Shared Memory Problem

[Bruce Holloway]

I'm having a problem with the shared memory routines under System V, and
I wonder if any gurus out there could help me....

I'm trying to create a shared memory segment, and put its base address into
a structure pointer. Everything seems fine until I actually try to use
the memory, when I get a core dump.

What am I doing wrong? (My code included).

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

#define	FALSE	0
#define	TRUE	1

#define	WAITING	1
#define	GOING	2

#define	MES	struct _mes
MES{
    int mes_flags;
    char mes_mes[80];
} *mes;

extern int errno;

extern char *shmat();

main(acnt,avar)
int acnt;
char *avar[];
{
    extern key_t ftok();
    key_t key;
    int shmid, fatal;
    char *mm;

    printf("Calling path is: %s\n",avar[0]);
    key = ftok(avar[0],'B');
    if(key == (key_t)-1){ printf("Couldn't make key\n"); return; }
    else printf("Key is %ld\n",(long)key);

    shmid = shmget(key, sizeof(MES), IPC_CREAT | IPC_EXCL);

    if(shmid<0){
	fatal = FALSE;
	switch(errno){
	    case EINVAL:
		mm = "Size is out of legal bounds";
		fatal = TRUE;
		break;
	    case EACCES:
		mm = "Access not granted";
		fatal = TRUE;
		break;
	    case ENOENT:
		mm = "Not creating a shmid, and one does not exist";
		fatal = TRUE;
		break;
	    case ENOSPC:
		mm = "Too many shmids already.";
		fatal = TRUE;
		break;
	    case ENOMEM:
		mm = "Not enough memory to fill request.";
		fatal = TRUE;
		break;
	    case EEXIST:
		mm = "A shmid already exists. We are not first.";
		break;
	    default:
		mm = "Undefined error code.";
		fatal = TRUE;
		break;
	    }
	printf("ERROR(%d): %s\n",errno,mm);
	if(fatal) return;
	shmid = shmget(key, sizeof(MES), IPC_CREAT);
	receive(shmid);
	return;
	}
    send(shmid);
}

send(shmid)
int shmid;
{
    int mesnum = 1;

    printf("We are first. Sending first message.\n\n");

    mes = (MES *)shmat(shmid, (char *)0, 0);
    if(!mes){ printf("SHMAT failed!\n"); return; }

/* Core is dumped on next statement */

    mes->mes_flags = 0;

    putchar('!');

    while(1){
	sprintf(mes->mes_mes, "Message #%d!\n", mesnum++);
	mes->mes_flags |= WAITING;
	while(mes->mes_flags & WAITING);
	}
}

receive(shmid)
int shmid;
{
    printf("We are second. Waiting for messages.\n\n");

    mes = (MES *)shmat(shmid, (char *)0, 0);
    if(!mes){ printf("SHMAT failed!\n"); return; }

    while(1){

/* Core is dumped on this statement. */

	while(!(mes->mes_flags & WAITING));

	printf(mes->mes_mes);
	mes->mes_flags &= ~WAITING;
	}
}

-- 

+----------------------------------------------------------------------------+
|Whatever I write are not the opinions or policies of Digital Research, Inc.,|
|and probably won't be in the foreseeable future.                            |
+----------------------------------------------------------------------------+

Bruce Holloway

....!ucbvax!hplabs!amdahl!drivax!holloway
(I'm not THAT Bruce Holloway, I'm the other one.)