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From: stan@drillsys (Stan Hanks)
Newsgroups: mod.computers.masscomp
Subject: Re: Shared memory examples sought
Message-ID: <939@soma.UUCP>
Date: Mon, 23-Jun-86 17:36:51 EDT
Article-I.D.: soma.939
Posted: Mon Jun 23 17:36:51 1986
Date-Received: Tue, 24-Jun-86 17:25:54 EDT
Sender: masscomp@soma.UUCP
Organization: Teleco Oilfield Services -- Drilling Systems Technology, Houston, Tx
Lines: 138
Approved: masscomp@soma.uucp
In-Reply-To: <923@soma.UUCP>




I recently finished a similar application for a positron emision 
scanner. I'm not convinced that this is the best way to solve the
problem, but it certainly is one way.

Being a casual reader of this news group (Stan Barber is a good friend
and I occasionally do joint work with him on their Masscomp) and not
normally running System V (I'm actually a Sun OEM), I can't well
include code and say "Here, this works". And as the previously mentioned
work is copyrighted and owned by the company who leased me to do it, 
I can't include it either. But, here is a scetch of how it goes. 
Holler if you need more detail; I'll get onto a System V machine and
pound out the details.

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

/* warning! good for 32 bit unsigned only !!!! */
#define BIS(flag, bit)	flag |= (1<<bit)
#define BIC(flag, bit)	flag &= ^(1<<bit)
#define BIT(flag, bit)  flag & (1<<bit)
#define INCMOD32(flag)	flag = (++flag % 32)

main()
{
	int flagd, bufd;		/* shared mem descriptors */
	char         *buf;		/* pointer to the buffer  */
	char         *buff[32];		/* sub-pointers to buffer */
	unsigned int *flag;		/* busy flags for buffer  */
	int i;

	/* allocate shared memory for 32-bit flag and 32 1K buffers */
	if ((flagd = shmget((key_t)0, 4, IPC_CREAT)) == -1) {
		printf("oops.\n");
		exit(1);
	}
	if ((bufd = shmget((key_t)1, 32*1024, IPC_CREAT)) == -1) {
		printf("oops.\n");
		exit(1);
	}

	/* attach the shared memory */
	if ((flag = (unsigned int *)shmat(flagd, 0, SHM_RND))) == -1) {
		printf("oops.\n");
		exit(1);
	}
	if ((buf = shmat(bufd, 0, SHM_RND))) == -1) {
		printf("oops.\n");
		exit(1);
	}

	/* clear the shared flags -- this is important!!! */
	*flag = 0;
	/* build array of 1K buffers */
	for (i=0; i<32; i++) buff[i] = buf + i*1024;

	/* pause to procreat */
	switch ((pid = fork()))) {
		case -1:
			printf("oops.\n");
			exit(1);
			break;
		case  0:
			do_child_stuff(flagd, bufd);
			break;
		default:
			break;
	}

	/* parent now stuffs things into shared memory */
	while (! done) {	 /* hey, what did you want for nothing??? */
		char sbuf[1024]; /* data that needs to be shared */
		int  which = 0;  /* which bucket are we using now?? */

		if (BIT(*flag, which)) {
			printf("Yow! We wrapped around. Abandon program\n");
			exit(1);
		}
		get_stuff_to_be_shared(sbuf);
		bcopy(sbuf, buff[which], 1024); /* bcopy(from, to, nbytes) */
		BIS(*flag, which);		/* set busy bit */
		INCMOD32(which);		/* move to next bucket */
	}
}

/* here we are in another process, waiting on this data to arrive */
do_child_stuff(xflagd, xbufd)
int xflagd, xbufd;
{
	char *xbuf;
	char *xbuff[32];
	unsigned int *xflag;
	int i;
	
	/* attach the shared memory */
	if ((xflag = (unsigned int *)shmat(xflagd, 0, SHM_RND))) == -1) {
		printf("oops.\n");
		exit(1);
	}
	if ((xbuf = shmat(xbufd, 0, SHM_RND))) == -1) {
		printf("oops.\n");
		exit(1);
	}
	/* build array of 1K buffers */
	for (i=0; i<32; i++) xbuff[i] = xbuf + i*1024;

	/* here we go.... */
	while (! done) {
		char rbuf[1024];	/* recieved data */
		int which = 0;		/* which buffer ? */

		/* we process in the same order as we fill, so */
		/* as to provide the same data ordering        */
		/* this also provides synchronization and      */
		/* eliminates the need for more serious mutex  */
		/* protocols. this of course changes if the    */
		/* data rates cause overruns                   */

		while(! BIT(*xflag, which)
			;			/* busy wait */
		bcopy(xbuff[which], rbuf, 1024);/* retrieve data */
		BIC(*xflag, which);		/* clear busy bit */
		INCMOD32(which);		/* look at next bucket */
		do_stuff_with_data(rbuf);
	}
}

=================================

Hope this helps.


Stan Hanks					...!drillsys!stan
Sr. Research Scientist				stan@rice.ARPA
Teleco Oilfield Services, Drilling Systems Technology
Houston TX					(713) 699-5594