#ifndef _NTRTL_H

/**
 * The RtlUpcaseUnicodeChar routine converts the specified Unicode character to uppercase.
 *
 * \param[in] SourceCharacter Specifies the character to convert.
 * \return The uppercase version of the specified Unicode character.
 * \sa https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/wdm/nf-wdm-rtlupcaseunicodechar
 */
NTSYSAPI
WCHAR
NTAPI
RtlUpcaseUnicodeChar(
    _In_ WCHAR SourceCharacter
    );

#endif
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// wdm.h

NTSYSAPI WCHAR RtlUpcaseUnicodeChar(
  [in] WCHAR SourceCharacter
);
View the official Windows Driver Kit DDI reference