This article describes the (documented) way to walk a callstack for any thread (own, other and remote). It has an abstraction layer, so the calling app does not need to know the internals.
This project was initially published on Codeproject (http://www.codeproject.com/KB/threads/StackWalker.aspx). But it is hard to maintain the article and the source on codeproject, so I was pushed to publish the source code on an "easier to modify" platform. Therefor I have chosen "codeplex" ;(
But time goes by, and codeplex went away ;)
So I now migrated to GitHub ;)
In some cases you need to display the callstack of the current thread or you are just interested in the callstack of other threads / processes. Therefore I wrote this project.
The goal for this project was the following:
To walk the callstack there is a documented interface: StackWalk64
Starting with Win9x/W2K, this interface is in the dbghelp.dll library (on NT, it is in imagehlp.dll).
But the function name (StackWalk64
) has changed starting with W2K (before it was called StackWalk
(without the 64
))!
This project only supports the newer Xxx64-functions. If you need to use it on older systems, you can download the redistributable for NT/W9x.
The latest dbghelp.dll can always be downloaded with the Debugging Tools for Windows. This also contains the symsrv.dll which enables the use of the public Microsoft symbols-server (can be used to retrieve debugging information for system-files; see below).
mkdir build-dir
cd build-dir
# batch
cmake -G "Visual Studio 15 2017 Win64" --config RelWithDebInfo -DCMAKE_INSTALL_PREFIX=%cd%/root ..
# powershell
cmake -G "Visual Studio 15 2017 Win64" --config RelWithDebInfo -DCMAKE_INSTALL_PREFIX="$($(get-location).Path)/root" ..
cmake --build . --config RelWithDebInfo
ctest.exe -V -C RelWithDebInfo
cmake --build . --target install --config RelWithDebInfo
The usage of the class is very simple. For example if you want to display the callstack of the current thread, just instantiate a StackWalk
object and call the ShowCallstack
member:
#include <windows.h>
#include "StackWalker.h"
void Func5() { StackWalker sw; sw.ShowCallstack(); }
void Func4() { Func5(); }
void Func3() { Func4(); }
void Func2() { Func3(); }
void Func1() { Func2(); }
int main()
{
Func1();
return 0;
}
This produces the following output in the debugger-output window:
[...] (output stripped)
d:\privat\Articles\stackwalker\stackwalker.cpp (736): StackWalker::ShowCallstack
d:\privat\Articles\stackwalker\main.cpp (4): Func5
d:\privat\Articles\stackwalker\main.cpp (5): Func4
d:\privat\Articles\stackwalker\main.cpp (6): Func3
d:\privat\Articles\stackwalker\main.cpp (7): Func2
d:\privat\Articles\stackwalker\main.cpp (8): Func1
d:\privat\Articles\stackwalker\main.cpp (13): main
f:\vs70builds\3077\vc\crtbld\crt\src\crt0.c (259): mainCRTStartup
77E614C7 (kernel32): (filename not available): _BaseProcessStart@4
You can now double-click on a line and the IDE will automatically jump to the desired file/line.
If you want to direct the output to a file or want to use some other output-mechanism, you simply need to derive from the StackWalker
class.
You have two options to do this: only overwrite the OnOutput
method or overwrite each OnXxx
-function.
The first solution (OnOutput
) is very easy and uses the default-implementation of the other OnXxx
-functions (which should be enough for most of the cases). To output also to the console, you need to do the following:
class MyStackWalker : public StackWalker
{
public:
MyStackWalker() : StackWalker() {}
protected:
virtual void OnOutput(LPCSTR szText) {
printf(szText); StackWalker::OnOutput(szText);
}
};
If you want detailed info about the callstack (like loaded-modules, addresses, errors, ...) you can overwrite the corresponding methods. The following methods are provided:
class StackWalker
{
protected:
virtual void OnSymInit(LPCSTR szSearchPath, DWORD symOptions, LPCSTR szUserName);
virtual void OnLoadModule(LPCSTR img, LPCSTR mod, DWORD64 baseAddr, DWORD size,
DWORD result, LPCSTR symType, LPCSTR pdbName,
ULONGLONG fileVersion);
virtual void OnCallstackEntry(CallstackEntryType eType, CallstackEntry &entry);
virtual void OnDbgHelpErr(LPCSTR szFuncName, DWORD gle, DWORD64 addr);
};
These methods are called during the generation of the callstack.
