vDbgPrintExWithPrefix - NtDoc

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

NTSYSAPI
ULONG
NTAPI
vDbgPrintExWithPrefix(
    _In_z_ PCCH Prefix,
    _In_ ULONG ComponentId,
    _In_ ULONG Level,
    _In_z_ PCCH Format,
    _In_ va_list arglist
    );

#endif

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This function is documented in Windows Driver Kit.