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MARSHAL - Custom Interface Standard Marshaling |
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The MARSHAL sample anticipates the interaction of a client and out-of-process server in the LOCCLIEN and LOCSERVE samples. For a client like LOCCLIEN to use the COM objects in an out-of-process server like LOCSERVE requires marshaling the car-related interfaces used in the previous COM samples of this series. The MARSHAL sample creates a proxy/stub DLL that provides standard marshaling for the ICar, IUtility, and ICruise custom interfaces.
The Microsoft® Interface Definition Language (MIDL) compiler is used to compile the interface specifications (expressed using the MIDL language in the MICARS.IDL file). MIDL.EXE is a utility provided as part of the Microsoft Platform SDK. The MIDL compilation of MICARS.IDL generates additional source files: MICARS.H, MICARS_I.C, MICARS_P.C, and DLLDATA.C.
In the series of COM Tutorial code samples, MARSHAL works with the LOCSERVE and LOCCLIEN samples to illustrate a COM client using the interfaces on COM objects in an out-of-process local server. In this sample series, only standard marshaling is used for the custom ICar, IUtility, and ICruise interfaces. Later samples in the series which use these interfaces across apartment, process, or machine boundaries (for example, APTSERVE, APTCLIEN, and REMCLIEN) also rely on MARSHAL.DLL for standard marshaling of these interfaces.
The MARSHAL sample demonstrates a simple and efficient way to create a marshaling DLL for custom interfaces. Default definitions are generated for the DllMain, DllGetClassObject, DllCanUnloadNow, GetProxyDllInfo, DllRegisterServer, and DllUnregisterServer functions. With this technique, all that is needed is a makefile, a .DEF file, and an input .IDL file specifying the interfaces. However, there may be occasions when you want more control over the content of the marshaling DLL. For example, you may want to perform some action within DllMain during DLL_PROCESS_ATTACH, you may want to code explicit control over the registration and unregistration of the marshaling server, or you may want to add standard module version information to the DLL resources. For coverage of these areas, see the subsequent MARSHAL2 sample. MARSHAL2 produces a marshaling DLL that marshals the same custom interfaces as MARSHAL. When registered, MARSHAL2.DLL is functionally equivalent to MARSHAL.DLL and replaces it. The makefile for the MARSHAL2.DLL does not automatically register the server to achieve this replacement. You must enable this in MARSHAL2's makefile. For the MARSHAL2 lesson, see MARSHAL2.HTM.
For functional descriptions and a tutorial code tour of MARSHAL, see the Code Tour section in MARSHAL.HTM. For details on setting up the programmatic usage of MARSHAL.DLL, see the Usage section in MARSHAL.HTM. To read MARSHAL.HTM, run TUTORIAL.EXE in the main tutorial directory and click the MARSHAL lesson in the table of lessons. You can also do the same thing by double-clicking the MARSHAL.HTM file after locating the main tutorial directory in Windows Explorer. For more details on the LOCCLIEN client and the LOCSERVE server and how MARSHAL.DLL supports their operation, see LOCSERVE.HTM and LOCCLIEN.HTM in the main tutorial directory. Because those code samples rely on MARSHAL.DLL, you must build MARSHAL DLL before building or running LOCCLIEN and LOCSERVE. MARSHAL's makefile automatically registers MARSHAL's proxy and stub interface marshalers in the system registry. This registration must be done before these interfaces can be used by COM clients or servers in LOCSERVE, LOCCLIEN, and later samples of the series.
MARSHAL's self-registration is done using the REGISTER.EXE utility built in the REGISTER sample. To build or run MARSHAL, you must build the REGISTER code sample first.
For details on setting up your system to build and test the code samples in this COM Tutorial series, see TUTORIAL.HTM. The supplied MAKEFILE is Microsoft NMAKE-compatible. To create a debug build, issue the NMAKE command in the Command Prompt window.
MARSHAL.DLL is built solely as a marshaling DLL for the specified interfaces. Although it can be implicitly loaded by linking to its associated .LIB file, it is normally used on behalf of a COM client that is using the interfaces across apartment, process, or machine boundaries. In these cases COM automatically loads this DLL as needed. Before COM can find and load the MARSHAL DLL to support marshaling of its supported interfaces, the MARSHAL server must be registered in the registry as the marshaling server for those interfaces. MARSHAL is a self-registering in-process server.
