Help about shared memory

[Andy Johnson]

In article <1739 at phoenix.Princeton.EDU>, asjoshi at phoenix.Princeton.EDU (Amit S. Joshi) writes:
> I wanted to share some data between two processes. I looked up the
> manual and found something called "shared memory". Could somebody tell
> me waht that is and how do I use it ?

I wrote a multi-user bbs and wanted to use shared memory to facilitate
communications between users, specifically for "chat".  I wrote this
simple program to test the shared memory facilities of Microport.  It
worked find and I have been using shared memory for over a year without
a problem.  Hope this helps.

Andy Johnson
{inhp4!codas!}!killer!ajohnson
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CUT HERE.......
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#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
 
#define MEMORY	0x62627321		/* string = "bbs!"  */
#define MAX_PRG 5
#define ERROR -1
 
typedef struct mpstr
{
    unsigned char	lst_in;
    unsigned char	in_recd;
    unsigned char	lst_ot;
    unsigned char	ot_recd;
    unsigned char	whoami[30];
    unsigned char	who_chat;
    unsigned char	on_line;
} MEMPRG;
 
typedef struct memstr
{
    int		wrtflg;
    MEMPRG	memprg[MAX_PRG];
} MEMSTR;
 
char name[80] ="THIS IS A STRING";
 
main(argc, argv)
int argc;
char **argv[];
{
    MEMSTR *memptr;
    char *memget();
    int pgmno;
 
    pgmno = 0;
    if (argc >1)
	pgmno = 1;
 
    printf("\nProgram = %d\n",pgmno);
    printf("\nName = %s\n",name);
    memptr = (MEMSTR*)memget();
    memptr->memprg[pgmno].on_line = 1;
    /* strcpy(memptr->memprg[pgmno].whoami,"USER XXX");*/
    printf("\nput it into memory!\n");
    printf("\nid=%s\n",memptr->memprg[0].whoami);
    printf("\nid=%s\n",memptr->memprg[1].whoami);
    printf("\nName = %s\n",name);
 
    remmem(memptr);
    printf("\nAll done!\n");
    exit(0);
}
 
 
char *memget()
{
    char *shmat();
    char *memadr;
    int mid;
 
    mid = shmget(MEMORY,sizeof(MEMSTR),(IPC_CREAT | 0666));
    if (mid == ERROR)
    {
	printf("\nError getting memory errno = %d\n",errno);
	exit(1);
    }
    printf("\nmid = %d\n",mid);
 
    memadr = shmat(mid,(char*)0,0);
    if (memadr == -1L)
    {
	printf("\nAttach failed %d\n",errno);
	exit(1);
    }
    printf("\nMemory attached at %lx\n",memadr);
 
    return(memadr);
 
}
 
 
remmem(memadr)
char *memadr;
{
    int result;
 
    result = shmdt(memadr);
    if (result == ERROR)
    {
	printf("\nCan't detach memory error %d\n",errno);
	exit(1);
    }
}