RegQueryValueExA function (winreg.h)

Retrieves the type and data for the specified value name associated with an open registry key.

To ensure that any string values (REG_SZ, REG_MULTI_SZ, and REG_EXPAND_SZ) returned are null-terminated, use the RegGetValue function.

Syntax

LSTATUS RegQueryValueExA(
  [in]                HKEY    hKey,
  [in, optional]      LPCSTR  lpValueName,
                      LPDWORD lpReserved,
  [out, optional]     LPDWORD lpType,
  [out, optional]     LPBYTE  lpData,
  [in, out, optional] LPDWORD lpcbData
);

Parameters

[in] hKey

A handle to an open registry key. The key must have been opened with the KEY_QUERY_VALUE access right. For more information, see Registry Key Security and Access Rights.

This handle is returned by the RegCreateKeyEx, RegCreateKeyTransacted, RegOpenKeyEx, or RegOpenKeyTransacted function. It can also be one of the following predefined keys:

HKEY_CLASSES_ROOT

HKEY_CURRENT_CONFIG

HKEY_CURRENT_USER

HKEY_LOCAL_MACHINE

HKEY_PERFORMANCE_DATA

HKEY_PERFORMANCE_NLSTEXT

HKEY_PERFORMANCE_TEXT

HKEY_USERS

[in, optional] lpValueName

The name of the registry value.

If lpValueName is NULL or an empty string, "", the function retrieves the type and data for the key's unnamed or default value, if any.

If lpValueName specifies a value that is not in the registry, the function returns ERROR_FILE_NOT_FOUND.

Keys do not automatically have an unnamed or default value. Unnamed values can be of any type. For more information, see Registry Element Size Limits.

lpReserved

This parameter is reserved and must be NULL.

[out, optional] lpType

A pointer to a variable that receives a code indicating the type of data stored in the specified value. For a list of the possible type codes, see Registry Value Types. The lpType parameter can be NULL if the type code is not required.

[out, optional] lpData

A pointer to a buffer that receives the value's data. This parameter can be NULL if the data is not required.

[in, out, optional] lpcbData

A pointer to a variable that specifies the size of the buffer pointed to by the lpData parameter, in bytes. When the function returns, this variable contains the size of the data copied to lpData.

The lpcbData parameter can be NULL only if lpData is NULL.

If the data has the REG_SZ, REG_MULTI_SZ or REG_EXPAND_SZ type, this size includes any terminating null character or characters unless the data was stored without them. For more information, see Remarks.

If the buffer specified by lpData parameter is not large enough to hold the data, the function returns ERROR_MORE_DATA and stores the required buffer size in the variable pointed to by lpcbData. In this case, the contents of the lpData buffer are undefined.

If lpData is NULL, and lpcbData is non-NULL, the function returns ERROR_SUCCESS and stores the size of the data, in bytes, in the variable pointed to by lpcbData. This enables an application to determine the best way to allocate a buffer for the value's data.

If hKey specifies HKEY_PERFORMANCE_DATA and the lpData buffer is not large enough to contain all of the returned data, RegQueryValueEx returns ERROR_MORE_DATA and the value returned through the lpcbData parameter is undefined. This is because the size of the performance data can change from one call to the next. In this case, you must increase the buffer size and call RegQueryValueEx again passing the updated buffer size in the lpcbData parameter. Repeat this until the function succeeds. You need to maintain a separate variable to keep track of the buffer size, because the value returned by lpcbData is unpredictable.

If the lpValueName registry value does not exist, RegQueryValueEx returns ERROR_FILE_NOT_FOUND and the value returned through the lpcbData parameter is undefined.

Return value

If the function succeeds, the return value is ERROR_SUCCESS.

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

If the lpData buffer is too small to receive the data, the function returns ERROR_MORE_DATA.

If the lpValueName registry value does not exist, the function returns ERROR_FILE_NOT_FOUND.

Remarks

An application typically calls RegEnumValue to determine the value names and then RegQueryValueEx to retrieve the data for the names.

If the data has the REG_SZ, REG_MULTI_SZ or REG_EXPAND_SZ type, the string may not have been stored with the proper terminating null characters. Therefore, even if the function returns ERROR_SUCCESS, the application should ensure that the string is properly terminated before using it; otherwise, it may overwrite a buffer. (Note that REG_MULTI_SZ strings should have two terminating null characters.) One way an application can ensure that the string is properly terminated is to use RegGetValue, which adds terminating null characters if needed.

If the data has the REG_SZ, REG_MULTI_SZ or REG_EXPAND_SZ type, and the ANSI version of this function is used (either by explicitly calling RegQueryValueExA or by not defining UNICODE before including the Windows.h file), this function converts the stored Unicode string to an ANSI string before copying it to the buffer pointed to by lpData.

When calling the RegQueryValueEx function with hKey set to the HKEY_PERFORMANCE_DATA handle and a value string of a specified object, the returned data structure sometimes has unrequested objects. Do not be surprised; this is normal behavior. When calling the RegQueryValueEx function, you should always expect to walk the returned data structure to look for the requested object.

Note that operations that access certain registry keys are redirected. For more information, see Registry Virtualization and 32-bit and 64-bit Application Data in the Registry.

Examples

Ensure that you reinitialize the value pointed to by the lpcbData parameter each time you call this function. This is very important when you call this function in a loop, as in the following code example.

#include <windows.h>
#include <malloc.h>
#include <stdio.h>

#define TOTALBYTES    8192
#define BYTEINCREMENT 4096

void main()
{
    DWORD BufferSize = TOTALBYTES;
    DWORD cbData;
    DWORD dwRet;

    PPERF_DATA_BLOCK PerfData = (PPERF_DATA_BLOCK) malloc( BufferSize );
    cbData = BufferSize;

    printf("\nRetrieving the data...");

    dwRet = RegQueryValueEx( HKEY_PERFORMANCE_DATA,
                             TEXT("Global"),
                             NULL,
                             NULL,
                             (LPBYTE) PerfData,
                             &cbData );
    while( dwRet == ERROR_MORE_DATA )
    {
        // Get a buffer that is big enough.

        BufferSize += BYTEINCREMENT;
        PerfData = (PPERF_DATA_BLOCK) realloc( PerfData, BufferSize );
        cbData = BufferSize;

        printf(".");
        dwRet = RegQueryValueEx( HKEY_PERFORMANCE_DATA,
                         TEXT("Global"),
                         NULL,
                         NULL,
                         (LPBYTE) PerfData,
                         &cbData );
    }
    if( dwRet == ERROR_SUCCESS )
        printf("\n\nFinal buffer size is %d\n", BufferSize);
    else printf("\nRegQueryValueEx failed (%d)\n", dwRet);
}

 备注

The winreg.h header defines RegQueryValueEx 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.

Requirements

REQUIREMENTS

Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Target Platform	Windows
Header	winreg.h (include Windows.h)
Library	Advapi32.lib
DLL	Advapi32.dll