#ifndef _NTIOAPI_H

/**
 * The FILE_MAILSLOT_QUERY_INFORMATION structure contains information about a mailslot.
 * \sa https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_file_mailslot_query_information
 */
typedef struct _FILE_MAILSLOT_QUERY_INFORMATION
{
    ULONG MaximumMessageSize;       // The maximum size, in bytes, of a single message that can be written to the mailslot, or 0 for a message of any size.
    ULONG MailslotQuota;            // The size, in bytes, of the in-memory pool that is reserved for writes to this mailslot.
    ULONG NextMessageSize;          // The next message size, in bytes.
    ULONG MessagesAvailable;        // The total number of messages waiting to be read from the mailslot.
    LARGE_INTEGER ReadTimeout;      // The time, in milliseconds, that a read operation can wait for a message to be written to the mailslot before a time-out occurs.
} FILE_MAILSLOT_QUERY_INFORMATION, *PFILE_MAILSLOT_QUERY_INFORMATION;

#endif
View code on GitHub

// ntifs.h

typedef struct _FILE_MAILSLOT_QUERY_INFORMATION {
  ULONG         MaximumMessageSize;
  ULONG         MailslotQuota;
  ULONG         NextMessageSize;
  ULONG         MessagesAvailable;
  LARGE_INTEGER ReadTimeout;
} FILE_MAILSLOT_QUERY_INFORMATION, *PFILE_MAILSLOT_QUERY_INFORMATION;
View the official Windows Driver Kit DDI reference