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/*-------------------------------------------------------------------------
*
* shmqueue.c
* shared memory linked lists
*
* Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
*
* IDENTIFICATION
* src/backend/storage/ipc/shmqueue.c
*
* NOTES
*
* Package for managing doubly-linked lists in shared memory.
* The only tricky thing is that SHM_QUEUE will usually be a field
* in a larger record. SHMQueueNext has to return a pointer
* to the record itself instead of a pointer to the SHMQueue field
* of the record. It takes an extra parameter and does some extra
* pointer arithmetic to do this correctly.
*
* NOTE: These are set up so they can be turned into macros some day.
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "storage/shmem.h"
/*
* ShmemQueueInit -- make the head of a new queue point
* to itself
*/
void
SHMQueueInit(SHM_QUEUE *queue)
{
Assert(ShmemAddrIsValid(queue));
queue->prev = queue->next = queue;
}
/*
* SHMQueueIsDetached -- true if element is not currently
* in a queue.
*/
bool
SHMQueueIsDetached(const SHM_QUEUE *queue)
{
Assert(ShmemAddrIsValid(queue));
return (queue->prev == NULL);
}
/*
* SHMQueueElemInit -- clear an element's links
*/
void
SHMQueueElemInit(SHM_QUEUE *queue)
{
Assert(ShmemAddrIsValid(queue));
queue->prev = queue->next = NULL;
}
/*
* SHMQueueDelete -- remove an element from the queue and
* close the links
*/
void
SHMQueueDelete(SHM_QUEUE *queue)
{
SHM_QUEUE *nextElem = queue->next;
SHM_QUEUE *prevElem = queue->prev;
Assert(ShmemAddrIsValid(queue));
Assert(ShmemAddrIsValid(nextElem));
Assert(ShmemAddrIsValid(prevElem));
prevElem->next = queue->next;
nextElem->prev = queue->prev;
queue->prev = queue->next = NULL;
}
/*
* SHMQueueInsertBefore -- put elem in queue before the given queue
* element. Inserting "before" the queue head puts the elem
* at the tail of the queue.
*/
void
SHMQueueInsertBefore(SHM_QUEUE *queue, SHM_QUEUE *elem)
{
SHM_QUEUE *prevPtr = queue->prev;
Assert(ShmemAddrIsValid(queue));
Assert(ShmemAddrIsValid(elem));
elem->next = prevPtr->next;
elem->prev = queue->prev;
queue->prev = elem;
prevPtr->next = elem;
}
/*
* SHMQueueInsertAfter -- put elem in queue after the given queue
* element. Inserting "after" the queue head puts the elem
* at the head of the queue.
*/
void
SHMQueueInsertAfter(SHM_QUEUE *queue, SHM_QUEUE *elem)
{
SHM_QUEUE *nextPtr = queue->next;
Assert(ShmemAddrIsValid(queue));
Assert(ShmemAddrIsValid(elem));
elem->prev = nextPtr->prev;
elem->next = queue->next;
queue->next = elem;
nextPtr->prev = elem;
}
/*--------------------
* SHMQueueNext -- Get the next element from a queue
*
* To start the iteration, pass the queue head as both queue and curElem.
* Returns NULL if no more elements.
*
* Next element is at curElem->next. If SHMQueue is part of
* a larger structure, we want to return a pointer to the
* whole structure rather than a pointer to its SHMQueue field.
* For example,
* struct {
* int stuff;
* SHMQueue elem;
* } ELEMType;
* When this element is in a queue, prevElem->next points at struct.elem.
* We subtract linkOffset to get the correct start address of the structure.
*
* calls to SHMQueueNext should take these parameters:
* &(queueHead), &(queueHead), offsetof(ELEMType, elem)
* or
* &(queueHead), &(curElem->elem), offsetof(ELEMType, elem)
*--------------------
*/
Pointer
SHMQueueNext(const SHM_QUEUE *queue, const SHM_QUEUE *curElem, Size linkOffset)
{
SHM_QUEUE *elemPtr = curElem->next;
Assert(ShmemAddrIsValid(curElem));
if (elemPtr == queue) /* back to the queue head? */
return NULL;
return (Pointer) (((char *) elemPtr) - linkOffset);
}
/*--------------------
* SHMQueuePrev -- Get the previous element from a queue
*
* Same as SHMQueueNext, just starting at tail and moving towards head.
* All other comments and usage applies.
*/
Pointer
SHMQueuePrev(const SHM_QUEUE *queue, const SHM_QUEUE *curElem, Size linkOffset)
{
SHM_QUEUE *elemPtr = curElem->prev;
Assert(ShmemAddrIsValid(curElem));
if (elemPtr == queue) /* back to the queue head? */
return NULL;
return (Pointer) (((char *) elemPtr) - linkOffset);
}
/*
* SHMQueueEmpty -- true if queue head is only element, false otherwise
*/
bool
SHMQueueEmpty(const SHM_QUEUE *queue)
{
Assert(ShmemAddrIsValid(queue));
if (queue->prev == queue)
{
Assert(queue->next == queue);
return true;
}
return false;
}
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