loadlibrary加载动态连接库问题
我写了个QT程序，然后一个功能加载动态链接库，使用LoadLibrary函数，如下：

HINSTANCE hInstance = NULL;
hInstance = LoadLibrary((LPCWSTR)"CameraDll.dll");
cap = (Cap)GetProcAddress(hInstance,"CameraSubArea");
(*cap)(0);
FreeLibrary(hInstance);

但是调试的时候那个hInstance始终是空指针，最后执行出错，请问这是为什么呢？我将dll文件拷贝到debug里面的 动态链接库 loadLibrary
[解决办法]
hInstance = LoadLibrary((LPCWSTR)"CameraDll.dll");
失败了
看看errno是啥？
[解决办法]
LoadLibrary
The LoadLibrary function maps the specified executable module into the address space of the calling process.

HINSTANCE LoadLibrary(
LPCTSTR lpLibFileName // address of filename of executable module
);

Parameters
lpLibFileName
Pointer to a null-terminated string that names the executable module (either a .DLL or .EXE file). The name specified is the filename of the module and is not related to the name stored in the library module itself, as specified by the LIBRARY keyword in the module-definition (.DEF) file.
If the string specifies a path but the file does not exist in the specified directory, the function fails. When specifying a path, be sure to use backslashes (\), not forward slashes (/).

If the string does not specify a path, the function uses a standard search strategy to find the file. See the Remarks for more information.

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

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

Remarks
LoadLibrary can be used to map a DLL module and return a handle that can be used in GetProcAddress to get the address of a DLL function. LoadLibrary can also be used to map other executable modules. For example, the function can specify an .EXE file to get a handle that can be used inFindResource orLoadResource. Do not use LoadLibrary to "run" a .EXE file.

If the module is a DLL not already mapped for the calling process, the system calls the DLL's DllMain function with the DLL_PROCESS_ATTACH value. If the DLL's entry-point function does not return TRUE, LoadLibrary fails and returns NULL.

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

Module handles are not global or inheritable. A call to LoadLibrary by one process does not produce a handle that another process can use — for example, in calling GetProcAddress. The other process must make its own call to LoadLibrary for the module before calling GetProcAddress.

If no filename extension is specified in the lpLibFileName parameter, the default library extension .DLL is appended. However, the filename string can include a trailing point character (.) to indicate that the module name has no extension. When no path is specified, the function searches for loaded modules whose base name matches the base name of the module to be loaded. If the name matches, the load succeeds. Otherwise, the function searches for the file in the following sequence:

The directory from which the application loaded.

The current directory.
Windows 95 and Windows 98: The Windows system directory. Use theGetSystemDirectory function to get the path of this directory.
Windows NT: The 32-bit Windows system directory. Use the GetSystemDirectory function to get the path of this directory. The name of this directory is SYSTEM32.

Windows NT: The 16-bit Windows system directory. There is no function that obtains the path of this directory, but it is searched. The name of this directory is SYSTEM.
The Windows directory. Use theGetWindowsDirectory function to get the path of this directory.
The directories that are listed in the PATH environment variable.
The first directory searched is the one directory containing the image file used to create the calling process (for more information, see the CreateProcess function). Doing this allows private dynamic-link library (DLL) files associated with a process to be found without adding the process's installed directory to the PATH environment variable.

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 LoadLibrary or LoadLibraryEx. If your DLL will be loaded explicitly, you must use the thread local storage functions instead of _declspec(thread).

Windows 95: If you are using LoadLibrary to load a module that contains a resource whose numeric identifier is greater than 0x7FFF, LoadLibrary fails.

Windows CE: Two different modules cannot have the same filename, given that the extensions are different. These effectively have the same "module" name. For example, if LoadLibrary is made on "Sample.cpl", the operating system will not load Sample.cpl, but instead will again load Sample.dll. A similar limitation exists for modules with the same name but residing in different directories. For example, if LoadLibrary is called on "\\Windows\Sample.dll", and then LoadLibrary is called on "\\MyDir\Sample.dll", "\\Windows\Sample.dll" will simply be reloaded.

A search path to the executable module cannot be specified. Unless the full path to the module is given, Windows CE will search the following path for the module:

The root directory of the PC Card RAM expansion card, if one exists.
The windows directory (\Windows).
The root directory (\).
QuickInfo
Windows NT: Requires version 3.1 or later.
Windows: Requires Windows 95 or later.
Windows CE: Requires version 1.0 or later.
Header: Declared in winbase.h.
Import Library: Use kernel32.lib.
Unicode: Implemented as Unicode and ANSI versions on Windows NT.

See Also
Dynamic-Link Libraries Overview, Dynamic-Link Library Functions, DllMain,FindResource, FreeLibrary, GetProcAddress,GetSystemDirectory,GetWindowsDirectory, LoadLibraryEx,LoadResource

GetLastError
The GetLastError function returns the calling thread's last-error code value. The last-error code is maintained on a per-thread basis. Multiple threads do not overwrite each other's last-error code.

DWORD GetLastError(VOID)

Parameters
This function has no parameters.

Return Values
The return value is the calling thread's last-error code value. Functions set this value by calling the SetLastError function. The Return Value section of each reference page notes the conditions under which the function sets the last-error code.

Windows 95 and Windows 98: Because SetLastError is a 32-bit function only, Win32 functions that are actually implemented in 16-bit code do not set the last-error code. You should ignore the last-error code when you call these functions. They include window management functions, GDI functions, and Multimedia functions.

Remarks
You should call the GetLastError function immediately when a function's return value indicates that such a call will return useful data. That is because some functions call SetLastError(0) when they succeed, wiping out the error code set by the most recently failed function.

Most functions in the Win32 API that set the thread's last error code value set it when they fail; a few functions set it when they succeed. Function failure is typically indicated by a return value error code such as FALSE, NULL, 0xFFFFFFFF, or 1. Some functions call SetLastError under conditions of success; those cases are noted in each function's reference page.

Error codes are 32-bit values (bit 31 is the most significant bit). Bit 29 is reserved for application-defined error codes; no system error code has this bit set. If you are defining an error code for your application, set this bit to one. That indicates that the error code has been defined by an application, and ensures that your error code does not conflict with any error codes defined by the system.

To obtain an error string for system error codes, use the FormatMessage function. For a complete list of error codes, see Error Codes or the SDK header file WINERROR.H.

QuickInfo
Windows NT: Requires version 3.1 or later.
Windows: Requires Windows 95 or later.
Windows CE: Requires version 1.0 or later.
Header: Declared in winbase.h.
Import Library: Use kernel32.lib.

See Also
Errors Overview, Error Functions, FormatMessage, SetLastError, SetLastErrorEx