The search path when DLL is loading on Windows

First Rules:

• If a DLL with the same module name is already loaded in memory, the system checks only for redirection and a manifest before resolving to the loaded DLL, no matter which directory it is in. The system does not search for the DLL.
• If the DLL is on the list of known DLLs for the version of Windows on which the application is running, the system uses its copy of the known DLL (and the known DLL's dependent DLLs, if any) instead of searching for the DLL. For a list of known DLLs on the current system, see the following registry key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\KnownDLLs.
• If a DLL has dependencies, the system searches for the dependent DLLs as if they were loaded with just their module names. This is true even if the first DLL was loaded by specifying a full path.

If the DLLs don't fall "First Rules" category, then

The standard DLL search order used by the system depends on whether safe DLL search mode is enabled or disabled. Safe DLL search mode places the user's current directory later in the search order.

Safe DLL search mode is enabled by default. To disable this feature, create theHKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\SafeDllSearchMode registry value and set it to 0. Calling theSetDllDirectory function effectively disables SafeDllSearchMode while the specified directory is in the search path and changes the search order as described in this topic.

Windows XP:  Safe DLL search mode is disabled by default. To enable this feature, create the SafeDllSearchMode registry value and set it to 1. Safe DLL search mode is enabled by default starting with Windows XP with Service Pack 2 (SP2).

If SafeDllSearchMode is enabled, the search order is as follows:

1. The directory from which the application loaded.
2. The system directory. Use the GetSystemDirectory function to get the path of this directory.
3. The 16-bit system directory. There is no function that obtains the path of this directory, but it is searched.
4. The Windows directory. Use the GetWindowsDirectory function to get the path of this directory.
5. The current directory.
6. The directories that are listed in the PATH environment variable. Note that this does not include the per-application path specified by theApp Paths registry key. The App Paths key is not used when computing the DLL search path.

If SafeDllSearchMode is disabled, the search order is as follows:

1. The directory from which the application loaded.
2. The current directory.
3. The system directory. Use the GetSystemDirectory function to get the path of this directory.
4. The 16-bit system directory. There is no function that obtains the path of this directory, but it is searched.
5. The Windows directory. Use the GetWindowsDirectory function to get the path of this directory.
6. The directories that are listed in the PATH environment variable. Note that this does not include the per-application path specified by theApp Paths registry key. The App Paths key is not used when computing the DLL search path.

Reference:

http://msdn.microsoft.com/en-us/library/ms682586%28VS.85%29.aspx

http://blog.csdn.net/clever101/article/details/5557538