Using WCT

The following sample code demonstrates the use of the wait chain traversal API. It enumerates all threads in the system and prints the wait chain for each thread.

To enumerate all threads in the system, run the sample with no parameters. To enumerate only the threads from a specified process, run the sample and pass the process identifier as a parameter. The sample performs the following steps:

1. Calls the RegisterWaitChainCOMCallback function to register the COM callback functions.
2. Calls the OpenThreadWaitChainSession function to create the wait chain session.
3. Calls the AdjustTokenPrivileges function to enable the SE_DEBUG_NAME privilege.
4. Calls the EnumProcesses and CreateToolhelp32Snapshot functions to enumerate the specified threads.
5. Calls the GetThreadWaitChain to retrieve an array of WAITCHAIN_NODE_INFO structures that contain the nodes of the wait chain.
6. Prints information from the wait chain.
7. Calls the CloseThreadWaitChainSession function to clean up the wait chain.

// Copyright (c) Microsoft Corporation. All rights reserved.

/*
 * Sample code for the Wait Chain Traversal (WCT) API.
 *
 * This program enumerates all threads in the system and prints the
 * wait chain for each of them.  It should be run from an elevated
 * command prompt to get results for services.
 *
 */

#define UNICODE

#include <windows.h>
#include <wct.h>
#include <psapi.h>
#include <tlhelp32.h>
#include <wchar.h>
#include <stdio.h>
#include <stdlib.h>

#pragma comment(lib, "Psapi.lib")
#pragma comment(lib, "Advapi32.lib")

typedef struct _STR_ARRAY
{
    CHAR Desc[32];
} STR_ARRAY;

// Human-readable names for the different synchronization types.
STR_ARRAY STR_OBJECT_TYPE[] =
{
    {"CriticalSection"},
    {"SendMessage"},
    {"Mutex"},
    {"Alpc"},
    {"Com"},
    {"ThreadWait"},
    {"ProcWait"},
    {"Thread"},
    {"ComActivation"},
    {"Unknown"},
    {"Max"}
};

// Global variable to store the WCT session handle
HWCT g_WctHandle = NULL;

// Global variable to store OLE32.DLL module handle.
HMODULE g_Ole32Hnd = NULL;

//
// Function prototypes
//

void
PrintWaitChain (
    __in DWORD ThreadId
    );


BOOL
GrantDebugPrivilege ( )
/*++

Routine Description:

    Enables the debug privilege (SE_DEBUG_NAME) for this process.
    This is necessary if we want to retrieve wait chains for processes
    not owned by the current user.

Arguments:

    None.

Return Value:

    TRUE if this privilege could be enabled; FALSE otherwise.

--*/
{
    BOOL             fSuccess    = FALSE;
    HANDLE           TokenHandle = NULL;
    TOKEN_PRIVILEGES TokenPrivileges;

    if (!OpenProcessToken(GetCurrentProcess(),
                          TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY,
                          &TokenHandle))
    {
        printf("Could not get the process token");
        goto Cleanup;
    }

    TokenPrivileges.PrivilegeCount = 1;

    if (!LookupPrivilegeValue(NULL,
                              SE_DEBUG_NAME,
                              &TokenPrivileges.Privileges[0].Luid))
    {
        printf("Couldn't lookup SeDebugPrivilege name");
        goto Cleanup;
    }

    TokenPrivileges.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;

    if (!AdjustTokenPrivileges(TokenHandle,
                               FALSE,
                               &TokenPrivileges,
                               sizeof(TokenPrivileges),
                               NULL,
                               NULL))
    {
        printf("Could not revoke the debug privilege");
        goto Cleanup;
    }

    fSuccess = TRUE;

 Cleanup:

    if (TokenHandle)
    {
        CloseHandle(TokenHandle);
    }

    return fSuccess;
}

BOOL
CheckThreads (
    __in DWORD ProcId
    )
/*++

Routine Description:

    Enumerates all threads (or optionally only threads for one
    process) in the system.  It the calls the WCT API on each of them.

Arguments:

    ProcId--Specifies the process ID to analyze.  If '0' all processes
        in the system will be checked.

Return Value:

    TRUE if processes could be checked; FALSE if a general failure
    occurred.