The makefile that builds this sample automatically registers the server in the registry. You can manually initiate its self-registration by issuing the following command at the command prompt in the MARSHAL directory:
nmake register
This assumes that you have a compilation environment set up. If not, you can also directly invoke the REGISTER.EXE command at the command prompt while in the MARSHAL directory.
..\register\register.exe marshal.dll
These registration commands require a prior build of the REGISTER sample in this series, as well as a prior build of MARSHAL.DLL.
In this series, the makefiles use the REGISTER.EXE utility from the REGISTER sample. Recent releases of the Microsoft® Platform SDK and Visual C++® include a utility, REGSVR32.EXE, which can be used in a similar fashion to register in-process servers and marshaling DLLs.
Files Description
MARSHAL.TXT Short description of the sample.
MAKEFILE The generic makefile for building the MARSHAL.DLL
code sample.
MARSHAL.DEF The module definition file. Exports server housing
functions.
MICARS.IDL The MIDL interface specifications for ICar, IUtility,
and ICruise.
MICARS.H Generated by compiling MICARS.IDL. The interface
include
file for the specified interfaces.
MICARS_I.C Generated by compiling MICARS.IDL. The data definitions
of the GUIDs for the marshaled interfaces.
MICARS_P.C Generated by compiling MICARS.IDL. The actual proxy and
stub functions for the interface methods.
DLLDATA.C Generated by compiling MICARS.IDL. DLL data routines
for
the proxies and default definitions for the DllMain,
DllRegisterServer, and DllUnregisterServer functions.
This code sample introduces standard marshaling of custom COM interfaces. Before custom interfaces in a COM server can be used at run-time by a client across apartment, process, or machine boundaries, they must be marshaled. Method calls must be translated from the client's address space to that of the server, and arguments must be packaged in the client and converted in the server's environment. The return value from the method call must also be packaged in the server and converted in the client for use in the client's environment. A proxy on the client side works with a stub on the server side to achieve this packaging and unpackaging.
This sample builds a proxy/stub marshaling DLL to serve three interfaces that will be used in subsequent samples in this series. The LOCCLIEN and LOCSERVE samples will be the first that require this marshaling. The two EXE applications built in those samples, LOCSERVE.EXE and LOCCLIEN.EXE, are a client and out-of-process local server that manipulate the car-related components introduced in earlier lessons. The MARSHAL.DLL server built in this sample provies standard marshaling support for the ICar, IUtility, and ICruise custom interfaces.
MARSHAL.DLL provides this support by implementing interface proxy and stub functions for each method of each interface. The internal details of these proxy/stub pairs, while worthy of study, are beyond the scope of this lesson. Standard marshaling largely hides the details, so you can take a black-box approach. For more information, see the COM documentation in the Platform SDK.
MARSHAL.DLL registers itself as the provider of standard marshaling for the interfaces specified in MICARS.IDL. Each interface is registered using to the GUID specified for it (for example, IID_ICar for the ICar interface). The makefile for MARSHAL invokes the REGISTER.EXE utility to register the interfaces. Later, when a client calls COM to create an instance of a component, the call specifies an execution context (for example, CLSCTX_LOCAL_SERVER). This context indicates whether marshaling is required for the interfaces on the created component, and if so, whether the out-of-process server is on the local machine. COM thus loads MARSHAL.DLL as needed, locating it by consulting the registry entries for the marshaled interfaces.
The MARSHAL.DLL developed in this sample is very similar to the in-process COM servers produced in previous samples. It is a COM server that offers standard marshaling services for the interfaces it is registered to handle. MARSHAL.DLL is produced by compiling some source files that are automatically generated by the MIDL compiler when it compiles MICARS.IDL, which specifies the custom interfaces.
The sequence of build operations in the makefile is different from that seen in such previous COM server code samples, such as the DLLSERVE in-process server. This is because most of the source code for MARSHAL.DLL is automatically generated by the MIDL compiler. The dependencies in the makefile guarantee that these generated source files (in this case, MICARS.H, MICARS_I.C, MICARS_P.C, and DLLDATA.C) are created before any attempt is made to compile and link them. Here is the MIDL compilation from the makefile:
# Generate the proxy/stub source from the .IDL file.
micars.h micars_p.c micars_i.c dlldata.c: micars.idl
midl /ms_ext /app_config /c_ext micars.idl
The generated source files all depend on MICARS.IDL. If any of them are absent, all will be regenerated by MIDL. If MICARS.IDL has a more recent time stamp than the C files, all the files will be regenerated.