In the constructor of the class, you need to specify if you want to generate callstacks for the current process or for another process. The following constructors are available:
{
public:
StackWalker(int options = OptionsAll,
LPCSTR szSymPath = NULL,
DWORD dwProcessId = GetCurrentProcessId(),
HANDLE hProcess = GetCurrentProcess());
// Just for other processes with default-values for options and symPath
StackWalker(DWORD dwProcessId, HANDLE hProcess);
// For showing stack trace after __except or catch
StackWalker(ExceptType extype, int options = OptionsAll, PEXCEPTION_POINTERS exp = NULL);
};
To do the actual stack-walking you need to call the following functions:
class StackWalker
{
public:
BOOL ShowCallstack(HANDLE hThread = GetCurrentThread(), CONTEXT *context = NULL,
PReadProcessMemoryRoutine readMemoryFunction = NULL, LPVOID pUserData = NULL);
};
With this StackWalker
you can also display the callstack inside an structured exception handler. You only need to write a filter-function which does the stack-walking:
// The exception filter function:
LONG WINAPI ExpFilter(EXCEPTION_POINTERS* pExp, DWORD dwExpCode)
{
StackWalker sw;
sw.ShowCallstack(GetCurrentThread(), pExp->ContextRecord);
return EXCEPTION_EXECUTE_HANDLER;
}
// This is how to catch an exception:
__try
{
// do some ugly stuff...
}
__except (ExpFilter(GetExceptionInformation(), GetExceptionCode()))
{
}
Display the callstack inside an C++ exception handler (two ways):
// This is how to catch an exception:
try
{
// do some ugly stuff...
}
catch (std::exception & ex)
{
StackWalker sw;
sw.ShowCallstack(GetCurrentThread(), sw.GetCurrentExceptionContext());
}
catch (...)
{
StackWalker sw(StackWalker::AfterCatch);
sw.ShowCallstack();
}
To walk the callstack of a given thread, you need at least two facts:
The context is used to retrieve the current Instruction Pointer and the values for the Stack Pointer (SP) and sometimes the Frame Pointer (FP). The difference between SP and FP is in short: SP points to the latest address on the stack. FP is used to reference the arguments for a function. See also Difference Between Stack Pointer and Frame Pointer. But only the SP is essential for the processor. The FP is only used by the compiler. You can also disable the usage of FP (see: (/Oy Frame-Pointer Omission).
The callstack is a memory-region which contains all the data/addresses of the callers. This data must be used to retrieve the callstack (as the name says). The most important part is: this data-region must not change until the stack-walking is finished! This is also the reason why the thread must be in the Suspended state to retrieve a valid callstack. If you want to do a stack-walking for the current thread, then you must not change the callstack memory after the addresses which are declared in the context record.
To successfully walk the callstack with StackWalk64, you need to initialize the STACKFRAME64-structure with meaningful values. In the documentation of StackWalk64
there is only a small note about this structure:
AddrPC
and AddrFrame
members of the STACKFRAME64
structure passed in the StackFrame
parameter are not initialized.According to this documentation, most programs only initialize AddrPC
and AddrFrame
and this had worked until the newest dbghelp.dll (v5.6.3.7). Now, you also need to initialize AddrStack
. After having some trouble with this (and other problems) I talked to the dbghelp-team and got the following answer (2005-08-02; my own comments are written in italics!):
AddrStack
should always be set to the stack pointer value for all platforms. You can certainly publish that AddrStack
should be set. You're also welcome to say that new releases of dbghelp are now requiring this.AddrPC
to the current instruction pointer (Eip
on x86, Rip
on x64 and StIIP
on IA64)AddrStack
to the current stack pointer (Esp
on x86, Rsp
on x64 and IntSp
on IA64)AddrFrame
to the current frame pointer when meaningful. On x86 this is Ebp
, on x64 you can use Rbp
(but is not used by VC2005B2; instead it uses Rdi
!) and on IA64 you can use RsBSP
. StackWalk64 will ignore the value when it isn't needed for unwinding.AddrBStore
to RsBSP
for IA64.On x86 systems (prior to XP), there is no direct supported function to retrieve the context of the current thread.
The recommended way is to throw an exception and catch it. Now you will have a valid context-record.
The default way of capturing the context of the current thread is by doing some inline-assembler to retrieve EIP
, ESP
and EBP
.
If you want to use the documented way, then you need to define CURRENT_THREAD_VIA_EXCEPTION
for the project.
But you should be aware of the fact, that GET_CURRENT_CONTEXT
is a macro which internally uses __try __except
.
Your function must be able to contain these statements.
Starting with XP and on x64 and IA64 systems, there is a documented function to retrieve the context of the current thread: RtlCaptureContext.
