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LoadLibraryEx function

Loads the specified module into the address space of the calling process. The specified module may cause other modules to be loaded.

Syntax


HMODULE WINAPI LoadLibraryEx(
  _In_        LPCTSTR lpFileName,
  _Reserved_  HANDLE hFile,
  _In_        DWORD dwFlags
);

Parameters

lpFileName [in]

A string that specifies the file name of the module to load. This name is not related to the name stored in a library module itself, as specified by the LIBRARY keyword in the module-definition (.def) file.

The module can be a library module (a .dll file) or an executable module (an .exe file). If the specified module is an executable module, static imports are not loaded; instead, the module is loaded as if DONT_RESOLVE_DLL_REFERENCES was specified. See the dwFlags parameter for more information.

If the string specifies a module name without a path and the file name extension is omitted, the function appends the default library extension .dll to the module name. To prevent the function from appending .dll to the module name, include a trailing point character (.) in the module name string.

If the string specifies a fully qualified path, the function searches only that path for the module. When specifying a path, be sure to use backslashes (\), not forward slashes (/). For more information about paths, see Naming Files, Paths, and Namespaces.

If the string specifies a module name without a path and more than one loaded module has the same base name and extension, the function returns a handle to the module that was loaded first.

If the string specifies a module name without a path and a module of the same name is not already loaded, or if the string specifies a module name with a relative path, the function searches for the specified module. The function also searches for modules if loading the specified module causes the system to load other associated modules (that is, if the module has dependencies). The directories that are searched and the order in which they are searched depend on the specified path and the dwFlags parameter. For more information, see Remarks.

If the function cannot find the module or one of its dependencies, the function fails.

hFile

This parameter is reserved for future use. It must be NULL.

dwFlags [in]

The action to be taken when loading the module. If no flags are specified, the behavior of this function is identical to that of the LoadLibrary function. This parameter can be one of the following values.

ValueMeaning
DONT_RESOLVE_DLL_REFERENCES
0x00000001

If this value is used, and the executable module is a DLL, the system does not call DllMain for process and thread initialization and termination. Also, the system does not load additional executable modules that are referenced by the specified module.

Note  Do not use this value; it is provided only for backward compatibility. If you are planning to access only data or resources in the DLL, use LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE or LOAD_LIBRARY_AS_IMAGE_RESOURCE or both. Otherwise, load the library as a DLL or executable module using the LoadLibrary function.

LOAD_IGNORE_CODE_AUTHZ_LEVEL
0x00000010

If this value is used, the system does not check AppLocker rules or apply Software Restriction Policies for the DLL. This action applies only to the DLL being loaded and not to its dependencies. This value is recommended for use in setup programs that must run extracted DLLs during installation.

Windows Server 2008 R2 and Windows 7:  On systems with KB2532445 installed, the caller must be running as "LocalSystem" or "TrustedInstaller"; otherwise the system ignores this flag. For more information, see "You can circumvent AppLocker rules by using an Office macro on a computer that is running Windows 7 or Windows Server 2008 R2" in the Help and Support Knowledge Base at http://support.microsoft.com/kb/2532445.

Windows Server 2008, Windows Vista, Windows Server 2003, and Windows XP:  AppLocker was introduced in Windows 7 and Windows Server 2008 R2.

LOAD_LIBRARY_AS_DATAFILE
0x00000002

If this value is used, the system maps the file into the calling process's virtual address space as if it were a data file. Nothing is done to execute or prepare to execute the mapped file. Therefore, you cannot call functions like GetModuleFileName, GetModuleHandle or GetProcAddress with this DLL. Using this value causes writes to read-only memory to raise an access violation. Use this flag when you want to load a DLL only to extract messages or resources from it.

This value can be used with LOAD_LIBRARY_AS_IMAGE_RESOURCE. For more information, see Remarks.

LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE
0x00000040

Similar to LOAD_LIBRARY_AS_DATAFILE, except that the DLL file is opened with exclusive write access for the calling process. Other processes cannot open the DLL file for write access while it is in use. However, the DLL can still be opened by other processes.

This value can be used with LOAD_LIBRARY_AS_IMAGE_RESOURCE. For more information, see Remarks.

Windows Server 2003 and Windows XP:  This value is not supported until Windows Vista.

LOAD_LIBRARY_AS_IMAGE_RESOURCE
0x00000020

If this value is used, the system maps the file into the process's virtual address space as an image file. However, the loader does not load the static imports or perform the other usual initialization steps. Use this flag when you want to load a DLL only to extract messages or resources from it.

Unless the application depends on the file having the in-memory layout of an image, this value should be used with either LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE or LOAD_LIBRARY_AS_DATAFILE. For more information, see the Remarks section.

Windows Server 2003 and Windows XP:  This value is not supported until Windows Vista.