MICARS.IDL is thus the principal source for this entire executable. The following are the interface definitions from MICARS.IDL:
[uuid(0002da00-0000-0000-c000-000000000046),
object
]
interface ICar : IUnknown
{
import "unknwn.idl";
HRESULT Shift([in] short nGear);
HRESULT Clutch([in] short nEngaged);
HRESULT Speed([in] short nMph);
HRESULT Steer([in] short nAngle);
}
[uuid(0002da01-0000-0000-c000-000000000046),
object
]
interface IUtility : IUnknown
{
import "unknwn.idl";
HRESULT Offroad([in] short nGear);
HRESULT Winch([in] short nRpm);
}
[uuid(0002da02-0000-0000-c000-000000000046),
object
]
interface ICruise : IUnknown
{
import "unknwn.idl";
HRESULT Engage([in] BOOL bOnOff);
HRESULT Adjust([in] BOOL bUpDown);
}
MIDL's specialized interface syntax is similar to that of C++. Because each interface is derived from IUnknown, a common .IDL file specifying it (UNKNWN.IDL, provided as part of the Platform SDK) is imported. The [in] directional attributes direct how the method arguments are marshaled. Other directional attributes in MIDL are [out] and [in,out]. Complete details on MIDL are beyond the scope of this lesson's introduction to standard marshaling. For more information on MIDL, see the Platform SDK.
Note that in previous code samples in this series, where marshaling was not involved, car-related interfaces like ICar could have used the int parameter type without problems. In this and the following lessons that involve marshaling, these parameter types are short (as in the ICar Shift method above). In the past this use of short paramters in marshaled interfaces was necessary when compiling 16-bit applications. Consistent with this, earlier versions of MIDL would generate compiler errors and not produce marshaling support for parameter types such as int whose final machine representation may be hardware-dependent. However, for Win32 applications, this restriction is no longer enforced by MIDL and the int parameter type can be used in interfaces that require marshaling.
MICARS.H is generated by the MIDL compiler when it compiles MICARS.IDL. (MICARS.H looks very similar to the ICARS.H interface declaration file from previous samples in this series.) MICARS.H is used as an include file when compiling the clients and servers that require the interface marshaling it supports. For example, LOCCLIEN.CPP includes this file for its declarations of the ICar, IUtility, and ICruise interfaces. Because the later samples like LOCSERVE and LOCCLIEN rely on MARSHAL.DLL, the MARSHAL makefile copies the generated MICARS.H file to the sibling \INC directory.
The C files (MICARS_I.C, MICARS_P.C, and DLLDATA.C) created by the MIDL compiler are later compiled and linked to produce MARSHAL.DLL. Here are the rules for these compilations from the makefile.
# Compilation/Dependency rules for the .DLL source files.
#
$(TDIR)\micars_i.obj: micars.h
$(cc) $(cvarsdll) $(cflags) $(CDBG) -Fo$@ micars_i.c
$(TDIR)\micars_p.obj: micars.h
$(cc) $(cvarsdll) $(cflags) $(CDBG) -Fo$@ micars_p.c
# Note the compiler switch that defines REGISTER_PROXY_DLL.
# This turns on conditional compilaiton of default definitions for
the
# DllMain, DllRegisterServer, and DllUnregisterServer functions.
This
# means that these functions do not need to be explicitly defined
# in a separate source module.
$(TDIR)\dlldata.obj: micars.h
$(cc) $(cvarsdll) $(cflags) $(CDBG) -DREGISTER_PROXY_DLL -Fo$@
dlldata.c
The automatically generated C files are the only files that need be compiled. DLLDATA.C has some special features that greatly ease the task of producing the DLL. These special features are controlled in the rpcproxy.h file that is included by DLLDATA.C in the the following line:
#include <rpcproxy.h>
RPCPROXY.H supports some important conditional compilation that is controlled by defining the appropriate macros prior to compilation. The most important of these is the REGISTER_PROXY_DLL macro. By defining this, you can enable default definitions for the DllMain, DllRegisterServer, and DllUnregisterServer functions. By relying on these default function definitions you don't need to explicitly define them as you normally would for an in-process COM server.
