IsDBCSLeadByteEx function (winnls.h)

Article
10/13/2021

Determines if a specified character is potentially a lead byte. A lead byte is the first byte of a two-byte character in a double-byte character set (DBCS) for the code page.

Syntax

BOOL IsDBCSLeadByteEx(
  [in] UINT CodePage,
  [in] BYTE TestChar
);

Parameters

[in] CodePage

Identifier of the code page used to check lead byte ranges. This parameter can be one of the code page identifiers defined in Unicode and Character Set Constants or one of the following predefined values. This function validates lead byte values only in code pages 932, 936, 949, 950, and 1361.

Value	Meaning
CP_ACP	Use system default Windows ANSI code page.
CP_MACCP	Use the system default Macintosh code page.
CP_OEMCP	Use system default OEM code page.
CP_THREAD_ACP	Use the Windows ANSI code page for the current thread.

[in] TestChar

The character to test.

Return value

Returns a nonzero value if the byte is a lead byte. The function returns 0 if the byte is not a lead byte or if the character is a single-byte character. To get extended error information, the application can call GetLastError.

Remarks

Note This function does not validate the presence or validity of a trail byte. Therefore, MultiByteToWideChar might not recognize a sequence that the application using IsDBCSLeadByte reports as a lead byte. The application can easily become unsynchronized with the results of MultiByteToWideChar, potentially leading to unexpected errors or buffer size mismatches.

In general, instead of attempting low-level manipulation of code page data, applications should use MultiByteToWideChar to convert the data to UTF-16 and work with it in that encoding.

Lead byte values are specific to each distinct DBCS. Some byte values can appear in a single code page as both the lead and trail byte of a DBCS character. Thus, IsDBCSLeadByteEx can only indicate a potential lead byte value.

To make sense of a DBCS string, an application normally starts at the beginning of the string and scans forward, keeping track when it encounters a lead byte, and treating the next byte as the trailing part of the same character. To back up, the application should use CharPrevExA instead of attempting to develop its own algorithm.

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps \| UWP apps]
Minimum supported server	Windows 2000 Server [desktop apps \| UWP apps]
Target Platform	Windows
Header	winnls.h (include Windows.h)
Library	Kernel32.lib
DLL	Kernel32.dll