INFO: hInstance Differences Under Win 3.1 and Win32 Platforms
ID: Q103644
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The information in this article applies to:
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Microsoft Win32 Software Development Kit (SDK)
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Microsoft Windows 2000
SUMMARY
In 16-bit Windows (Win16) applications running on Windows 3.1, an instance
handle can be used to uniquely identify the instance of an application
because instance handles are unique within the scope of an address space.
Because each instance of a 32-bit Windows (Win32) application runs in its
own address space, instance handles cannot be used to uniquely identify an
instance of an application running on the system. This article explains
why, and includes some alternative calls that might assist in uniquely
identifying an Win32 application instance.
MORE INFORMATION
Although the concepts for an instance handle are similar between Win32 and
Win16, the results you see regarding them might be very different from what
you expect.
With Win16, when you start several instances of the same application, they
all share the same address space. You have multiple instances of the same
code segment. However, each of these instances has a unique data segment
associated with it. Using an instance handle (hInstance) is a way to
uniquely identify these different instances and data segments in the
address space.
Instance handles are unique to the address space. On Windows NT, when
looking at the value of the instance handle, or the value returned from
GetWindowLong(hWnd, GWL_HINSTANCE), a developer coming from a Win16
background might be surprised to see that most of the windows on the
desktop return the same value. This is because the return value is the
hInstance for the instance of the application, which is running it its own
address space. (An interesting side note: The hInstance value is the base
address where the application's module was able to load: either the default
address or the fixed up address.)
In Win32, running several instances of the same application causes the
instances to start and run in their own separate address space. To
emphasize the difference: multiple instances of the same application on
Windows 3.1 run in the same address space; in Win32 applications running on
Windows NT or Windows 9x, each instance has its own, separate address
space. Using an instance handle to uniquely identify an application
instance, as is possible in Win16, does not apply in Win32. (Another
interesting side note: Remember that even if there are multiple instances
of an application, if they are able to load at their default virtual
address spaces, the virtual address pages of the different applications'
executable code will map to the same physical memory pages.)
In Win32, instance handles are not unique in the global scope of the
system; however, window handles, thread IDs, and process IDs are. Here are
some calls that may assist in alternative methods to uniquely identify
instance of applications on Windows NT:
- GetWindowThreadProcessID() retrieves the identifier of the thread
that created the given window and, optionally, the identifier of
the process that created the window.
- OpenProcess() returns a handle to a process specified by a process
ID.
- GetCurrentProcessID() returns the calling process's ID.
- EnumThreadWindows() returns all of the windows associated with a
thread.
- The FindWindow() function retrieves the handle of the top-level
window specified by class name and window name.
- Windows NT only: To enumerate all of the processes on the system,
you can query the Registry using RegQueryValueEx() with key
HKEY_PERFORMANCE_DATA, and the Registry database index associated with
the database string "Process".
- Windows NT 4.0 only: To enumerate all of the processes on the system,
you can use the EnumProcesses PSAPI function. See the Platform SDK
documentation for more information on EnumProcesses
- Windows 95 and later, Windows 2000 and later: To enumerate all of
the processes on the system, you can use the 32-bit Toolhelp APIs.
See the Platform SDK documentation on the following APIs:
CreateToolhelp32Snapshot, Process32First, and Process32Next
For further details on using these calls, please see the Platform SDK
documentation.
Additional query words:
Keywords : kbNTOS kbGrpUser kbWinOS
Version : WINDOWS:
Platform : WINDOWS
Issue type : kbinfo
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