Since MICARS.IDL specified the interfaces completely, enough information is available when DLLDATA.C is compiled to automatically generate the code within DllRegisterServer and DllUnregisterServer to register and unregister the interfaces. In contrast to the registration process in previous server samples, it is interfaces, rather than components, that are now being registered and unregistered. From the standpoint of this sample, you can regard this process as a black box. By enabling the default functions, MARSHAL.DLL will contain appropriate code to fully register and unregister the interfaces. MARSHAL.DLL will also have a default DllMain definition. For more details on writing explicit functions for DllMain, DllRegisterServer, and DllUnregisterServer, see the MARSHAL2 sample.
Whether or not you define the REGISTER_PROXY_DLL macro, the MIDL-generated C files will have default definitions for the other required functions: DllGetClassObject, DllCanUnloadNow, and GetProxyDllInfo. The GetProxyDllInfo function is used internally by COM. The DllGetClassObject and DllCanUnloadNow functions should be familiar as standard fixtures for COM in-process servers. Thus, MARSHAL.DLL is actually an in-process server with a class factory as well as special COM components that support the operation of standard marshaling for those interfaces originally specified in MICARS.IDL. These default functions are defined for you but you must still export them from the DLL during the final link of the object modules. The following is the final link of MARSHAL.DLL from the makefile:
# Link the object and resource binaries into the target DLL
binary.
# Build the import LIB file so apps can link to and use this DLL.
$(DLL).dll: $(DLLOBJS)
$(LINK) @<<
$(LINKFLAGS) $(dlllflags)
-out:$@
-base:0x1C000000
-def:$*.def
-implib:$*.lib
-map:$(TDIR)\$*.map
$(DLLOBJS)
$(olelibsdll) $(APPLIBS)
<<
The Linker's -def:$*.def command line switch is used above. This reduces to -def:MARSHAL.DEF when the makefile is processed. Thus, the link relies on MARSHAL.DEF for the necessary export specifications. Here is MARSHAL.DEF:
LIBRARY MARSHAL
DESCRIPTION 'MARSHAL: COM Tutorial Sample. Copyright Microsoft
Corp., 1997'
EXPORTS
DllGetClassObject PRIVATE
DllCanUnloadNow PRIVATE
DllRegisterServer PRIVATE
DllUnregisterServer PRIVATE
GetProxyDllInfo PRIVATE
The requied exports for the default functions in this server DLL are marked for internal, private use only by COM.
After you build (and thus register) this code sample, you can view the registered entries in the Registry Editor (REGEDT32.EXE for Windows NT®, REGEDIT.EXE for Windows® 95). Its entries for the ICar interface look like this.
HKEY_CLASSES_ROOT
\CLSID
\{0002DA00-0000-0000-C000-000000000046} =
"PSFactoryBuffer"
\InprocServer32 =
"D:\TUTSAMP\MARSHAL\MARSHAL.DLL"
ThreadingModel = "Both"
HKEY_CLASSES_ROOT
\Interface
\{0002DA00-0000-0000-C000-000000000046} = "ICar"
\NumMethods = "7"
\ProxyStubClsid32 =
"{0002DA00-0000-0000-C000-000000000046}"
Under HKEY_CLASSES_ROOT\CLSID, this marshaling DLL appears with the in-process COM server entry, InprocServer32. COM treats this standard marshaling server as an in-process server. Note that this server is registered to support both the Apartment and Free threading models. For more information on the significance of this designation see the FRESERVE sample.
The default registration code uses the GUID of the first interface encountered (for example, IID_ICar) as the CLSID for registering the entire proxy/stub server. COM later uses this CLSID to locate and load MARSHAL.DLL for the marshaling of any of the interfaces it is registered to handle (that is, ICar, IUtility, ICruise). When an application makes an interface method call that crosses apartment, process, or machine boundaries, COM uses the interface's IID (that is, a GUID) registry entry to locate the CLSID registry entry for the MARSHAL.DLL server. It then uses this CLSID to load the server (if it isn't already loaded) so the interface call can then be marshaled using the appropriate proxy/stub pair in the server.
In the preceding registry entries the CLSID used for the server is the GUID of the first interface encountered (in this case, IID_ICar). This double use of the GUID is permitted because the interface IID will not be confused with the server's CLSID--the same GUID can be safely used for completely different things. The default behavior of using the IID of the first interface encountered as the CLSID of the proxy/stub server is adequate for most programming.
