RtlDecompressBufferEx - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTRTL_H
#if (PHNT_VERSION >= PHNT_WIN8)

NTSYSAPI
NTSTATUS
NTAPI
RtlDecompressBufferEx(
    _In_ USHORT CompressionFormat,
    _Out_writes_bytes_to_(UncompressedBufferSize, *FinalUncompressedSize) PUCHAR UncompressedBuffer,
    _In_ ULONG UncompressedBufferSize,
    _In_reads_bytes_(CompressedBufferSize) PUCHAR CompressedBuffer,
    _In_ ULONG CompressedBufferSize,
    _Out_ PULONG FinalUncompressedSize,
    _In_opt_ PVOID WorkSpace
    );

#endif
#endif

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This function is documented in Windows Driver Kit.