MOUNTDEV_NAME - NtDoc

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

// Output structure for IOCTL_MOUNTDEV_QUERY_DEVICE_NAME.
typedef struct _MOUNTDEV_NAME
{
    USHORT NameLength;
    _Field_size_bytes_(NameLength) WCHAR Name[1];
} MOUNTDEV_NAME, * PMOUNTDEV_NAME;

#endif

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This structure is documented in Windows Driver Kit.