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postgresql的共享内存维护主要依靠以下三个结构体：

HHDR：

typedef struct hashhdr {
    long bsize;			/* Bucket/Page Size */
    long bshift;		/* Bucket shift */
    long dsize;			/* Directory Size */
    long ssize;			/* Segment Size */
    long sshift;		/* Segment shift */
    long max_bucket;		/* ID of Maximum bucket in use */
    long high_mask;		/* Mask to modulo into entire table */
    long low_mask;		/* Mask to modulo into lower half of table */
    long ffactor;		/* Fill factor */
    long nkeys;			/* Number of keys in hash table */
    long nsegs;			/* Number of allocated segments */
    long keysize;		/* hash key length in bytes */
    long datasize;		/* elem data length in bytes */
    long max_dsize;		/* 'dsize' limit if directory is fixed size */ 
    BUCKET_INDEX freeBucketIndex;
    /* index of first free bucket */
#ifdef HASH_STATISTICS
    long accesses;
    long collisions;
#endif
} HHDR;

这个结构体定义了hash结构的目录。

HTAB：

typedef struct htab {
    HHDR	*hctl;		/* shared control information */
    long 	(*hash)(); 	/* Hash Function */
    char 	*segbase; 	/* segment base address for 
				 * calculating pointer values 
				 */
    SEG_OFFSET	*dir;		/* 'directory' of segm starts */
    long 	*(*alloc)(); 	/* memory allocator 
				 * (long * for alignment reasons)
				 */

} HTAB;

这个结构体定义了hash table（动态、静态属性）。

HASHCTL：

typedef struct hashctl {
    long bsize;		/* Bucket Size */
    long ssize;		/* Segment Size */
    long dsize;		/* Dirsize Size */
    long ffactor;	/* Fill factor */
    long (*hash)();	/* Hash Function */
    long keysize;	/* hash key length in bytes */
    long datasize;	/* elem data length in bytes */
    long max_size;	/* limit to dsize if directory size is limited */
    long *segbase;	/* base for calculating bucket + seg ptrs */
    long * (*alloc)();	/* memory allocation function */
    long *dir;		/* directory if allocated already */
    long *hctl;		/* location of header information in shd mem */
} HASHCTL;

其中，HHDR是HTAB的一个元素。