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) 1996-2025, 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 PGDLLIMPORT int shared_memory_type;
extern PGDLLIMPORT int huge_pages;
extern PGDLLIMPORT int huge_page_size;
 
/* Possible values for huge_pages and huge_pages_status */
typedef enum
{
    HUGE_PAGES_OFF,
    HUGE_PAGES_ON,
    HUGE_PAGES_TRY,             /* only for huge_pages */
    HUGE_PAGES_UNKNOWN,         /* only for huge_pages_status */
}           HugePagesType;
 
/* Possible values for shared_memory_type */
typedef enum
{
    SHMEM_TYPE_WINDOWS,
    SHMEM_TYPE_SYSV,
    SHMEM_TYPE_MMAP,
}           PGShmemType;
 
#ifndef WIN32
extern PGDLLIMPORT unsigned long UsedShmemSegID;
#else
extern PGDLLIMPORT HANDLE UsedShmemSegID;
extern PGDLLIMPORT void *ShmemProtectiveRegion;
#endif
extern PGDLLIMPORT 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);
extern void GetHugePageSize(Size *hugepagesize, int *mmap_flags);
 
#endif                          /* PG_SHMEM_H */