SetFirmwareEnvironmentVariableA function (winbase.h)

Article
02/09/2023

Sets the value of the specified firmware environment variable.

Syntax

BOOL SetFirmwareEnvironmentVariableA(
  [in] LPCSTR lpName,
  [in] LPCSTR lpGuid,
  [in] PVOID  pValue,
  [in] DWORD  nSize
);

Parameters

[in] lpName

The name of the firmware environment variable. The pointer must not be NULL.

[in] lpGuid

The GUID that represents the namespace of the firmware environment variable. The GUID must be a string in the format "{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}". If the system does not support GUID-based namespaces, this parameter is ignored.

[in] pValue

A pointer to the new value for the firmware environment variable.

[in] nSize

The size of the pBuffer buffer, in bytes. If this parameter is zero, the firmware environment variable is deleted.

Return value

If the function succeeds, the return value is a nonzero value.

If the function fails, the return value is zero. To get extended error information, call GetLastError. Possible error codes include ERROR_INVALID_FUNCTION.

Remarks

Starting with Windows 10, version 1803, Universal Windows apps can read and write UEFI firmware variables. See Access UEFI firmware variables from a Universal Windows App for details.

Starting with Windows 10, version 1803, reading UEFI firmware variables is also supported from User-Mode Driver Framework (UMDF) drivers. Writing UEFI firmware variables from UMDF drivers is not supported.

To write a firmware environment variable, the user account that the app is running under must have the SE_SYSTEM_ENVIRONMENT_NAME privilege. A Universal Windows app must be run from an administrator account and follow the requirements outlined in Access UEFI firmware variables from a Universal Windows App.

The exact set of firmware environment variables is determined by the boot firmware. The location of these environment variables is also specified by the firmware. For example, on a UEFI-based system, NVRAM contains firmware environment variables that specify system boot settings. For information about specific variables used, see the UEFI specification. For more information about UEFI and Windows, see UEFI and Windows.

Firmware variables are not supported on a legacy BIOS-based system. The SetFirmwareEnvironmentVariable function will always fail on a legacy BIOS-based system, or if Windows was installed using legacy BIOS on a system that supports both legacy BIOS and UEFI. To identify these conditions, call the function with a dummy firmware environment name such as an empty string ("") for the lpName parameter and a dummy GUID such as "{00000000-0000-0000-0000-000000000000}" for the lpGuid parameter. On a legacy BIOS-based system, or on a system that supports both legacy BIOS and UEFI where Windows was installed using legacy BIOS, the function will fail with ERROR_INVALID_FUNCTION. On a UEFI-based system, the function will fail with an error specific to the firmware, such as ERROR_NOACCESS, to indicate that the dummy GUID namespace does not exist.

SetFirmwareEnvironmentVariable is the user-mode equivalent of the ExSetFirmwareEnvironmentVariable kernel-mode routine.

Note

The winbase.h header defines SetFirmwareEnvironmentVariable as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.

Requirements

Requirement	Value
Minimum supported client	Windows Vista, Windows XP with SP1 [desktop apps \| UWP apps]
Minimum supported server	Windows Server 2003 [desktop apps \| UWP apps]
Target Platform	Windows
Header	winbase.h (include Windows.h)
Library	Kernel32.lib
DLL	Kernel32.dll