--*/
{
    DWORD processes[1024];
    DWORD numProcesses;
    DWORD i;

    // Try to enable the SE_DEBUG_NAME privilege for this process. 
    // Continue even if this fails--we just won't be able to retrieve
    // wait chains for processes not owned by the current user.
    if (!GrantDebugPrivilege())
    {
        printf("Could not enable the debug privilege");
    }

    // Get a list of all processes currently running.
    if (EnumProcesses(processes, sizeof(processes), &numProcesses) == FALSE)
    {
        printf("Could not enumerate processes");
        return FALSE;
    }

    for (i = 0; i < numProcesses / sizeof(DWORD); i++)
    {
        HANDLE process;
        HANDLE snapshot;

        if (processes[i] == GetCurrentProcessId())
        {
            continue;
        }

        // If the caller specified a Process ID, check if we have a match.
        if (ProcId != 0)
        {
            if (processes[i] != ProcId)
            {
                continue;
            }
        }

        // Get a handle to this process.
        process = OpenProcess(PROCESS_ALL_ACCESS, FALSE, processes[i]);
        if (process)
        {
            WCHAR file[MAX_PATH];

            printf("Process 0x%x - ", processes[i]);

            // Retrieve the executable name and print it.
            if (GetProcessImageFileName(process, file, ARRAYSIZE(file)) > 0)
            {
                PCWSTR filePart = wcsrchr(file, L'\\');
                if (filePart)
                {
                    filePart++;
                }
                else
                {
                    filePart = file;
                }

                printf("%S", filePart);
            }

            printf("\n----------------------------------\n");

            // Get a snapshot of this process. This enables us to
            // enumerate its threads.
            snapshot = CreateToolhelp32Snapshot(TH32CS_SNAPTHREAD,
                                                processes[i]);
            if (snapshot)
            {
                THREADENTRY32 thread;
                thread.dwSize = sizeof(thread);

                // Walk the thread list and print each wait chain
                if (Thread32First(snapshot, &thread))
                {
                    do
                    {
                        if (thread.th32OwnerProcessID == processes[i])
                        {
                            // Open a handle to this specific thread
                            HANDLE threadHandle = OpenThread(THREAD_ALL_ACCESS,
                                                             FALSE,
                                                             thread.th32ThreadID);
                            if (threadHandle)
                            {
                                // Check whether the thread is still running
                                DWORD exitCode;
                                GetExitCodeThread(threadHandle, &exitCode);

                                if (exitCode == STILL_ACTIVE)
                                {
                                    // Print the wait chain.
                                    PrintWaitChain(thread.th32ThreadID);
                                }

                                CloseHandle(threadHandle);
                            }
                        }
                    } while (Thread32Next(snapshot, &thread));
                }
                CloseHandle(snapshot);
            }

            CloseHandle(process);
            printf("\n");
        }
    }
    return TRUE;
}

void
PrintWaitChain (
    __in DWORD ThreadId
    )
/*++

Routine Description:

    Enumerates all threads (or optionally only threads for one
    process) in the system. It the calls the WCT API on each thread.

Arguments:

    ThreadId--Specifies the thread ID to analyze.

Return Value:

    (none)

--*/
{
    WAITCHAIN_NODE_INFO NodeInfoArray[WCT_MAX_NODE_COUNT];
    DWORD               Count, i;
    BOOL                IsCycle;

    printf("%d: ", ThreadId);

    Count = WCT_MAX_NODE_COUNT;

    // Make a synchronous WCT call to retrieve the wait chain.
    if (!GetThreadWaitChain(g_WctHandle,
                            NULL,
                            WCTP_GETINFO_ALL_FLAGS,
                            ThreadId,
                            &Count,
                            NodeInfoArray,
                            &IsCycle))
    {
        printf("Error (0X%x)\n", GetLastError());
        return;
    }

    // Check if the wait chain is too big for the array we passed in.
    if (Count > WCT_MAX_NODE_COUNT)
    {
        printf("Found additional nodes %d\n", Count);
        Count = WCT_MAX_NODE_COUNT;
    }

    // Loop over all the nodes returned and print useful information.
    for (i = 0; i < Count; i++)
    {
        switch (NodeInfoArray[i].ObjectType)
        {
            case WctThreadType:
                // A thread node contains process and thread ID.
                printf("[%x:%x:%s]->",
                       NodeInfoArray[i].ThreadObject.ProcessId,
                       NodeInfoArray[i].ThreadObject.ThreadId,
                       ((NodeInfoArray[i].ObjectStatus == WctStatusBlocked) ? "b" : "r"));
                break;

            default:
                // A synchronization object.

