System V shared memory and Ultrix ?

[Dan Ts'o]

In article <547 at rna.UUCP> dan at rna.UUCP (Dan Ts'o) writes:
>x
>	Has anyone used the System V-style shared memory facility in
>Ultrix-32m (Microvax II), especially in double-buffered I/O operations ?
>For example, the continuous DMA'ing in of data while DMA'ing out to
>magtape. one process DMA's into shared memory and then signals the second
>process for it to DMA's to magtape while the first process fills the second
>buffer.
>
>	Can someone mail me a simple C program in which to processes communicate
>using these System V shared memory calls ?  Please educate a poor 4.XBSD user
>to the wonders of shared memory. Thanks.
 
	Well, here I am answering my own question - sort of. I came up with a
simple shm*() program below which seems to work. However I'm still confused
about a number of the facilities of shm*(). For example, what role does "key"
play ? How do I pass the shared memory segment to non-related process ?
Perhaps what I should do (it doesn't seem to be documented) is stick a non
zero long in key and require that the communicating process shmget() with the
same key. Right ?
	Is the shared memory segment automatically destroyed if no process has
it "open" ? What constitutes the "open" - the shmget() or the shmat() ? If it
isn't destroyed then probably the shmat() is the "open" and the shmget() is
a lookup/create.
	Sorry if these questions are trivial but the documentation in Ultrix
seems scarce. I would still like to know if there are problems with using
these operations for the style of I/O mentioned above.


					Cheers,
					Dan Ts'o
					Dept. Neurobiology
					Rockefeller Univ.
					1230 York Ave.
					NY, NY 10021
					212-570-7671
					...cmcl2!rna!dan
					rna!dan at cmcl2.arpa


#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>

#define	NSHM	10

char *shmat();

main()
{
	key_t key;
	int size, shmflg;
	int shmid;
	int pid;
	register char *p;
	register int *v, i;

	size = NSHM * (sizeof *v);
	shmflg = IPC_CREAT + 0666;
	key = IPC_PRIVATE;
	shmid = shmget(key, size, shmflg);
	if (shmid < 0) {
		fprintf(stderr, "%d: Bad shmget(), ", shmid);
		perror("");
		exit(1);
	}
	pid = fork();
	if (pid == -1) {
		perror("Bad fork()");
		exit(1);
	}
	shmflg = SHM_RND;
	p = shmat(shmid, (char *)0, shmflg);
	if (p == (char *)-1) {
		perror("Bad shmat()");
		exit(1);
	}
	v = (int *)p;
	if (pid == 0) {
		for (i = 0; i < NSHM; i++)
			printf("%d ", v[i]);
		printf("\n");
		sleep(10);
		for (i = 0; i < NSHM; i++)
			printf("%d ", v[i]);
		printf("\n");
		exit(0);
	}
	sleep(3);
	for (i = 0; i < NSHM; i++)
		v[i] = i*i;
	sleep(10);
	exit(0);
}