RegSetValueExA function (winreg.h)

Sets the data and type of a specified value under a registry key.


  [in]           HKEY       hKey,
  [in, optional] LPCSTR     lpValueName,
                 DWORD      Reserved,
  [in]           DWORD      dwType,
  [in]           const BYTE *lpData,
  [in]           DWORD      cbData


[in] hKey

A handle to an open registry key. The key must have been opened with the KEY_SET_VALUE access right. For more information, see Registry Key Security and Access Rights.

This handle is returned by the RegCreateKeyEx, RegCreateKeyTransacted, RegOpenKeyEx, or RegOpenKeyTransacted function. It can also be one of the following predefined keys:

The Unicode version of this function supports the following additional predefined keys:

[in, optional] lpValueName

The name of the value to be set. If a value with this name is not already present in the key, the function adds it to the key.

If lpValueName is NULL or an empty string, "", the function sets the type and data for the key's unnamed or default value.

For more information, see Registry Element Size Limits.

Registry keys do not have default values, but they can have one unnamed value, which can be of any type.


This parameter is reserved and must be zero.

[in] dwType

The type of data pointed to by the lpData parameter. For a list of the possible types, see Registry Value Types.

[in] lpData

The data to be stored.

For string-based types, such as REG_SZ, the string must be null-terminated. With the REG_MULTI_SZ data type, the string must be terminated with two null characters.

Note  lpData indicating a null value is valid, however, if this is the case, cbData must be set to '0'.

[in] cbData

The size of the information pointed to by the lpData parameter, in bytes. If the data is of type REG_SZ, REG_EXPAND_SZ, or REG_MULTI_SZ, cbData must include the size of the terminating null character or characters.

Return value

If the function succeeds, the return value is ERROR_SUCCESS.

If the function fails, the return value is a nonzero error code defined in Winerror.h. You can use the FormatMessage function with the FORMAT_MESSAGE_FROM_SYSTEM flag to get a generic description of the error.


Value sizes are limited by available memory. However, storing large values in the registry can affect its performance. Long values (more than 2,048 bytes) should be stored as files, with the locations of the files stored in the registry.

Application elements such as icons, bitmaps, and executable files should be stored as files and not be placed in the registry.

If dwType is the REG_SZ, REG_MULTI_SZ, or REG_EXPAND_SZ type and the ANSI version of this function is used (either by explicitly calling RegSetValueExA or by not defining UNICODE before including the Windows.h file), the data pointed to by the lpData parameter must be an ANSI character string. The string is converted to Unicode before it is stored in the registry.

Note that operations that access certain registry keys are redirected. For more information, see Registry Virtualization and 32-bit and 64-bit Application Data in the Registry.

Consider using the RegSetKeyValue function, which provides a more convenient way to set the value of a registry key.


The winreg.h header defines RegSetValueEx 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.


Requirement Value
Minimum supported client Windows 2000 Professional [desktop apps only]
Minimum supported server Windows 2000 Server [desktop apps only]
Target Platform Windows
Header winreg.h (include Windows.h)
Library Advapi32.lib
DLL Advapi32.dll

See also






Registry Functions

Registry Overview