SetDefaultDllDirectories

SetDefaultDllDirectories is a Windows API function that configures the default DLL search order for the calling process. It is implemented in Kernel32.dll and is used to influence how LoadLibrary and related functions resolve DLLs when a full path is not specified. The function does not load any libraries by itself; instead, it establishes the default locations that will be considered during subsequent DLL loading operations.

The function takes a single parameter, DirectoryFlags, which is a bitmask of flags that specify which locations to include in the default search order. Typical flags control whether to search the application directory, the system directory (such as System32), user directories, per-process DLL load directories added via AddDllDirectory, and other standard locations. The exact composition of the effective search order can vary by Windows version, and you can combine flags to tailor the behavior to your needs. After setting these defaults, subsequent LoadLibrary and related calls use the chosen search locations unless a full path is provided.

In practice, SetDefaultDllDirectories is often used at process startup to adopt a safer, more predictable DLL loading policy and to mitigate DLL search order hijacking risks. It is commonly paired with AddDllDirectory or SetDllDirectory to permit or restrict specific directories. Applications should call it early, before loading libraries, and then perform any additional path configuration as needed.

Return value is a BOOL: nonzero indicates success, zero indicates failure. On failure, use GetLastError to retrieve the error code. The function is available on Windows 7 and later; it is not present on Windows XP and earlier. See also LoadLibrary, AddDllDirectory, and SetDllDirectory.