#ifndef _NTEXAPI_H
#if (PHNT_MODE != PHNT_MODE_KERNEL)

/**
 * The NtCancelTimer routine Cancels a timer object.
 *
 * \param TimerHandle A handle to the timer object.
 * \param CurrentState A pointer to a variable that receives the current state of the timer object.
 * \return NTSTATUS Successful or errant status.
 */
NTSYSCALLAPI
NTSTATUS
NTAPI
NtCancelTimer(
    _In_ HANDLE TimerHandle,
    _Out_opt_ PBOOLEAN CurrentState
    );

#endif
#endif
View code on GitHub

#ifndef _NTZWAPI_H

NTSYSCALLAPI
NTSTATUS
NTAPI
ZwCancelTimer(
    _In_ HANDLE TimerHandle,
    _Out_opt_ PBOOLEAN CurrentState
    );

#endif
View code on GitHub