Description of the .PDB files and of the .DBG files

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The .PDB extension stands for "program database." It holds the new format for storing debugging information that was introduced in Visual C++ version 1.0. In the future, the .PDB file will also hold other project state information. One of the most important motivations for the change in format was to allow incremental linking of debug versions of programs, a change first introduced in Visual C++ version 2.0.

The .DBG extension stands for "debug." The .DBG files created with the 32-bit NT toolset are in the Portable Executable (PE) file format. They contain sections with COFF, FPO, and in some cases Codeview information. The Visual C++ integrated debugger can read .DBG files in this format, however it ignores the COFF symbol sections and looks for Codeview information.

If you need to determine what symbol information is contained in a .DBG file, you can type the following at the command prompt:

Dumpbin sample.dbg/symbol.
Note The path might need to include directories for Dumpbin.exe and MSdis100.dll:

Path=%Path%;C:\Program Files\DevStudio\VC\bin;C:\Msssdk\bin
For more information about DUMPBIN, click the following article number to view the article in the Microsoft Knowledge Base:

177429 Examples of DUMPBIN output

More Information

The .PDB files

While earlier, 16-bit versions of Visual C++ used .PDB files, the debugging information stored in them was appended to the end of the .EXE or .DLL file by the linker. In the versions of Visual C++ mentioned above, both the linker and the integrated debugger were modified to allow .PDB files to be used directly during the debugging process, thereby eliminating substantial amounts of work for the linker and also bypassing the cumbersome CVPACK limit of 64K types.

For more information about CVPACK limitations, click the following article number to view the article in the Microsoft Knowledge Base:

112335 BUG: CK1020 or CK4009 encountered when type info exceeds 64K

By default, when you build projects generated by the Visual Workbench, the compiler switch /Fd is used to rename the .PDB file to <project>.PDB. Therefore, you will have only one .PDB file for the entire project.

When you run makefiles that were not generated by the Visual Workbench, and the /Fd is not used with /Zi, you will end up with two .PDB files:
  • VCx0.PDB (where "x" refers to the major version of the corresponding Visual C++, either "2" or "4"), which stores all debugging information for the individual .OBJ files. It resides in the directory where the project makefile resides.
  • <project>.PDB, which stores all debugging information for the resulting .EXE file. It resides in the \WINDEBUG subdirectory.
Why two files? When the compiler is run, it doesn't know the name of the .EXE file into which the .OBJ files will be linked, so the compiler can't put the information into <project>.PDB. The two files store different information. Each time you compile an .OBJ file, the compiler merges the debugging information into VCX0.PDB. It does not put in symbol information such as function definitions. It only puts in information concerning types. One benefit of this is that when every source file includes common header files such as <windows.h>, all the typedefs from these headers are only stored once, rather than in every .OBJ file.

When you run the linker, it creates <project>.PDB, which holds the debugging information for the project's .EXE file. All debugging information, including function prototypes and everything else, is placed into <project>.PDB, not just the type information found in VCX0.PDB. The two kinds of .PDB files share the same extension because they are architecturally similar; they both allow incremental updates. Nevertheless, they actually store different information.

The new Visual C++ debugger uses the <project>.PDB file created by the linker directly, and embeds the absolute path to the .PDB in the .EXE or .DLL file. If the debugger can't find the .PDB file at that location or if the path is invalid (if, for example, the project was moved to another computer), the debugger looks for it in the current directory.

The .DBG files

The Visual C++ integrated debugger can also use .DBG files as long as they are made from a binary containing Codeview format debugging output. These are useful for debugging when the source code is not available. Even without the source, .DBG files allow you to set breakpoints on functions, watch variables, and see the functions in the call stack. They are also required for OLE RPC debugging.

One caveat needs to be pointed out: when working with symbols from a .DBG file, you must use the fully decorated names. For example, to set a breakpoint on a call to the Windows sndPlaySound function, you would specify _sndPlaySoundA@8 as the location.

There are actually two .DBG file formats. The old format has existed for quite a while in the 16-bit world. For example, because the format of .COM files is a simple binary image loaded into memory, the Codeview debugging information could not be appended to the end of the file because the file size might exceed the 64K limit for a .COM file. Therefore the symbolic information was instead put into a separate .DBG file, which had only Codeview information in it. The .DBG files could also be generated by running CVPACK on an .EXE file using the /strip option.

For 32-bit .EXEs, the Visual C++ version 2.x and 4.x debugger's symbol handler does not read the old format. Instead, it reads the format used in the Windows NT .DBG files, supplied for use with its system .DLL files. These .DBG files are in the Portable Executable (PE) file format and contain sections with COFF, FPO, and in some cases Codeview symbolic information. The new Visual C++ debugger reads .DBG files in this format only. Furthermore, it only uses the Codeview information, ignoring the other symbol sections.

It is possible to strip debug information from a PE file and store it in a .DBG file for use by debuggers. For this to work, the debugger needs to know whether to find the debug information in a separate file or not and whether the information has been stripped from the file or not. One method would be for the debugger to search through the executable file looking for debug information. However, to save the debugger from having to search the file, a file characteristic field (IMAGE_FILE_DEBUG_STRIPPED) was invented that indicates that the file has been stripped. Debuggers can look for this field in the PE file header to quickly determine whether the debug information is present in the file or not.

To generate a .DBG file in this format, you can use REBASE.EXE, which is provided with the Win32 SDK. Please see the Win32 SDK documentation for more details.

During the Windows NT retail builds, debug symbols are stripped from the system binaries and drivers and stored in separate .DBG files. This is done because the Windows NT kernel debugger can use these .DBG files and provide debugging symbols even for optimized drivers. Remember, however, that the Visual C++ integrated debugger is not designed to debug protected mode kernel code.

The Windows NT symbol files can be found in a debug subdirectory of the \SUPPORT directory on the Windows NT retail CD-ROM. These files must be copied from the CD-ROM to your hard drive. For user mode debugging on the target debugger machine, .DBG symbols must be present in the Windows NT \<winnt>\SYMBOLS directory of the target system (<winnt> is the directory where Windows NT is installed). The new Visual C++ setup program installs an "NT System Symbols Setup" icon in your program group. You can use it to automatically copy the .DBG files from your Windows NT Workstation CD-ROM disk to the correct directory structure on your hard drive. This method does not work for the Windows NT Server 4.0 CD-ROM because the .DBG files are stored in compressed format.

For kernel debugging, place .DBG files into a symbols tree underneath the directory specified by the _NT_SYMBOL_PATH environment variable (for example, C:\DEBUG\SYMBOLS). Kernel debugging is possible with a minimal set of symbols consisting of symbols for all the drivers (*.SYS) in the SYMBOLS\SYS directory, and symbols for NTOSKRNL.EXE and HAL.DLL in the SYMBOLS\EXE and SYMBOLS\DLL directories, respectively. For more information on kernel debugging, refer to the discussion regarding kernel debugging in the Windows NT DDK Programmer's Guide.

While it is theoretically possible to convert from a .PDB file back to a .DBG file, it is not a trivial task. At this time, we know of no such tool. If we hear of such a tool, we will update this article in the Microsoft Knowledge Base.

Номер статьи: 121366 — последний просмотр: 22 сент. 2009 г. — редакция: 1

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