                // Some objects have names...
                if (NodeInfoArray[i].LockObject.ObjectName[0] != L'\0')
                {
                    printf("[%s:%S]->",
                           STR_OBJECT_TYPE[NodeInfoArray[i].ObjectType-1].Desc,
                           NodeInfoArray[i].LockObject.ObjectName);
                }
                else
                {
                    printf("[%s]->",
                           STR_OBJECT_TYPE[NodeInfoArray[i].ObjectType-1].Desc);
                }
                if (NodeInfoArray[i].ObjectStatus == WctStatusAbandoned)
                {
                    printf("<abandoned>");
                }
                break;
        }
    }

    printf("[End]");

    // Did we find a deadlock?
    if (IsCycle)
    {
        printf(" !!!Deadlock!!!");
    }

    printf("\n");
}

void
Usage ()
/*++

Routine Description:

  Print usage information to stdout.

--*/
{
    printf("\nPrints the thread wait chains for one or all processes in the system.\n\n");
    printf("\nUsage:\tWctEnum [ProcId]\n");
    printf("\t (no params) -- get the wait chains for all processes\n");
    printf("\t ProcId      -- get the wait chains for the specified process\n\n");
}

BOOL
InitCOMAccess ()
/*++

Routine Description:

    Register COM interfaces with WCT. This enables WCT to provide wait
    information if a thread is blocked on a COM call.

--*/
{
    HMODULE               ModuleHandle = NULL;
    PCOGETCALLSTATE       CallStateCallback;
    PCOGETACTIVATIONSTATE ActivationStateCallback;

    // Get a handle to OLE32.DLL. You must keep this handle around
    // for the life time for any WCT session.
    g_Ole32Hnd = LoadLibrary(L"ole32.dll");
    if (!g_Ole32Hnd)
    {
        printf("ERROR: GetModuleHandle failed: 0x%X\n", GetLastError());
        return FALSE;
    }

    // Retrieve the function addresses for the COM helper APIs.
    CallStateCallback = (PCOGETCALLSTATE)
        GetProcAddress(g_Ole32Hnd, "CoGetCallState");
    if (!CallStateCallback)
    {
        printf("ERROR: GetProcAddress failed: 0x%X\n", GetLastError());
        return FALSE;
    }

    ActivationStateCallback = (PCOGETACTIVATIONSTATE)
        GetProcAddress(g_Ole32Hnd, "CoGetActivationState");
    if (!ActivationStateCallback)
    {
        printf("ERROR: GetProcAddress failed: 0x%X\n", GetLastError());
        return FALSE;
    }

    // Register these functions with WCT.
    RegisterWaitChainCOMCallback(CallStateCallback,
                                 ActivationStateCallback);
    return TRUE;
}

int _cdecl
wmain (
    __in int argc,
    __in_ecount(argc) PWSTR* argv
    )
/*++

Routine Description:

  Main entry point for this application.

--*/
{
    int rc = 1;

    // Initialize the WCT interface to COM. Continue if this
    // fails--there just will not be COM information.
    if (!InitCOMAccess())
    {
        printf("Could not enable COM access\n");
    }

    // Open a synchronous WCT session.
    g_WctHandle = OpenThreadWaitChainSession(0, NULL);
    if (NULL == g_WctHandle)
    {
        printf("ERROR: OpenThreadWaitChainSession failed\n");
        goto Cleanup;
    }

    if (argc < 2)
    {
        // Enumerate all threads in the system.
        CheckThreads(0);
    }
    else
    {
        // Only enumerate threads in the specified process.
        //
        // Take the first command line parameter as the process ID.
        DWORD  ProcId = 0;

        ProcId = _wtoi(argv[1]);
        if (ProcId == 0)
        {
            Usage();
            goto Cleanup;
        }

        CheckThreads(ProcId);
    }

    // Close the WCT session.
    CloseThreadWaitChainSession(g_WctHandle);

    rc = 0;

 Cleanup:

    if (NULL != g_Ole32Hnd)
    {
        FreeLibrary(g_Ole32Hnd);
    }

    return rc;
}