RegSetValueEx function

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


  _In_             HKEY    hKey,
  _In_opt_         LPCTSTR lpValueName,
  _Reserved_       DWORD   Reserved,
  _In_             DWORD   dwType,
  _In_       const BYTE    *lpData,
  _In_             DWORD   cbData


hKey [in]

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:
lpValueName [in, optional]

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.

dwType [in]

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

lpData [in]

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. String-literal values must be formatted using a backslash preceded by another backslash as an escape character. For example, specify "C:\\mydir\\myfile" to store the string "C:\mydir\myfile".

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

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.


Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]


Winreg.h (include Windows.h)





Unicode and ANSI names

RegSetValueExW (Unicode) and RegSetValueExA (ANSI)

See also

Registry Functions
Registry Overview



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