Error 1 when trying to upload to board

PlatformIO IDE
3

With a platformio.ini of

[env:heltec_wifi_kit_32]
platform = espressif32
board = heltec_wifi_kit_32
framework = arduino
lib_deps =
   olikraus/U8g2 @ ^2.28.8
   fastled/FastLED @ ^3.3.3

The error message for your program reads

Indexing .pio\build\heltec_wifi_kit_32\liba1b\libWire.a
In file included from src\main.cpp:13:0:
src\marquee.h: In function 'void DrawMarquee()':
src\marquee.h:15:9: error: 'g_LEDs' was not declared in this scope
         g_LEDs[i] = c.setHue(k+=8);
         ^
src\marquee.h:21:9: error: 'g_LEDs' was not declared in this scope
         g_LEDs[i] = CRGB::Black;
         ^
src\main.cpp: In function 'void loop()':
src\main.cpp:58:11: warning: unused variable 'initialHue' [-Wunused-variable]
   uint8_t initialHue = 0;
           ^
src\main.cpp:59:17: warning: unused variable 'deltaHue' [-Wunused-variable]
   const uint8_t deltaHue = 16;
                 ^
src\main.cpp:60:17: warning: unused variable 'hueDensity' [-Wunused-variable]
   const uint8_t hueDensity = 8;
                 ^

which tells you exactly what’s wrong: the global variable g_LEDs is unknown in the function DrawMarquee(). When you look at the code, you write:

So you first pull in the function code that uses g_LEDs and then you actually declare and create the g_LEDs variable. So, at the position in which DrawMarquee() is defined, there is no mention of the global variable g_LEDs before that.

An extremely cheap way to fix that is to simply declare the global variable before you use it, i.e. reverse the order of the #include line and the one below. Then you get

Building .pio\build\heltec_wifi_kit_32\firmware.bin
Checking size .pio\build\heltec_wifi_kit_32\firmware.elf
Advanced Memory Usage is available via "PlatformIO Home > Project Inspect"
RAM:   [=         ]   5.5% (used 18148 bytes from 327680 bytes)
Flash: [==        ]  20.8% (used 272662 bytes from 1310720 bytes)
esptool.py v2.6
========================= [SUCCESS] Took 17.16 seconds =========================

However I don’t consider that a proper fix, since global variable sharing accross files is not done according to best practices at all. This forum already has a lot of info on splitting source code files and the associated techinques with sharing global variables: Tutorial for creating multi cpp file arduino project

I can also give my reference solution for that problem. The basic technique is:

  • declare global variables as extern ... in a header file
  • split code into header file and implementation file