StartTraceA function (evntrace.h)

The StartTrace function registers and starts an event tracing session.


Cross-process event tracing sessions are a limited system resource. Developers should avoid starting event tracing sessions on customer machines. When event tracing sessions are needed, they should be limited to the smallest possible scope: use as few sessions as possible, use an in-process-only session if possible (EVENT_TRACE_PRIVATE_LOGGER_MODE | EVENT_TRACE_PRIVATE_IN_PROC), only start the trace when collection is specifically needed for a scenario, stop the trace as soon as the scenario is completed, limit the amount of memory used by the session, and use strict event filters so you do not collect unnecessary events.


            CONTROLTRACE_ID         *TraceId,
  [in]      LPCSTR                  InstanceName,
  [in, out] PEVENT_TRACE_PROPERTIES Properties



[in] InstanceName

Null-terminated string that contains the name of the event tracing session. The session name is limited to 1,024 characters, is case-insensitive, and must be unique.


Use a descriptive name for your session so that the session's ownership and usage can be determined from the session name. Do not use a GUID or other non-deterministic or non-descriptive value. Do not append random digits to make your session name unique. ETW sessions are a limited resource so your component should not be starting multiple sessions. If your component's session is already running when your component starts, your component should clean up the orphaned session rather than creating a second session.

This function copies the session name that you provide to the offset that the LoggerNameOffset member of Properties points to.

[in, out] Properties

Pointer to an EVENT_TRACE_PROPERTIES structure that specifies the behavior of the session. The following are key members of the structure to set:

  • Wnode.BufferSize
  • Wnode.Guid
  • Wnode.ClientContext
  • Wnode.Flags
  • LogFileMode
  • LogFileNameOffset
  • LoggerNameOffset

Depending on the type of log file you choose to create, you may also need to specify a value for MaximumFileSize. See the Remarks section for more information on setting the Properties parameter and the behavior of the session.

Starting with Windows 10, version 1703: For better performance in cross process scenarios, you can now pass filtering in to StartTrace when starting system wide private loggers. You will need to pass in the new EVENT_TRACE_PROPERTIES_V2 structure to include filtering information. See Configuring and Starting a Private Logger Session for more details.

Return value

If the function succeeds, the return value is ERROR_SUCCESS.

If the function fails, the return value is one of the system error codes. The following are some common errors and their causes.


    One of the following is true:

    • The Wnode.BufferSize member of Properties specifies an incorrect size.
    • Properties does not have sufficient space allocated to hold a copy of InstanceName.

    One of the following is true:

    • Properties is NULL.
    • TraceHandle is NULL.
    • The LogFileNameOffset member of Properties is not valid.
    • The LoggerNameOffset member of Properties is not valid.
    • The LogFileMode member of Properties specifies a combination of flags that is not valid.
    • The Wnode.Guid member is SystemTraceControlGuid, but the InstanceName parameter is not KERNEL_LOGGER_NAME.

    A session with the same name or GUID is already running.


    You can receive this error for one of the following reasons:

    • Another session is already using the file name specified by the LogFileNameOffset member of the Properties structure.
    • Both LogFileMode and LogFileNameOffset are zero.

    There is not enough free space on the drive for the log file. This occurs if:

    • MaximumFileSize is nonzero and there is not MaximumFileSize bytes available for the log file
    • the drive is a system drive and there is not an additional 200 MB available
    • MaximumFileSize is zero and there is not an additional 200 MB available

    Choose a drive with more space, or decrease the size specified in MaximumFileSize (if used).


    Only users with administrative privileges, users in the Performance Log Users group, and services running as LocalSystem, LocalService, NetworkService can control event tracing sessions. To grant a restricted user the ability to control trace sessions, add them to the Performance Log Users group. Only users with administrative privileges and services running as LocalSystem can control an NT Kernel Logger session.

    If the user is a member of the Performance Log Users group, they may not have permission to create the log file in the specified folder.


    One of the following is true:

    • The logging session uses the EVENT_TRACE_SYSTEM_LOGGER_MODE flag and the maximum number of system loggers (8) has been reached.

