213534
3fffff10: 00000000 00000000 3fff1adc 402018f4
3fffff20: 40000000 00000000 00000000 00000000
3fffff30: 00000000 00000000 00000000 40212d30
3fffff40: 3ffeedd8 3ffeee40 3ffeedd8 40212d41
3fffff50: 3fff1adc 40213468 3fff1adc 402134df
3fffff60: f3f2f101 3ffef348 3fff1afc 40201060
3fffff70: 3fff1b18 3ffef348 3ffefe38 402147fe
3fffff80: 3fffdad0 3ffef348 3ffeedd8 402010e2
3fffff90: feefeffe feefeffe feefeffe feefeffe
3fffffa0: 3fffdad0 00000000 3ffeee00 40201b20
3fffffb0: feefeffe feefeffe 3ffe85b4 40100c11
<<<stack<<<

--------------- CUT HERE FOR EXCEPTION DECODER ---------------

in the fixed version the output is correct.

[maxgerhardt]

@valeros I see that your PR in Arduino-ESP8266 has landed and added PIO_FRAMEWORK_ARDUINO_ENABLE_EXCEPTIONS but the docs were not updated yet at https://github.com/platformio/platformio-docs/commits/develop, right?