LOAD_LIBRARY_SEARCH_APPLICATION_DIR
0x00000200

If this value is used, the application's installation directory is searched for the DLL and its dependencies. Directories in the standard search path are not searched. This value cannot be combined with LOAD_WITH_ALTERED_SEARCH_PATH.

Windows 7, Windows Server 2008 R2, Windows Vista, and Windows Server 2008:  This value requires KB2533623 to be installed.

Windows Server 2003 and Windows XP:  This value is not supported.

LOAD_LIBRARY_SEARCH_DEFAULT_DIRS
0x00001000

This value is a combination of LOAD_LIBRARY_SEARCH_APPLICATION_DIR, LOAD_LIBRARY_SEARCH_SYSTEM32, and LOAD_LIBRARY_SEARCH_USER_DIRS. Directories in the standard search path are not searched. This value cannot be combined with LOAD_WITH_ALTERED_SEARCH_PATH.

This value represents the recommended maximum number of directories an application should include in its DLL search path.

Windows 7, Windows Server 2008 R2, Windows Vista, and Windows Server 2008:  This value requires KB2533623 to be installed.

Windows Server 2003 and Windows XP:  This value is not supported.

LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR
0x00000100

If this value is used, the directory that contains the DLL is temporarily added to the beginning of the list of directories that are searched for the DLL's dependencies. Directories in the standard search path are not searched.

The lpFileName parameter must specify a fully qualified path. This value cannot be combined with LOAD_WITH_ALTERED_SEARCH_PATH.

For example, if Lib2.dll is a dependency of C:\Dir1\Lib1.dll, loading Lib1.dll with this value causes the system to search for Lib2.dll only in C:\Dir1. To search for Lib2.dll in C:\Dir1 and all of the directories in the DLL search path, combine this value with LOAD_LIBRARY_DEFAULT_DIRS.

Windows 7, Windows Server 2008 R2, Windows Vista, and Windows Server 2008:  This value requires KB2533623 to be installed.

Windows Server 2003 and Windows XP:  This value is not supported.

LOAD_LIBRARY_SEARCH_SYSTEM32
0x00000800

If this value is used, %windows%\system32 is searched for the DLL and its dependencies. Directories in the standard search path are not searched. This value cannot be combined with LOAD_WITH_ALTERED_SEARCH_PATH.

Windows 7, Windows Server 2008 R2, Windows Vista, and Windows Server 2008:  This value requires KB2533623 to be installed.

Windows Server 2003 and Windows XP:  This value is not supported.

LOAD_LIBRARY_SEARCH_USER_DIRS
0x00000400

If this value is used, directories added using the AddDllDirectory or the SetDllDirectory function are searched for the DLL and its dependencies. If more than one directory has been added, the order in which the directories are searched is unspecified. Directories in the standard search path are not searched. This value cannot be combined with LOAD_WITH_ALTERED_SEARCH_PATH.

Windows 7, Windows Server 2008 R2, Windows Vista, and Windows Server 2008:  This value requires KB2533623 to be installed.

Windows Server 2003 and Windows XP:  This value is not supported.

LOAD_WITH_ALTERED_SEARCH_PATH
0x00000008

If this value is used and lpFileName specifies an absolute path, the system uses the alternate file search strategy discussed in the Remarks section to find associated executable modules that the specified module causes to be loaded. If this value is used and lpFileName specifies a relative path, the behavior is undefined.

If this value is not used, or if lpFileName does not specify a path, the system uses the standard search strategy discussed in the Remarks section to find associated executable modules that the specified module causes to be loaded.

This value cannot be combined with any LOAD_LIBRARY_SEARCH flag.

 

Return value

If the function succeeds, the return value is a handle to the loaded module.

If the function fails, the return value is NULL. To get extended error information, call GetLastError.

Remarks

The LoadLibraryEx function is very similar to the LoadLibrary function. The differences consist of a set of optional behaviors that LoadLibraryEx provides:

  • LoadLibraryEx can load a DLL module without calling the DllMain function of the DLL.
  • LoadLibraryEx can load a module in a way that is optimized for the case where the module will never be executed, loading the module as if it were a data file.
  • LoadLibraryEx can find modules and their associated modules by using either of two search strategies or it can search a process-specific set of directories.

You select these optional behaviors by setting the dwFlags parameter; if dwFlags is zero, LoadLibraryEx behaves identically to LoadLibrary.

The calling process can use the handle returned by LoadLibraryEx to identify the module in calls to the GetProcAddress, FindResource, and LoadResource functions.

To enable or disable error messages displayed by the loader during DLL loads, use the SetErrorMode function.

It is not safe to call LoadLibraryEx from DllMain. For more information, see the Remarks section in DllMain.

