PM_COLLECT_PROC callback function (winperf.h)

Article
10/13/2021

Collects the performance data and returns it to the consumer. Implement and export this function if you are writing a performance DLL to provide performance data. The system calls this function whenever a consumer queries the registry for performance data.

The CollectPerformanceData function is a placeholder for the application-defined function name.

Syntax

PM_COLLECT_PROC PmCollectProc;

DWORD PmCollectProc(
                 LPWSTR pValueName,
                 void **ppData,
                 DWORD *pcbTotalBytes,
                 DWORD *pNumObjectTypes
)
{...}

Parameters

pValueName

ppData

pcbTotalBytes

pNumObjectTypes

Return value

One of the following values:

Return code	Description
ERROR_MORE_DATA	The size of the pData buffer (where pData refers to the pointer pointed to by lppData) as specified by lpcbTotalBytes is not large enough to store the data. Leave pData unchanged, and set lpcbTotalBytes and lpNumObjectTypes to zero. No attempt is made to indicate the required buffer size, because this can change before the next call.
ERROR_SUCCESS	Return this value in all cases other than the ERROR_MORE_DATA case, even if no data is returned or an error occurs. To report errors other than insufficient buffer size, use the Application Event Log.

Remarks

If the requested objects specified in the lpValueName parameter do not correspond to any of the object indexes that your performance DLL supports, leave the pData parameter unchanged (where pData refers to the pointer pointed to by lppData), and set the lpcbTotalBytes and lpNumObjectTypes parameters to zero. This indicates that no data was returned.

If you support one or more of the queried objects, determine whether the size of the pData buffer as specified by lpcbTotalBytes is large enough to store the data. If not, leave pData unchanged, and set lpcbTotalBytes and lpNumObjectTypes to zero. No attempt is made to indicate the required buffer size, because this may change before the next call. Return ERROR_MORE_DATA.

If your data collection is time-consuming, you should respond only to queries for specific objects, or costly queries. You should also lower the priority of the thread collecting the data, so that it does not adversely affect system performance. For the query string format, see Using the Registry Functions to Consume Counter Data.

If the consumer is running on another computer (remotely), then the OpenPerformanceData, ClosePerformanceData, and CollectPerformanceData functions are called in the context of the Winlogon process, which handles the server side of the remote connection. This distinction is important when troubleshooting problems that occur only remotely.

After your function returns successfully, the system can perform some basic tests to ensure the integrity of the data. By default, no tests are performed. If a test fails, the system generates an event log message and the data is discarded to prevent any further problems due to pointers that are not valid. The following registry value controls the test level: HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Perflib\ExtCounterTestLevel.

The following are the possible test levels for ExtCounterTestLevel.

Level	Meaning
1	Test the pointers and buffers of trusted counter DLLs. Sends a copy of the user's buffer.
2	Test pointers and buffer lengths but does not test pointer references or buffer contents. Sends a copy of the user's buffer.
3	Do not test pointers or buffers. Sends a copy of the user's buffer.
4	Do not test pointers or buffers. Sends the user's buffer, not a copy. This is the default value.

The following tests are performed at levels 1 and 2:

Verifies that the value of lpcbTotalBytes is consistent with the returned buffer pointer, pData. If you add the lpcbTotalBytes value to the original buffer pointer passed to this function, you should end up with the same buffer pointer returned by this function. If they are not the same, an error message is logged and the data is ignored.
Verify that a buffer overrun did not occur. The system adds a 1-KB guard page before and after the consumer-allocated buffer. If the returned buffer pointer, pData, points past the first byte of the appended guard page, then it is assumed that the buffer is not valid and the data is ignored. If the buffer pointer exceeds the end of the buffer, but not the end of the guard page, then a buffer overrun error is logged. If the buffer pointer is past the end of the guard page, then a heap error is logged because the heap that the buffer was allocated from could have been corrupted, causing other memory errors.
Verify that the guard pages have not been corrupted. The 1-KB guard pages that were added before and after the buffer are initialized with a data pattern before this function is called. This data pattern is checked after the collect procedure returns. If any discrepancy is detected, a buffer overrun or other memory error is assumed and the data is ignored.

The following tests are performed only if test level 1 is used:

Verify that the sum of each object's TotalByteLength member is the same as the value of lpcbTotalBytes. If not, the data is ignored.
Verify that the ByteLength member of each instance is consistent. The lengths are consistent if the next object or end of buffer follows the last instance. If not, the data is ignored.

Examples

See Implementing CollectPerformanceData.

Requirements

Requirement	Value
Minimum supported client	Windows XP [desktop apps only]
Minimum supported server	Windows Server 2003 [desktop apps only]
Target Platform	Windows
Header	winperf.h

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