RtlUnicodeToUTF8N function (ntifs.h)

Article
03/13/2023

The RtlUnicodeToUTF8N routine converts a Unicode string to a UTF-8 string.

Syntax

NTSYSAPI NTSTATUS RtlUnicodeToUTF8N(
  [out] PCHAR  UTF8StringDestination,
  [in]  ULONG  UTF8StringMaxByteCount,
  [out] PULONG UTF8StringActualByteCount,
  [in]  PCWCH  UnicodeStringSource,
  [in]  ULONG  UnicodeStringByteCount
);

Parameters

[out] UTF8StringDestination

A pointer to a caller-allocated destination buffer into which the routine writes the UTF-8 output string. If this parameter is NULL, the routine writes the required size of the output buffer to *UTF8StringActualByteCount.

[in] UTF8StringMaxByteCount

Specifies the maximum number of bytes that the routine can write to the buffer that UTF8StringDestination points to. If UTF8StringDestination = NULL, set UTF8StringMaxByteCount = 0.

[out] UTF8StringActualByteCount

A pointer to a location into which the routine writes the actual number of bytes that it has written to the buffer that UTF8StringDestination points to. If UTF8StringDestination is non-NULL, this count never exceeds the value of UTF8StringMaxByteCount. If UTF8StringDestination is NULL, this count is the number of bytes that are required to contain the entire output string.

[in] UnicodeStringSource

A pointer to the Unicode source string.

[in] UnicodeStringByteCount

Specifies the number of bytes in the Unicode source string that the UnicodeStringSource parameter points to.

Return value

RtlUnicodeToUTF8N returns STATUS_SUCCESS if the call is successful and all Unicode character codes in the input string were converted to the corresponding UTF-8 character codes in the output string. It returns STATUS_SOME_NOT_MAPPED if the call is successful but one or more input characters were invalid and were replaced by the Unicode replacement character, U+FFFD, before they were converted to UTF-8. Possible error return values include the following error codes:

Return code	Description
STATUS_BUFFER_TOO_SMALL	The UTF8StringMaxByteCount parameter specifies a buffer size that is too small to contain the entire output string.
STATUS_INVALID_PARAMETER	The UTF8StringDestination and UTF8StringActualByteCount parameters are both NULL.
STATUS_INVALID_PARAMETER_4	The UnicodeStringSource parameter is NULL.
STATUS_INVALID_PARAMETER_5	UnicodeStringByteCount is not an integer multiple of sizeof(WCHAR).

Remarks

The UTF-8 output string is null-terminated only if the Unicode input string is null-terminated.

The routine returns STATUS_BUFFER_TOO_SMALL if the UTF8StringMaxByteCount parameter specifies a buffer size that is too small to contain the entire output string. In this case, the routine writes as many UTF-8 characters as will fit in the buffer, and the *UTF8StringActualByteCount value specifies the number of valid bytes that the routine has written to the buffer. The partial string that is contained in the output buffer might not include a terminating null character.

You can make an initial call to RtlUnicodeToUTF8N to obtain the required output buffer size, and then call RtlUnicodeToUTF8N again to obtain the Unicode output string. In the initial call, set UTF8StringDestination = NULL and UTF8StringMaxByteCount = 0, and the routine will write the required buffer size to *UTF8StringActualByteCount. Next, allocate a buffer of the required size and call RtlUnicodeToUTF8N a second time to obtain the UTF-8 output string.

RtlUnicodeToUTF8N supports Unicode surrogate pairs. However, a surrogate leading word value that is not followed by a trailing word value, or a trailing word value that is not preceded by a leading word value, is not recognized as a valid character code and is replaced by the Unicode replacement character, U+FFFD.

RtlUnicodeToUTF8N continues to convert the input string to an output string until it reaches the end of the source buffer or the end of the destination buffer, whichever occurs first. The routine converts any null characters in the input string to null characters in the output string. If the input string contains a terminating null character, but the null character is not located at the end of the source buffer, the routine continues past the terminating null character until it reaches the end of the available buffer space.

The RtlUTF8ToUnicodeN routine converts a UTF-8 string to a Unicode string.

You can use RtlUnicodeToUTF8N and RtlUTF8ToUnicode routines to perform a lossless conversion of valid text strings between the Unicode and UTF-8 formats. However, strings that have arbitrary data values are likely to violate the Unicode rules for encoding surrogate pairs, and any information that is contained in the invalid values in an input string is lost and cannot be recovered from the resulting output string.

Requirements

Requirement	Value
Minimum supported client	Windows 7
Target Platform	Universal
Header	ntifs.h (include Ntifs.h, Wdm.h, Ntifs.h)
Library	NtosKrnl.lib
DLL	NtosKrnl.exe
IRQL	PASSIVE_LEVEL