In a Windows application, I have a GetFirmwareEnvironmentVariableA function to read a firmware environment variable. Is there any way to write something in this variable in uefi driver and read from it later in Windows?
The function to set an NVRAM variable is called SetVariable() and is available to UEFI drivers via EFI_RUNTIME_SERVICES table.
To know more about it's interface and usage, read chapter 7.2 Variable Services of the UEFI 2.6 specification.