1 /*-------------------------------------------------------------------------
2  *
3  * shm_mq.h
4  *	  single-reader, single-writer shared memory message queue
5  *
6  * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
7  * Portions Copyright (c) 1994, Regents of the University of California
8  *
9  * src/include/storage/shm_mq.h
10  *
11  *-------------------------------------------------------------------------
12  */
13 #ifndef SHM_MQ_H
14 #define SHM_MQ_H
15 
16 #include "postmaster/bgworker.h"
17 #include "storage/dsm.h"
18 #include "storage/proc.h"
19 
20 /* The queue itself, in shared memory. */
21 struct shm_mq;
22 typedef struct shm_mq shm_mq;
23 
24 /* Backend-private state. */
25 struct shm_mq_handle;
26 typedef struct shm_mq_handle shm_mq_handle;
27 
28 /* Descriptors for a single write spanning multiple locations. */
29 typedef struct
30 {
31 	const char *data;
32 	Size		len;
33 } shm_mq_iovec;
34 
35 /* Possible results of a send or receive operation. */
36 typedef enum
37 {
38 	SHM_MQ_SUCCESS,				/* Sent or received a message. */
39 	SHM_MQ_WOULD_BLOCK,			/* Not completed; retry later. */
40 	SHM_MQ_DETACHED				/* Other process has detached queue. */
41 } shm_mq_result;
42 
43 /*
44  * Primitives to create a queue and set the sender and receiver.
45  *
46  * Both the sender and the receiver must be set before any messages are read
47  * or written, but they need not be set by the same process.  Each must be
48  * set exactly once.
49  */
50 extern shm_mq *shm_mq_create(void *address, Size size);
51 extern void shm_mq_set_receiver(shm_mq *mq, PGPROC *);
52 extern void shm_mq_set_sender(shm_mq *mq, PGPROC *);
53 
54 /* Accessor methods for sender and receiver. */
55 extern PGPROC *shm_mq_get_receiver(shm_mq *);
56 extern PGPROC *shm_mq_get_sender(shm_mq *);
57 
58 /* Set up backend-local queue state. */
59 extern shm_mq_handle *shm_mq_attach(shm_mq *mq, dsm_segment *seg,
60 									BackgroundWorkerHandle *handle);
61 
62 /* Associate worker handle with shm_mq. */
63 extern void shm_mq_set_handle(shm_mq_handle *, BackgroundWorkerHandle *);
64 
65 /* Break connection, release handle resources. */
66 extern void shm_mq_detach(shm_mq_handle *mqh);
67 
68 /* Get the shm_mq from handle. */
69 extern shm_mq *shm_mq_get_queue(shm_mq_handle *mqh);
70 
71 /* Send or receive messages. */
72 extern shm_mq_result shm_mq_send(shm_mq_handle *mqh,
73 								 Size nbytes, const void *data, bool nowait);
74 extern shm_mq_result shm_mq_sendv(shm_mq_handle *mqh,
75 								  shm_mq_iovec *iov, int iovcnt, bool nowait);
76 extern shm_mq_result shm_mq_receive(shm_mq_handle *mqh,
77 									Size *nbytesp, void **datap, bool nowait);
78 
79 /* Wait for our counterparty to attach to the queue. */
80 extern shm_mq_result shm_mq_wait_for_attach(shm_mq_handle *mqh);
81 
82 /* Smallest possible queue. */
83 extern PGDLLIMPORT const Size shm_mq_minimum_size;
84 
85 #endif							/* SHM_MQ_H */
86