Generating debug information with Visual C++
Kinds of debug information
Debug information formats
Generating debug information (general)
Generating debug information with Visual C++ 6.0
Generating debug information with Visual C++ 2002, 2003 and 2005
Debug information for static libraries
Debug information and size of the executable
Debuggers and debug information formats
Operating system symbols
When we use a debugger to debug an application, we expect that it will allow us to step through the source code, set breakpoints in source files, inspect values of various variables, including even complex user-defined types. But we also know that a typical executable is nothing more than a sequence of raw bytes (mostly machine language instructions, plus various headers and tables that help the operating system to run the executable). When the executable is loaded by the operating system and running, it uses additional pieces of memory for various purposes (stack, heap, etc.). But again, these are all raw bytes. How can the debugger know what source file and line corresponds to the currently executing CPU instruction? Or how can it know what address in stack memory corresponds to local variable X in function Y? The answer is debug information, the glue that links the expressions of a high level programming language to the raw bytes of the running program.
Now lets stop and try to determine what kinds of information about the program are needed by the debugger to do its job. For simplicity, lets limit our discussion to the existing Microsoft debuggers and Intel x86 platform. The following table lists all kinds of debug information that I am currently aware of.
|Kind of information||Description|
|Public functions and variables||This kind of information describes functions and variables that are visible across several compilation units (source files). For every function or variable, debug information stores its location and name.|
|Private functions and variables||This kind of information describes all functions and variables, including those that are not visible across several compilation units (therefore it also describes static functions, static and local variables, function parameters). For every function or variable, debug information stores its location, size and name.|
|Source file and line information||This kind of information maps every line of every source file to the corresponding location in the executable. (Of course it is possible that a source line does not have the corresponding location at all (e.g. a comment line). Such lines are not present in debug information).|
|Type information||For every function or variable, debug information can store additional information about its type. For a variable or a function parameter, this information will tell the debugger whether it is an integer, or a string, or a user defined type, and so on. For a function, it will tell the number of parameters, calling convention, and the type of the function’s return value.|
|FPO information||For a function compiled with FPO (frame pointer omission) optimisation, debug information also stores some data that helps the debugger to determine the size of the function’s stack frame, even when the frame pointer is not available. Without this information, debuggers would not be able to show correct call stacks when debugging optimised applications.|
|Edit and Continue information||This kind of information helps Visual Studio system to implement Edit and Continue feature.|
The term “location” can have different meanings. For functions, it is always the address of the first byte of the function. For global and static variables, it is the address of the first byte of the variable in memory. For local variables and function parameters, it is usually the offset of the first byte of the variable from some predefined place in the function’s stack frame. Other types of locations are also possible (e.g. register, TLS slot, metadata token).
By this time we already know what kinds of information debuggers use. Now we will explore
how debug information is stored. In the past decade, Microsoft development tools made use
of several different formats of packaging debug information. Here we will talk about COFF,
CodeView and Program Database formats, which are the most widely used. For every format
we are going to discuss, the following characteristics are especially interesting:
- What kinds of debug information can be stored using this format?
- Where is the debug information stored (in the executable itself, or separately)?
- Is the format documented or not?
This is the oldest of all debug information formats discussed here. It can contain
only three kinds of debug information – public functions and variables, source file
and line information, and FPO information.
COFF debug information is always stored in the executable itself, it cannot be stored in a separate file.
The format is documented; the documentation can be found in Microsoft Portable Executable and Common Object File Format Specification.
This is a newer, and more complex, format. It can store all available kinds of debug information,
except for Edit and Continue data.
CodeView information is usually stored in the executable itself, but it can also be extracted from the executable and stored in a file (usually with .DBG extension).
CodeView format is partially documented; the documentation can be found in Visual C++ 5.0 Symbolic Debug Information Specification document available on MSDN.
This is the newest debug information format of the three. It can store all possible kinds
of debug information, including Edit and Continue data. It also supports incremental linking,
which is not possible with other formats.
Program Database information is always stored separately from the executable, in a file (usually with .PDB extension).
Program Database format is not documented. Instead, special programming interfaces (DbgHelp and DIA) are available to work with it.
There are currently two versions of Program Database format. The first version, often called PDB 2.0, is used by Visual Studio 6.0. The second and newer version (called PDB 7.0) is used by Visual Studio.NET. PDB 7.0 format is not backward compatible – Visual Studio 6.0 debugger cannot read it.
