RtlCreateUnicodeString - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTRTL_H

/**
 * The RtlCreateUnicodeString routine creates a new counted Unicode string.
 *
 * \param DestinationString Pointer to the newly allocated and initialized Unicode string.
 * \param SourceString Pointer to a null-terminated Unicode string with which to initialize the new string.
 * \return TRUE if the Unicode string was successfully created, FALSE otherwise.
 * \sa https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/nf-ntifs-rtlcreateunicodestring
 */
_Success_(return != 0)
_Must_inspect_result_
NTSYSAPI
BOOLEAN
NTAPI
RtlCreateUnicodeString(
    _Out_ PUNICODE_STRING DestinationString,
    _In_z_ PCWSTR SourceString
    );

#endif

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This function is documented in Windows Driver Kit.