    • The maximum number of logging sessions on the system has been reached. No new loggers can be created until a logging session has been stopped. On most systems, the maximum number of logging sessions is 64.

      You can change the maximum number of logging sessions for a system by editing the REG_DWORD key at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\WMI@EtwMaxLoggers. Permissible values are 32 through 256, inclusive. A reboot is required for any change to take effect.

      Note that Loggers use system resources. Increasing the number of loggers on the system will come at a performance cost if those slots are filled. This limit exists to prevent excessive use of system resources.


      The limit should only be manually adjusted by a system administrator to enable specific scenarios. The EtwMaxLoggers setting must not be automatically modified by a program or driver.

      Prior to Windows 10, version 1709, this is a fixed cap of 64 loggers for non-private loggers.


Event trace controllers call this function.

The session remains active until the session is stopped, the computer is restarted, an I/O error occurs, or the maximum file size is reached for non-circular logs. To stop an event tracing session, call the ControlTrace function and set the ControlCode parameter to EVENT_TRACE_CONTROL_STOP.

You cannot start more than one session with the same session GUID (as specified by Properties.Wnode.Guid). In most cases, you will set Properties.Wnode.Guid to all-zero (i.e. GUID_NULL) to allow the ETW system to generate a new GUID for the session.

To specify a private logger session, set Wnode.Guid member of Properties to the provider's control GUID, not the private logger session's control GUID. The provider must have registered the GUID before you call StartTrace.

You do not use this function to start a global logger session (deprecated). For details on starting a global logger session, see Configuring and Starting the Global Logger Session.

System Loggers

For the logger to be a system logger and receive events from SystemTraceProvider or other system providers, any of the following must be true:

  • The Properties member LogFileMode includes the EVENT_TRACE_SYSTEM_LOGGER_MODE flag (preferred).
  • The Properties member Wnode.Guid is set to SystemTraceControlGuid or GlobalLoggerGuid (deprecated - should not be used for new code because it will conflict with other components that also try to use these GUIDs).
  • InstanceName is set to KERNEL_LOGGER_NAME (deprecated - should not be used for new code because it will conflict with other components that also try to use this name).


 A system logger must set the EnableFlags member of the EVENT_TRACE_PROPERTIES structure to indicate which SystemTraceProvider events should be included in the trace.

Because system loggers receive special kernel events, they are subject to additional restrictions:

  • There can be no more than 8 system loggers active on the same system.
  • System loggers cannot be created within a Windows Server container.
  • System loggers cannot use the EVENT_TRACE_USE_PAGED_MEMORY flag.
  • Prior to Windows 10, version 1703, no more than 2 distinct clock types can be used simultaneously by any system loggers. For example, if one active system logger is using the "CPU cycle counter" clock type, and another active system logger is using the "Query performance counter" clock type, then any attempt to start a system logger using the "System time" clock type will fail because it would require the activation of a third clock type. Because of this limitation, Microsoft strongly recommends that system loggers do not use the "System time" clock type.
  • Starting with Windows 10, version 1703, the clock type restriction has been removed. All three clock types can now be used simultaneously by system loggers.

To specify an NT Kernel Logger session (deprecated), set InstanceName to KERNEL_LOGGER_NAME and the Wnode.Guid member of Properties to SystemTraceControlGuid. If you do not specify the GUID as SystemTraceControlGuid, ETW will override the GUID value and set it to SystemTraceControlGuid.


For an example that uses StartTrace, see Configuring and Starting an Event Tracing Session.


The evntrace.h header defines StartTrace as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.


Requirement Value
Minimum supported client Windows Vista [desktop apps | UWP apps]
Minimum supported server Windows Server 2008 [desktop apps | UWP apps]
Target Platform Windows
Header evntrace.h
Library Sechost.lib on Windows 8.1 and Windows Server 2012 R2; Advapi32.lib on Windows 8, Windows Server 2012, Windows 7, Windows Server 2008 R2, Windows Server 2008, Windows Vista
DLL Sechost.dll on Windows 8.1 and Windows Server 2012 R2; Advapi32.dll on Windows 8, Windows Server 2012, Windows 7, Windows Server 2008 R2, Windows Server 2008, Windows Vista

See also