RegOpenKeyExA function (winreg.h)

Opens the specified registry key. Note that key names are not case sensitive.

To perform transacted registry operations on a key, call the RegOpenKeyTransacted function.


  [in]           HKEY   hKey,
  [in, optional] LPCSTR lpSubKey,
  [in]           DWORD  ulOptions,
  [in]           REGSAM samDesired,
  [out]          PHKEY  phkResult


[in] hKey

A handle to an open registry key. This handle is returned by the RegCreateKeyEx or RegOpenKeyEx function, or it can be one of the following predefined keys:


[in, optional] lpSubKey

The name of the registry subkey to be opened.

Key names are not case sensitive.

If the lpSubKey parameter is NULL or a pointer to an empty string, and if hKey is a predefined key, then the system refreshes the predefined key, and phkResult receives the same hKey handle passed into the function. Otherwise, phkResult receives a new handle to the opened key.

For more information, see Registry Element Size Limits.

[in] ulOptions

Specifies the option to apply when opening the key. Set this parameter to zero or the following:

Value Meaning
The key is a symbolic link. Registry symbolic links should only be used when absolutely necessary.

[in] samDesired

A mask that specifies the desired access rights to the key to be opened. The function fails if the security descriptor of the key does not permit the requested access for the calling process. For more information, see Registry Key Security and Access Rights.

[out] phkResult

A pointer to a variable that receives a handle to the opened key. If the key is not one of the predefined registry keys, call the RegCloseKey function after you have finished using the handle.

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.


On legacy versions of Windows, this API is also exposed by kernel32.dll.


Unlike the RegCreateKeyEx function, the RegOpenKeyEx function does not create the specified key if the key does not exist in the registry.

Certain registry operations perform access checks against the security descriptor of the key, not the access mask specified when the handle to the key was obtained. For example, even if a key is opened with a samDesired of KEY_READ, it can be used to create registry keys if the key's security descriptor permits. In contrast, the RegSetValueEx function specifically requires that the key be opened with the KEY_SET_VALUE access right.

If your service or application impersonates different users, do not use this function with HKEY_CURRENT_USER. Instead, call the RegOpenCurrentUser function.

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.


lResult = RegOpenKeyEx (hKeyRoot, lpSubKey, 0, KEY_READ, &hKey);

if (lResult != ERROR_SUCCESS) 
    if (lResult == ERROR_FILE_NOT_FOUND) {
        printf("Key not found.\n");
        return TRUE;
    else {
        printf("Error opening key.\n");
        return FALSE;

To see this example in context, see Deleting a Key with Subkeys.


The winreg.h header defines RegOpenKeyEx 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