The following table shows comparative data for all three formats:
|Format||Documented||Storage||Public functions and variables||Private functions and variables||Source files and lines||Type information||FPO information||EnC information|
|CodeView||Yes||Executable or separate file (.DBG)||+||+||+||+||+||-|
|Program Database||No||Separate file (.PDB)||+||+||+||+||+||+|
The build process of a typical executable consists of two steps – compiling and linking. First, the compiler parses source files and generates machine instructions, which are stored in object files. Next, the linker combines all available object files into the final executable. In addition to object files, the linker can also be asked to use libraries (which are actually collections of object files). The whole process is shown in the following figure.
If we want to produce debug information for the executable, we also have to perform two steps. First, we ask the compiler to produce debug information for every source file. Next, we ask the linker to combine debug information for every file into the final set of debug information for the executable. This process is shown in the following figure.
By default, compiler and linker do not produce debug information. Therefore at every step we should be able to specify proper compiler and linker options to let them know that we want debug information to be generated. We can also specify what kinds of debug information should be produced, what debug information formats should be used, and where to store the resulting debug information.
Below we will discuss the corresponding compiler and linker options of the two popular development systems – Microsoft Visual C++ 6.0 and Microsoft Visual C++.NET (2002 and 2003).
We can ask the compiler to produce debug information for a source file using one
of the following options: /Zd, /Z7, /Zi, /ZI (all options can be also configured in the IDE).
/Zd option asks the compiler to produce debug information in COFF format and store it in the resulting object file.
/Z7 option asks the compiler to produce debug information in CodeView format and store it in the resulting object file.
/Zi option asks the compiler to produce debug information in Program Database format and store it in a separate file with .PDB extension.
/ZI option is almost identical to /Zi, but the resulting debug information produced with /ZI will also contain Edit and Continue data.
The name of the .PDB file used by /Zi and /ZI options is VC60.PDB by default, but it can be changed using /Fd compiler option.
Summary information is collected in the following table.
|/Zi||Program Database||.PDB file||
|/ZI||Program Database||.PDB file||
The following options can be used to configure the linker to produce debug information for the executable: /debug, /debugtype, /pdb, /pdbtype (all options can be configured in the IDE).
/debug option asks the linker to generate debug information for the executable. If this option is not specified, debug information is not produced and other options have no effect.
/debugtype option allows to specify the format of the generated debug information. The following uses of the option are possible:
- use COFF format
- use CodeView or Program Database format (depends on /pdb option)
- use both COFF and CodeView/Program Database formats
One important limitation of /debugtype:coff option is that the resulting debug information will not contain file and line information, even if it was included in the debug information produced for object files. If you need source file and line information, use /debugtype:cv or /debugtype:both.
/pdb option tells the linker whether CodeView or Program Database format should be used. /pdb:none asks the linker to use CodeView format, while /pdb:filename asks the linker to use Program Database format and also specifies the name of the .PDB file. If /debugtype:coff option is also specified, /pdb option has no effect.
/pdbtype option makes sense only if debug information for one or more object files and libraries is also stored in a .PDB file. If that’s the case, /pdbtype:sept option will ask the linker to leave the debug information in the original .PDB files, and do not copy it into the final .PDB file for the executable. As a result, the link process will complete faster, but all .PDB files will be needed to successfully debug the executable. If it is not desirable to keep debug information separated into several .PDB files, /pdbtype:con option should be used. With this option, the linker will copy the contents of all .PDB files into the final .PDB file for the executable.
If you have difficulties to understand all available combinations of linker options, I must say that I had them too. To make it simpler to understand how the options should be used, I prepared some summary data in the following table.
|coff||/pdb:none (has no effect)||COFF||In the executable|
|coff||/pdb:filename (has no effect)||COFF||In the executable|
|cv||/pdb:none||CodeView||In the executable|
|cv||/pdb:filename||Program Database||In .PDB file|
|both||/pdb:none||COFF and CodeView||In the executable|
|both||/pdb:filename||COFF and Program Database||COFF information in the executable, Program Database information in .PDB file|
We can ask the compiler to produce debug information for a source file using one of the following options: /Zd, /Z7, /Zi, /ZI (all options can be configured in the IDE; /Zd option is not supported by Visual C++ 2005).
