C:\Users\ats37.platformio\packages\framework-arduinoespressif8266\cores\esp8266/Arduino.h:27:10: fatal error: stdlib.h: No such file or directory
Since this morning I have this problem.
I tried pio pkg update -g -p espressif8266 with no Success
Sounds like corrupted toolchain to me. Delete all folders
C:\Users\ats37.platformio\packages\toolchain-xtensa (or similiarly xtensa named) and rebiuld.
I already did this, same result. I deleted espressif8266 Folder.
I deleted framework-arduinoespressif8266 PlatformIO installs the core but in this folder ist not one Lib what starts with std*.h
I deleted the whole PlatFormIO Folders and now it works again how odd.