PdhOpenLogA function (pdh.h)

Article
02/09/2023

Opens the specified log file for reading or writing.

Syntax

PDH_FUNCTION PdhOpenLogA(
  [in]  LPCSTR     szLogFileName,
  [in]  DWORD      dwAccessFlags,
  [in]  LPDWORD    lpdwLogType,
  [in]  PDH_HQUERY hQuery,
  [in]  DWORD      dwMaxSize,
  [in]  LPCSTR     szUserCaption,
  [out] PDH_HLOG   *phLog
);

Parameters

[in] szLogFileName

Null-terminated string that specifies the name of the log file to open. The name can contain an absolute or relative path.

If the lpdwLogType parameter is PDH_LOG_TYPE_SQL, specify the name of the log file in the form, SQL:DataSourceName!LogFileName.

[in] dwAccessFlags

Type of access to use to open the log file. Specify one of the following values.

Value	Meaning
PDH_LOG_READ_ACCESS	Open the log file for reading.
PDH_LOG_WRITE_ACCESS	Open a new log file for writing.
PDH_LOG_UPDATE_ACCESS	Open an existing log file for writing.

You can use the bitwise inclusive OR operator (|) to combine the access type with one or more of the following creation flags.

Value	Meaning
PDH_LOG_CREATE_NEW	Creates a new log file with the specified name.
PDH_LOG_CREATE_ALWAYS	Creates a new log file with the specified name. If the log file already exists, the function removes the existing log file before creating the new file.
PDH_LOG_OPEN_EXISTING	Opens an existing log file with the specified name. If a log file with the specified name does not exist, this is equal to PDH_LOG_CREATE_NEW.
PDH_LOG_OPEN_ALWAYS	Opens an existing log file with the specified name or creates a new log file with the specified name.
PDH_LOG_OPT_CIRCULAR	Creates a circular log file with the specified name. When the file reaches the value of the dwMaxSize parameter, data wraps to the beginning of the log file. You can specify this flag only if the lpdwLogType parameter is PDH_LOG_TYPE_BINARY.
PDH_LOG_USER_STRING	Used with PDH_LOG_TYPE_TSV to write the user caption or log file description indicated by the szUserString parameter of PdhUpdateLog or PdhOpenLog. The user caption or log file description is written as the last column in the first line of the text log.

[in] lpdwLogType

Type of log file to open. This parameter can be one of the following values.

Value	Meaning
PDH_LOG_TYPE_UNDEFINED	Undefined log file format. If specified, PDH determines the log file type. You cannot specify this value if the dwAccessFlags parameter is PDH_LOG_WRITE_ACCESS.
PDH_LOG_TYPE_CSV	Text file containing column headers in the first line, and individual data records in each subsequent line. The fields of each data record are comma-delimited. The first line also contains information about the format of the file, the PDH version used to create the log file, and the names and paths of each of the counters.
PDH_LOG_TYPE_SQL	The data source of the log file is an SQL database.
PDH_LOG_TYPE_TSV	Text file containing column headers in the first line, and individual data records in each subsequent line. The fields of each data record are tab-delimited. The first line also contains information about the format of the file, the PDH version used to create the log file, and the names and paths of each of the counters.
PDH_LOG_TYPE_BINARY	Binary log file format.

[in] hQuery

Specify a query handle if you are writing query data to a log file. The PdhOpenQuery function returns this handle.

This parameter is ignored and should be NULL if you are reading from the log file.

[in] dwMaxSize

Maximum size of the log file, in bytes. Specify the maximum size if you want to limit the file size or if dwAccessFlags specifies PDH_LOG_OPT_CIRCULAR; otherwise, set to 0.

For circular log files, you must specify a value large enough to hold at least one sample. Sample size depends on data being collected. However, specifying a value of at least one megabyte will cover most samples.

[in] szUserCaption

Null-terminated string that specifies the user-defined caption of the log file. A log file caption generally describes the contents of the log file. When an existing log file is opened, the value of this parameter is ignored.

[out] phLog

Handle to the opened log file.

Return value

If the function succeeds, it returns ERROR_SUCCESS.

If the function fails, the return value is a system error code or a PDH error code.

Remarks

To use this function to write performance data to a log file, you must open a query using PdhOpenQuery and add the desired counters to it, before calling this function.

Newer operating systems can read log files that were generated on older operating systems; however, log files that were created on Windows Vista and later operating systems cannot be read on earlier operating systems.

The following rules apply to log files

READ_ACCESS requires OPEN_EXISTING.
UPDATE_ACCESS cannot be used with file-based logs. It can only be used with database logs.
WRITE_ACCESS requires one of CREATE_NEW, CREATE_ALWAYS, OPEN_EXISTING, OPEN_ALWAYS.

Examples

For an example, see Writing Performance Data to a Log File.

Note

The pdh.h header defines PdhOpenLog as an alias that 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 is not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.

Requirements

Requirement	Value
Minimum supported client	Windows XP [desktop apps only]
Minimum supported server	Windows Server 2003 [desktop apps only]
Target Platform	Windows
Header	pdh.h
Library	Pdh.lib
DLL	Pdh.dll

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