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diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
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+++ b/src/include/storage/pg_shmem.h
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+/*-------------------------------------------------------------------------
+ *
+ * pg_shmem.h
+ *	  Platform-independent API for shared memory support.
+ *
+ * Every port is expected to support shared memory with approximately
+ * SysV-ish semantics; in particular, a memory block is not anonymous
+ * but has an ID, and we must be able to tell whether there are any
+ * remaining processes attached to a block of a specified ID.
+ *
+ * To simplify life for the SysV implementation, the ID is assumed to
+ * consist of two unsigned long values (these are key and ID in SysV
+ * terms).  Other platforms may ignore the second value if they need
+ * only one ID number.
+ *
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/storage/pg_shmem.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PG_SHMEM_H
+#define PG_SHMEM_H
+
+#include "storage/dsm_impl.h"
+
+typedef struct PGShmemHeader	/* standard header for all Postgres shmem */
+{
+	int32		magic;			/* magic # to identify Postgres segments */
+#define PGShmemMagic  679834894
+	pid_t		creatorPID;		/* PID of creating process (set but unread) */
+	Size		totalsize;		/* total size of segment */
+	Size		freeoffset;		/* offset to first free space */
+	dsm_handle	dsm_control;	/* ID of dynamic shared memory control seg */
+	void	   *index;			/* pointer to ShmemIndex table */
+#ifndef WIN32					/* Windows doesn't have useful inode#s */
+	dev_t		device;			/* device data directory is on */
+	ino_t		inode;			/* inode number of data directory */
+#endif
+} PGShmemHeader;
+
+/* GUC variables */
+extern int	shared_memory_type;
+extern int	huge_pages;
+extern int	huge_page_size;
+
+/* Possible values for huge_pages */
+typedef enum
+{
+	HUGE_PAGES_OFF,
+	HUGE_PAGES_ON,
+	HUGE_PAGES_TRY
+}			HugePagesType;
+
+/* Possible values for shared_memory_type */
+typedef enum
+{
+	SHMEM_TYPE_WINDOWS,
+	SHMEM_TYPE_SYSV,
+	SHMEM_TYPE_MMAP
+}			PGShmemType;
+
+#ifndef WIN32
+extern unsigned long UsedShmemSegID;
+#else
+extern HANDLE UsedShmemSegID;
+extern void *ShmemProtectiveRegion;
+#endif
+extern void *UsedShmemSegAddr;
+
+#if !defined(WIN32) && !defined(EXEC_BACKEND)
+#define DEFAULT_SHARED_MEMORY_TYPE SHMEM_TYPE_MMAP
+#elif !defined(WIN32)
+#define DEFAULT_SHARED_MEMORY_TYPE SHMEM_TYPE_SYSV
+#else
+#define DEFAULT_SHARED_MEMORY_TYPE SHMEM_TYPE_WINDOWS
+#endif
+
+#ifdef EXEC_BACKEND
+extern void PGSharedMemoryReAttach(void);
+extern void PGSharedMemoryNoReAttach(void);
+#endif
+
+extern PGShmemHeader *PGSharedMemoryCreate(Size size,
+										   PGShmemHeader **shim);
+extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
+extern void PGSharedMemoryDetach(void);
+
+#endif							/* PG_SHMEM_H */