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From: pthonda@bgsuvax.UUCP (Killer B)
Newsgroups: comp.unix.programmer
Subject: Shared Memory
Keywords: SHMGET
Message-ID: <7070@bgsuvax.UUCP>
Date: 1 Mar 91 00:36:27 GMT
Organization: Bowling Green State University B.G., Oh.
Lines: 105


The program  given below is running fine on SPARC station, however when I
tried to execute the same program on Vax 785 with Unix 4.3 BSD,Iam 
encountering the message "Shmget failed :Invalid argument".I tried to 
typecast the argumnets to call 'shmget' to the right types but the 
problem is still persistent. However after going through manuals I figured
out that 'shmget' gives this whenever the programs specifies a 
shared memory segment size which is less than or greater than the system 
imposed limits.Could anyone tell me how to get the system information 
about default allowable shared memory segment sizes.

Thanx in advance.

Pratap.

And here is the code..
/*=========================================================================*/

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

#define SHMKEY  (key_t)0x10	/* Establish a key for shared memory */
#define IPCFLAGS IPC_CREAT | IPC_EXCL 
#define RDWR  0666
#define ERROR 	-1

struct shmid_ds *buf ;

static int  shmId ;	
static int  prId;
long	*shmPtr ;

main(argc,argv)
int	argc ;
char	*argv[] ;
{
	char *shmat(), *malloc() ;
	int	retValue ;

	
	/* Get the shared memory segment */
	shmId = shmget(SHMKEY,sizeof(long),RDWR|IPCFLAGS) ;
	if(shmId < 0) {

/* It's failing here. *********/
		perror("shmget failed") ;
		exit(ERROR) ;
	}

/* Set the required permissions */
/*	buf = (struct shmid_ds *) malloc(sizeof(struct shmid_ds)) ;
	buf->shm_perm.cuid = getpid() ;
	buf->shm_perm.mode = 0600 ;
	if(shmctl(shmId,IPC_SET,buf) < 0) {
		perror("Shmctl failed");
		exit(ERROR) ;
	}
*/
	/* Attach the shared memory segment */
	shmPtr =  (long *) shmat(shmId,0,0) ;	
	if((char *) shmPtr  == (char *)-1) {
		perror("Shmat failed") ;
		exit(ERROR) ;
	}

/* Initialize to a value for testing ....*/
	*shmPtr = 120L ;
 
	if((prId = fork()) < 0) {
		perror("Fork failed") ;
		exit(ERROR) ;
	}


	if(prId == 0) 
		consumer() ;
	else
		producer() ;
	shmdt(shmPtr) ;
	shmctl(shmId,IPC_RMID,0) ;
}


int producer() 
{
	int i ;
	
	for(i = 0 ; i < 100 ; i++) {
		++*shmPtr  ;
		printf("Producer incremented. Current value is %ld\n",*shmPtr);
	}
}

int consumer()
{
	int k ;

	for(k = 0 ; k < 100 ; k++) {
		--*shmPtr ;
		printf("Consumer decremented. Current value is %ld\n",*shmPtr);
	}
}