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1========================================================================================================================
2Methods for Debugging DLLs
3========================================================================================================================
4If you have the source for both the DLL and the calling program, open the project for the calling executable file and
5debug the DLL from there. If you load a DLL dynamically, you must specify it in the Additional DLLs category of the
6Debug tab in the Project Settings dialog box.
7
8If you have the source for the DLL only, open the project that builds the DLL. Use the Debug tab in the Project
9Settings dialog box to specify the executable file that calls the DLL.
10
11You can also debug a DLL without a project. For example, maybe you just picked up a DLL and source code but you
12don’t have an associated project or workspace. You can use the Open command on the File menu to select the .DLL
13file you want to debug. The debug information should be in either the .DLL or the related .PDB file. After
14Visual C++ opens the file, on the Build menu click Start Debug and Go to begin debugging.
15
16To debug a DLL using the project for the executable file
17
18From the Project menu, click Settings.
19The Project Settings dialog box appears.
20
21Choose the Debug tab.
22
23
24In the Category drop-down list box, select General.
25
26
27In the Program Arguments text box, type any command-line arguments required by the executable file.
28
29
30In the Category drop-down list box, select Additional DLLs.
31
32
33In the Local Name column, type the names of DLLs to debug.
34If you are debugging remotely, the Remote Name column appears. In this column, type the complete path for the
35remote module to map to the local module name.
36
37In the Preload column, select the check box if you want to load the module before debugging begins.
38
39
40Click OK to store the information in your project.
41
42
43From the Build menu, click Start Debug and Go to start the debugger.
44You can set breakpoints in the DLL or the calling program. You can open a source file for the DLL and set breakpoints
45in that file, even though it is not a part of the executable file’s project.
46
47To debug a DLL using the project for the DLL
48
49From the Project menu, click Settings.
50The Project Settings dialog box appears.
51
52Choose the Debug tab.
53
54
55In the Category drop-down list box, select General.
56
57
58In the Executable For Debug Session text box, type the name of the executable file that calls the DLL.
59
60
61In the Category list box, select Additional DLLs.
62
63
64In the Local Module Name column, type the name of the DLLs you want to debug.
65
66
67Click OK to store the information in your project.
68
69
70Set breakpoints as required in your DLL source files or on function symbols in the DLL.
71
72
73From the Build menu, click Start Debug and Go to start the debugger.
74To debug a DLL created with an external project
75
76From the Project menu, click Settings.
77The Project Settings dialog box appears.
78
79Choose the Debug tab.
80
81
82In the Category drop-down list box, select General.
83
84
85In the Executable For Debug Session text box, type the name of the DLL that your external makefile builds.
86
87
88Click OK to store the information in your project.
89
90
91Build a debug version of the DLL with symbolic debugging information, if you don’t already have one.
92
93
94Follow one of the two procedures immediately preceding this one to debug the DLL.
95
96========================================================================================================================
97Why Don’t My DLL Breakpoints Work?
98========================================================================================================================
99Some reasons why your breakpoints don’t work as expected are listed here, along with solutions or work-arounds for each.
100If you follow the instructions in one topic and are still having breakpoint problems, look at some of the other topics.
101Often breakpoint problems result from a combination of conditions.
102
103You can't set a breakpoint in a source file when the corresponding symbolic information isn't loaded into memory by
104the debugger.
105You cannot set a breakpoint in any source file when the corresponding symbolic information will not be loaded into memory
106by the debugger.
107Symptoms include messages such as "the breakpoint cannot be set" or a simple, noninformational beep.
108
109When setting breakpoints before the code to be debugged has been started, the debugger uses a breakpoint list to keep
110track of how and where to set breakpoints. When you actually begin the debugging session, the debugger loads the symbolic
111information for all the code to be debugged and then walks through its breakpoint list, attempting to set the
112breakpoints.
113
114However, if one or more of the code modules have not been designated to the debugger, there will be no symbolic
115information for the debugger to use when walking through its breakpoint list. Situations where this is likely to
116occur include:
117
118Attempts to set breakpoints in a DLL before the call to LoadLibrary.
119
120Setting a breakpoint in an ActiveX server before the container has started the server.
121
122Other similar cases.
123
124To prevent this behavior in Visual C++, specify all additional DLLs and COM servers in the Additional DLLs field
125in the Debug/Options dialog box to notify the debugger that you want it to load symbolic debug information for
126additional .DLL files. When this has been done, breakpoints set in code that has not yet been loaded into memory
127will be "virtual" breakpoints. When the code is actually loaded into memory by the loader, these become physical
128breakpoints. Make sure that these additional debugging processes are not already running when you start your
129debugging session. The debugging process and these additional processes must be sychronized at the same beginning
130point to work correctly, hitting all breakpoints.
131
132Breakpoints are missed when more than one copy of a DLL is on your hard disk.
133Having more than one copy of a DLL on your hard drive, especially if it is in your Windows directory, can cause
134debugger confusion. The debugger will load the symbolic information for the DLL specified to it at run time (with the
135Additional DLLs field in the Debug/Options dialog box), while Windows has actually loaded a different copy of the
136DLL itself into memory. Because there is no way to force the debugger to load a specific DLL, it is a good idea to
137keep only one version of a DLL at a time in your path, current directory, and Windows directory.
138
139You can’t set "Break When Expression Has Changed" breakpoints on a variable local to a DLL.
140Setting a "Break When Expression Has Changed" breakpoint on a variable local to a DLL function before the call
141to LoadLibrary causes the breakpoint to be virtual (there are no physical addresses for the DLL in memory yet).
142Virtual breakpoints involving expressions pose a special problem. The DLL must be specified to the debugger at
143startup (causing its symbolic information to be loaded). In addition, the DLL's executable code must also be loaded
144into memory before this kind of breakpoint can be set. This means that the calling application's code must be
145executed to the point after its call to LoadLibrary before the debugger will allow this type of breakpoint to be set.