Visual C++:  The Visual C++ compiler supports a syntax that enables you to declare thread-local variables: _declspec(thread). If you use this syntax in a DLL, you will not be able to load the DLL explicitly using LoadLibraryEx on versions of Windows prior to Windows Vista. If your DLL will be loaded explicitly, you must use the thread local storage functions instead of _declspec(thread). For an example, see Using Thread Local Storage in a Dynamic Link Library.

Loading a DLL as a Data File or Image Resource

The LOAD_LIBRARY_AS_DATAFILE, LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE, and LOAD_LIBRARY_AS_IMAGE_RESOURCE values affect the per-process reference count and the loading of the specified module. If any of these values is specified for the dwFlags parameter, the loader checks whether the module was already loaded by the process as an executable DLL. If so, this means that the module is already mapped into the virtual address space of the calling process. In this case, LoadLibraryEx returns a handle to the DLL and increments the DLL reference count. If the DLL module was not already loaded as a DLL, the system maps the module as a data or image file and not as an executable DLL. In this case, LoadLibraryEx returns a handle to the loaded data or image file but does not increment the reference count for the module and does not make the module visible to functions such as CreateToolhelp32Snapshot or EnumProcessModules.

If LoadLibraryEx is called twice for the same file with LOAD_LIBRARY_AS_DATAFILE, LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE, or LOAD_LIBRARY_AS_IMAGE_RESOURCE, two separate mappings are created for the file.

When the LOAD_LIBRARY_AS_IMAGE_RESOURCE value is used, the module is loaded as an image using portable executable (PE) section alignment expansion. Relative virtual addresses (RVA) do not have to be mapped to disk addresses, so resources can be more quickly retrieved from the module. Specifying LOAD_LIBRARY_AS_IMAGE_RESOURCE prevents other processes from modifying the module while it is loaded.

Unless an application depends on specific image mapping characteristics, the LOAD_LIBRARY_AS_IMAGE_RESOURCE value should be used with either LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE or LOAD_LIBRARY_AS_DATAFILE. This allows the loader to choose whether to load the module as an image resource or a data file, selecting whichever option enables the system to share pages more effectively. Resource functions such as FindResource can use either mapping.

To determine how a module was loaded, use one of the following macros to test the handle returned by LoadLibraryEx.


#define LDR_IS_DATAFILE(handle)      (((ULONG_PTR)(handle)) &  (ULONG_PTR)1)
#define LDR_IS_IMAGEMAPPING(handle)  (((ULONG_PTR)(handle)) & (ULONG_PTR)2)
#define LDR_IS_RESOURCE(handle)      (LDR_IS_IMAGEMAPPING(handle) || LDR_IS_DATAFILE(handle))


The following table describes these macros.

MacroDescription
LDR_IS_DATAFILE(handle)If this macro returns TRUE, the module was loaded as a data file (LOAD_LIBRARY_AS_DATAFILE or LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE).
LDR_IS_IMAGEMAPPING(handle)If this macro returns TRUE, the module was loaded as an image file (LOAD_LIBRARY_AS_IMAGE_RESOURCE).
LDR_IS_RESOURCE(handle)If this macro returns TRUE, the module was loaded as either a data file or an image file.

 

Use the FreeLibrary function to free a loaded module, whether or not loading the module caused its reference count to be incremented. If the module was loaded as a data or image file, the mapping is destroyed but the reference count is not decremented. Otherwise, the DLL reference count is decremented. Therefore, it is safe to call FreeLibrary with any handle returned by LoadLibraryEx.

Searching for DLLs and Dependencies

The search path is the set of directories that are searched for a DLL. The LoadLibraryEx function can search for a DLL using a standard search path or an altered search path, or it can use a process-specific search path established with the SetDefaultDllDirectories and AddDllDirectory functions. For a list of directories and the order in which they are searched, see Dynamic-Link Library Search Order.

The LoadLibraryEx function uses the standard search path in the following cases:

  • The file name is specified without a path and the base file name does not match the base file name of a loaded module, and none of the LOAD_LIBRARY_SEARCH flags are used.
  • A path is specified but LOAD_WITH_ALTERED_SEARCH_PATH is not used.
  • The application has not specified a default DLL search path for the process using SetDefaultDllDirectories.

If lpFileName specifies a relative path, the entire relative path is appended to every token in the DLL search path. To load a module from a relative path without searching any other path, use GetFullPathName to get a nonrelative path and call LoadLibraryEx with the nonrelative path. If the module is being loaded as a datafile and the relative path starts with ".\" or "..\", the relative path is treated as an absolute path.

If lpFileName specifies an absolute path and dwFlags is set to LOAD_WITH_ALTERED_SEARCH_PATH, LoadLibraryEx uses the altered search path. The behavior is undefined when LOAD_WITH_ALTERED_SEARCH_PATHflag is set, and lpFileName specifiies a relative path.

