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Overview of Process Snapshotting

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With process snapshotting, you can capture and analyze data about running processes. The data is typically used for diagnostic purposes. Various types of data are available, including data about threads, handles, virtual spaces and process performance.

The Process Snapshotting functions are in kernel32.dll, and are installed as part of Windows.

Capturing a snapshot

To capture process data, you call the PssCaptureSnapshot function, providing a handle to the process. The data that PssCaptureSnapshot captures is called a snapshot.

A snapshot can contain more than one type of data. You specify the types you want by setting the appropriate flags in the CaptureFlags parameter. The flags are members of the PSS_CAPTURE_FLAGS enumeration.

Though PssCaptureSnapshot captures the snapshot, it does not provide the snapshot data to you. Instead it returns a snapshot handle. To get data you call other functions, as described below.

Getting data

There are two functions that provide snapshot data to you. PssQuerySnapshot provides types of data for which there is only a single instance in the snapshot, and PssWalkSnapshot provides multi-instance data.

Getting single-instance data

The PSS_QUERY_INFORMATION_CLASS enumeration lists the types of single-instance data that PssQuerySnapshot provides. When you call PssQuerySnapshot, you set the InformationClass parameter to specify the type of data you want. You can only get one type per call.

The Buffer parameter is a pointer to a structure that receives the data. The type of structure that you provide depends on the type of data you request with the InformationClass parameter. The PSS_QUERY_INFORMATION_CLASS topic tells you what type of structure to provide for each type of data.

Getting multi-instance data

The PSS_WALK_INFORMATION_CLASS enumeration lists the types of multi-instance data that PssWalkSnapshot provides. For these types there are a variable number of instances in a snapshot. You retrieve the instances one at a time, one per call to PssWalkSnapshot. You can call PssQuerySnapshot to determine the number of instances available.

Before you call PssWalkSnapshot for the first time, you call PssWalkMarkerCreate to create a walk marker. Each time you call PssWalkSnapshot, you pass the walk marker, which indicates to PssWalkSnapshot what instance to return. Before PssWalkSnapshot returns, it advances the marker to the next instance. You can create any number of walk markers for a snapshot.

You can call PssWalkMarkerGetPosition to get a marker position, and PssWalkMarkerSetPosition to set a marker to a position that PssWalkMarkerGetPosition provided. You call PssWalkMarkerSeekToBeginning to set the marker back to the beginning position (the position indicated by a newly-created walk marker).

When you call PssWalkSnapshot, the Buffer parameter is a pointer to a structure that receives the data. The type of structure that you provide depends on the type of data you request with the InformationClass parameter. The PSS_WALK_INFORMATION_CLASS topic tells you what type of structure to provide for each type of data.

Use the following steps to capture and process all instances of multi-instance data.

  1. Call PssCaptureSnapshot to capture a snapshot. You specify the types of data to capture with the CaptureFlags parameter. PssCaptureSnapshot returns a snapshot handle.

  2. Call PssWalkMarkerCreate to create a walk marker. PssWalkMarkerCreate returns a walk marker handle.

  3. Call PssWalkSnapshot, passing parameters as follows:

    • The SnapshotHandle parameter is set to the snapshot handle that PssCaptureSnapshot returned.
    • The InformationClass parameter is set to one of the members of PSS_WALK_INFORMATION_CLASS to specify the type of data to provide.
    • The WalkMarkerHandle is set to the walk marker handle that PssWalkMarkerCreate returned.
    • The Buffer parameter is a pointer to a structure that receives the data. The structure must be the proper type for the type of data being received.
    • The BufferLength parameter is set to the length of the receiving structure that Buffer points to. You can use sizeof to get this; for example, sizeof(PSS_HANDLE_ENTRY).

    These parameters can remain the same every time you call PssWalkSnapshot to get another instance, unless your processing requires a different buffer for each call.

  4. Handle the return value from PssWalkSnapshot as follows:

    • If the return value is ERROR_SUCCESS, the call succeeded, and you have data in your buffer. Process the data, then go back to step 3 to call PssWalkSnapshot again. Before PssWalkSnapshot returned, it advanced the walk marker to point to the next instance.
    • If the return value is ERROR_NO_MORE_ITEMS, you got and processed every instance. You are done.
    • If the return value is anything else, there is an error, and you must stop short of processing all the instances.