To do a stack-walking of the current thread, you simply need to do:
StackWalker sw;
sw.ShowCallstack();
To walk the callstack of another thread inside the same process, you need to suspend the target thread (so the callstack will not change during the stack-walking). But you should be aware that suspending a thread in the same process might lead to dead-locks! (See: Why you never should call Suspend/TerminateThread (Part I), [Part II](http://blog.kalmbachnet.de/?postid=16, Part III)
If you have the handle to the thread, you can do the following to retrieve the callstack:
MyStackWalker sw;
sw.ShowCallstack(hThread);
For a complete sample to retrieve the callstack of another thread, you can take a look at the demo-project.
The approach is almost the same as for walking the callstack for the current process.
You only need to provide the ProcessID
and a handle to the process (hProcess
). Then you also need to suspend the thread to do the stack-walking. A complete sample to retrieve the callstack of another process is in the demo-project.
StackWalk
instanceIt is no problem to reuse the StackWalk
instance, as long as you want to do the stack-walking for the same process.
If you want to do a lot of stack-walking it is recommended to reuse the instance.
The reason is simple: if you create a new instance, then the symbol-files must be re-loaded for each instance.
And this is really time-consuming. Also it is not allowed to access the StackWalk
functions from different threads (the dbghelp.dll is not thread-safe!). Therefore it makes no sense to create more than one instance...
By default, a symbol-search path (SymBuildPath
and SymUseSymSrv
) is provided to the dbghelp.dll. This path contains the following directories:
szSymPath
. If this parameter is provided, the option SymBuildPath
is automatically set. Each path must be separated with a semi-colon ";"_NT_SYMBOL_PATH
_NT_ALTERNATE_SYMBOL_PATH
SYSTEMROOT
SYSTEMROOT
appended with "\system32"If you want to use the public symbols for the OS-files from the Microsoft-Symbol-Server, you either need the Debugging Tools for Windows (then symsrv.dll and the latest dbghelp.dll will be found automatically) or you need to redistribute "dbghelp.dll" and "smysrv.dll" from this package!
To successfully walk the callstack of a thread, dbghelp.dll requires that the modules are known by the library. Therefore you need to "register" each module of the process via SymLoadModule64
. To accomplish this you need to enumerate the modules of the given process.
Starting with Win9x and W2K, it is possible to use the ToolHelp32-API. You need to make a snapshot (CreateToolhelp32Snapshot
) of the process and then you can enumerate the modules via Module32First and Module32Next. Normally the ToolHelp functions are located in the kernel32.dll but on Win9x it is located in a separate DLL: tlhelp32.dll. Therefore we need to check the functions in both DLLs.
If you have NT4, then the ToolHelp32-API
is not available. But in NT4 you can use the PSAPI. To enumerate all modules you need to call [EnumProcessModules](), but you only get the handles to the modules. To feed SymLoadModule64 you also need to query the ModuleBaseAddr
, SizeOfImage
(via GetModuleInformation), ModuleBaseName
(via GetModuleBaseName) and ModuleFileName(Path)
(via GetModuleFileNameEx).
There are a couple of issues with dbghelp.dll.
AddrPC
and AddrFrame
members of the STACKFRAME64
structure passed in the StackFrame
parameter are not initialized.and
ContextRecord
] parameter is required only when the MachineType
parameter is not IMAGE_FILE_MACHINE_I386
.But this is not true anymore.
Now the callstack on x86-systems cannot be retrieved if you pass NULL
as ContextRecord
.
From my point of view this is a major documentation change.
Now you either need to initialize the AddrStack
as well, or provide a valid ContextRecord
which contains the EIP
, EBP
and ESP
registers!
To do some kind of modification of the behavior, you can optionally specify some options. Here is the list of the available options:
typedef enum StackWalkOptions
{
// No additional info will be retrieved
// (only the address is available)
RetrieveNone = 0,
// Try to get the symbol name
RetrieveSymbol = 1,
// Try to get the line for this symbol
RetrieveLine = 2,
// Try to retrieve the module info
RetrieveModuleInfo = 4,
// Also retrieve the version for the DLL/EXE
RetrieveFileVersion = 8,
// Contains all the above
RetrieveVerbose = 0xF,
// Generate a "good" symbol-search-path
SymBuildPath = 0x10,
// Also use the public Microsoft-Symbol-Server
SymUseSymSrv = 0x20,
// Contains all the above "Sym"-options
SymAll = 0x30,
// Contains all options (default)
OptionsAll = 0x3F
} StackWalkOptions;
StackWalk64
function. If you need to use it on NT4/Win9x, you need to redistribute the dbghelp.dll for this platform.OpenThread
which is not available on NT4/W9x. To have an example of doing this in NT4/Win9x please refer to Remote Library./DEBUG:fastlink
option