The SetDllDirectory function can be used to modify the search path. This solution is better than using SetCurrentDirectory or hard-coding the full path to the DLL. However, be aware that using SetDllDirectory effectively disables safe DLL search mode while the specified directory is in the search path and it is not thread safe. If possible, it is best to use AddDllDirectory to modify a default process search path. For more information, see Dynamic-Link Library Search Order.

An application can specify the directories to search for a single LoadLibraryEx call by using the LOAD_LIBRARY_SEARCH_* flags. If more than one LOAD_LIBRARY_SEARCH flag is specified, the directories are searched in the following order:

  • The directory that contains the DLL (LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR). This directory is searched only for dependencies of the DLL to be loaded.
  • The application directory (LOAD_LIBRARY_SEARCH_APPLICATION_DIR).
  • Paths explicitly added to the application search path with the AddDllDirectory function (LOAD_LIBRARY_SEARCH_USER_DIRS) or the SetDllDirectory function. If more than one path has been added, the order in which the paths are searched is unspecified.
  • The System32 directory (LOAD_LIBRARY_SEARCH_SYSTEM32).

Windows 7, Windows Server 2008 R2, Windows Vista, and Windows Server 2008:  The LOAD_LIBRARY_SEARCH_* flags are available on systems that have KB2533623 installed. To determine whether the flags are available, use GetProcAddress to get the address of the AddDllDirectory, RemoveDllDirectory, or SetDefaultDllDirectories function. If GetProcAddress succeeds, the LOAD_LIBRARY_SEARCH_* flags can be used with LoadLibraryEx.

If the application has used the SetDefaultDllDirectories function to establish a DLL search path for the process and none of the LOAD_LIBRARY_SEARCH_* flags are used, the LoadLibraryEx function uses the process DLL search path instead of the standard search path.

If a path is specified and there is a redirection file associated with the application, the LoadLibraryEx function searches for the module in the application directory. If the module exists in the application directory, LoadLibraryEx ignores the path specification and loads the module from the application directory. If the module does not exist in the application directory, the function loads the module from the specified directory. For more information, see Dynamic Link Library Redirection.

If you call LoadLibraryEx with the name of an assembly without a path specification and the assembly is listed in the system compatible manifest, the call is automatically redirected to the side-by-side assembly.

Security Remarks

LOAD_LIBRARY_AS_DATAFILE does not prevent other processes from modifying the module while it is loaded. Because this can make your application less secure, you should use LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE instead of LOAD_LIBRARY_AS_DATAFILE when loading a module as a data file, unless you specifically need to use LOAD_LIBRARY_AS_DATAFILE. Specifying LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE prevents other processes from modifying the module while it is loaded. Do not specify LOAD_LIBRARY_AS_DATAFILE and LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE in the same call.

Do not use the SearchPath function to retrieve a path to a DLL for a subsequent LoadLibraryEx call. The SearchPath function uses a different search order than LoadLibraryEx and it does not use safe process search mode unless this is explicitly enabled by calling SetSearchPathMode with BASE_SEARCH_PATH_ENABLE_SAFE_SEARCHMODE. Therefore, SearchPath is likely to first search the user’s current working directory for the specified DLL. If an attacker has copied a malicious version of a DLL into the current working directory, the path retrieved by SearchPath will point to the malicious DLL, which LoadLibraryEx will then load.

Do not make assumptions about the operating system version based on a LoadLibraryEx call that searches for a DLL. If the application is running in an environment where the DLL is legitimately not present but a malicious version of the DLL is in the search path, the malicious version of the DLL may be loaded. Instead, use the recommended techniques described in Getting the System Version.

For a general discussion of DLL security issues, see Dynamic-Link Library Security.

Examples

For an example, see Looking Up Text for Error Code Numbers.

Requirements

Minimum supported client

Windows XP [desktop apps only]

Minimum supported server

Windows Server 2003 [desktop apps only]

Header

LibLoaderAPI.h on Windows 8 (include Windows.h);
WinBase.h on Windows 7, Windows Server 2008 R2, Windows Vista, Windows Server 2008, Windows XP, and Windows Server 2003 (include Windows.h)

Library

Kernel32.lib

DLL

Kernel32.dll

Unicode and ANSI names

LoadLibraryExW (Unicode) and LoadLibraryExA (ANSI)

See also

DllMain
Dynamic-Link Library Functions
Dynamic-Link Library Search Order
Dynamic-Link Library Security
FindResource
FreeLibrary
GetProcAddress
GetSystemDirectory
GetWindowsDirectory
LoadLibrary
LoadResource
OpenFile
Run-Time Dynamic Linking
SearchPath
SetDllDirectory
SetErrorMode

 

 

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