However, there are situations that may warrant explicit control over the default DLLDATA.C code that is generated. For example, instead of relying on the default use of the first interface IID as the marshaling server's CLSID, you can explicitly specify a CLSID. When REGISTER_PROXY_DLL is defined, RPCPROXY.H provides for additional conditional compilation control with the following macros: PROXY_CLSID=<clsid symbol>, PROXY_CLSID_IS=<explicit value of guid>, and ENTRY_PREFIX=<function name prefix>.
The PROXY_CLSID= macro explicitly specifies a CLSID for the marshaling server. The following is an example of how this macro is defined in a linker command-line switch:
-DREGISTER_PROXY_DLL -DPROXY_CLSID=CLSID_CarMarshal
This macro directly uses the CLSID definition symbol that you may have already defined elsewhere in an include file using the DEFINE_GUID macro. It assumes that you have declared and instantiated CLSID_CarMarshal elsewhere in this DLL. For more information, see the discussion of INITGUID.H in DLLSERVE.HTM.
The PROXY_CLSID_IS macro can be used instead of PROXY_CLSID to define the explicit value of the CLSID in the standard binary HEX form of a GUID. The following example illustrates this:
-DPROXY_CLSID_IS={0x0002da03,0x0000,0x0000,{0xc0,0x00,0x00,0x00,0x00
,0x00,0x00,0x46}}
The ENTRY_PREFIX macro is used to generate the marshaling server code in a way that combines the proxies with other COM objects in the same in-process server. In many cases the interfaces that are marshaled in the marshaling server are implemented in COM objects that are housed in different servers. A separate marshaling server is appropriate for these situations. However, there are cases when the interfaces exposed by the COM objects are uniquely associated with those objects. In these situations it can be efficient to combine the marshaling server with the in-process server that houses the application COM objects. Thus, a COM server--such as the previous DLLSERVE sample server--that would normally house application COM objects can also be used as the server housing for the marshaling proxy. The ENTRY_PREFIX provides a convenient way to create this combined server.
By specifying the ENTRY_PREFIX macro, each of the normal DLL entry points supplied in DLLDATA.C (via RPCPROXY.H) is renamed by adding the specified entry prefix. Thus, the DllGetClassObject generated for the proxy becomes <prefix>DllGetClassObject, and so on. The enveloping COM server would implement and expose the normal DllGetClassObject function. The main functions should call their renamed counterpart functions (that is, <prefix>DllMain, <prefix>DllGetClassObject, <prefix>DllCanUnloadNow, <prefix>DllRegisterServer, and <prefix>DllUnregisterServer) at the end of the server's main implementation of these functions.
For example, if ENTRY_PREFIX=Prxy during the compile of DLLDATA.C, then the server's main implementation of DllGetClassObject would look like the following (based on code in DLLSERVE.CPP):
STDAPI DllGetClassObject(
REFCLSID rclsid,
REFIID riid,
PPVOID ppv)
{
HRESULT hr = CLASS_E_CLASSNOTAVAILABLE;
IUnknown* pCob = NULL;
BOOL bProxyObject = TRUE;
if (CLSID_DllCar == rclsid)
{
hr = E_OUTOFMEMORY;
pCob = new CFCar(NULL, g_pServer);
bProxyObject = FALSE;
}
else if (CLSID_DllUtilityCar == rclsid)
{
hr = E_OUTOFMEMORY;
pCob = new CFUtilityCar(NULL, g_pServer);
bProxyObject = FALSE;
}
if (NULL != pCob)
{
g_pServer->ObjectsUp();
hr = pCob->QueryInterface(riid, ppv);
if (FAILED(hr))
{
g_pServer->ObjectsDown();
DELETE_POINTER(pCob);
}
}
if (bProxyObject)
hr = PrxyDllGetClassObject(rclsid, riid, ppv);
return hr;
}
You do not need to code any explicit linkage of the proxy object lifetimes to the combined server lifetime. The proxy objects will maintain an independent object count. Either object count will cause the DLL to stay loaded. When the server's object count reaches zero, DllCanUnloadNow should chain to PrxyDllCanUnloadNow in a manner similar to that shown above for DllGetClassObject.
There are other special macros supported in RPCPROXY.H. See the comments at the front of that file for more details. RPCPROXY.H is located in the \MSSDK\INCLUDE directory of the installed Microsoft Platform SDK.
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