/Z7 option asks the compiler to produce debug information in CodeView format
and store it in the resulting object file.
/Zd, /Zi and /ZI options ask the compiler to produce debug information in Program Database format and store it in a file with .PDB extension. The difference between these three options is in the contents of the generated debug information (see summary table below for details).
The name of the .PDB file used by /Zd, /Zi and /ZI options is VC70.PDB, VC71.PDB or VC80.PDB by default (depending on the version of Visual Studio), but it can be changed using /Fd compiler option.
Note also that these new versions of the compiler do not produce debug information in COFF format.
|/Zd||Program Database||.PDB file||
|/Zi||Program Database||.PDB file||
|/ZI||Program Database||.PDB file||
The following options are used to configure the linker to produce debug information for the executable: /debug, /pdb, /pdbstripped (all options can be configured in the IDE).
/debug option asks the linker to generate debug information for the executable. If this option is not specified, debug information is not produced and other options have no effect. The format of the resulting debug information is always Program Database, and therefore it is always stored in a file with .PDB extension. By default, the linker uses the executable’s file name as the name for the .PDB file. The .PDB file can include all available contents of debug information.
/pdb option allows to specify a non-default name for the .PDB file.
/pdbstripped option allows to produce an additional .PDB file, whose contents are restricted to include only the following:
- public functions and variables
- FPO information
Note that COFF and CodeView formats are no longer supported by the linker.
The process of producing debug information for static libraries is much simpler than for executables, because there is no link step. Regardless of the compiler version, we can use one of the available /Z* options (/Zd, /Z7, /Zi, /ZI) to ask the compiler to produce debug information for the static library.
One important consideration is where to store the debug information. When /Z7 or /Zd option is used, debug information will be stored in the resulting .LIB file. When /Zi or /ZI option is used, debug information is stored in a separate .PDB file (its name can be specified using /Fd option).
If we generate debug information for an executable, how will the executable’s size be affected? The answer depends on the place where we store the debug information, which in turn depends on the debug information format we use.
Using COFF or CodeView formats usually means that debug information is stored in the executable. In that case, the size of the executable can grow significantly (an executable with debug information can become two times bigger than the same executable without debug information, or even more).
If Program Database format is used, debug information is stored in a separate file. The size of the executable is almost unaffected; it increases only by several hundred bytes, because the executable now contains a small header that helps debuggers to located the file with debug information.
One more step is needed to avoid unnecessary inflation of the executable. A side effect of using /debug linker option is the change of the default /opt:ref option to /opt:noref. As a result, enabling debug information generation will disable size optimisations performed by the linker. To turn size optimisations on again, it is necessary to enable /opt:ref option explicitly.
When we use CodeView format, the linker always stores debug information in the executable. But with the help of a small utility called Rebase, it is possible to extract debug information from the executable and put it into a separate file with .DBG extension.
Rebase is included with Visual Studio. It can be used for various purposes, but when it comes to extracting debug information, its command line is pretty simple:
rebase –b BaseAddr –x SymbolDir [-p] ExeName
|Specifies the new base address of the executable. If you do not want to change the base address, specify the same address as the one currently used by the executable.|
|Specifies the directory where the .DBG file will be stored. It is also possible to specify . (dot), which means the current directory.|
If this option is used, the .DBG file will contain only the following kinds of debug information:
For example, the following command will extract debug information from the DLL and put it into .DBG file in the current directory:
rebase –b 0x60000000 –x . MyDll.dll
When we decide about using a particular debug information format in our application, we must ensure that our favourite debugger can understand it. The following table lists the most popular debuggers and their support of the debug information formats.
|Debugger||COFF||CodeView||Program Database (2.0)||Program Database (7.0)|
|Visual Studio 2002, 2003, 2005||-||+||+||+|
|Visual C++ 6.0||+||+||+||-|
WinDbg 6.3 debugger has partial support for CodeView format. It can read only the following kinds of debug information:
- public functions and variables
- FPO information
- source files and lines
The following table lists debug information formats used by publicly available symbol files of Windows operating system components.
|CodeView (.DBG files)|
|CodeView (.DBG files) and Program Database (2.0)|
|Program Database (2.0)|
|Program Database (7.0)|
|Program Database (2.0)|
Have questions or comments? Free free to contact Oleg Starodumov at email@example.com.