Shared memory communication program

pratham29 · 11-29-2013, 02:51 PM

Hello,
How to write a code for communication between two programs eg:A program "Read.c" takes input from user and other program "Write.c" prints the characters.
How to execute these in Ubuntu.
Thanks for your time and consideration.

Regards,
Pratham

NevemTeve · 11-30-2013, 01:29 AM

That's what named pipes are good for.

pratham29 · 12-01-2013, 01:46 PM

Code:

//SHMServer.C
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
 #define MAXSIZE     27
 
void die(char *s)
{  perror(s);
   exit(1);}
 
int main()
{   char c;
    int shmid;
    key_t key;
    char *shm, *s;
    key = 5678;
    if ((shmid = shmget(key, MAXSIZE, IPC_CREAT | 0666)) < 0)
    die("shmget");
    if ((shm = shmat(shmid, NULL, 0)) == (char *) -1)
    die("shmat");
    s = shm;                                       /* Put some things into the memory for the  other process to read.  */
    for (c = 'a'; c <= 'z'; c++)
    *s++ = c;
   
 /* Wait for process  changes the 1st character of  memory to '*', indicating that it has read what  we put there.  */
   
 while (*shm != '*')
        sleep(1);
        exit(0);
}

SHMClient.C

Code:

 
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#define MAXSIZE     27
 void die(char *s)
{   perror(s);
    exit(1);}
 
int main()
{
    int shmid;
    key_t key;
    char *shm, *s;
    key = 5678;
    if ((shmid = shmget(key, MAXSIZE, 0666)) < 0)
    die("shmget");
    if ((shm = shmat(shmid, NULL, 0)) == (char *) -1)
    die("shmat");
    /*Read what the server.c  put in the memory.
    for (s = shm; *s != '\0'; s++)
    putchar(*s);
    putchar('\n');

    /*Change the first character of the segment to '*', indicating we have read the segment.  */
   
 *shm = '*';
   exit(0);}

I am getting an error with the above code, as "command not found " when I enter alphabets in server.c program.
Kindly help me with this. Thanks for time and consideration,awaiting response.
Regards,
Pratham

dwhitney67 · 12-01-2013, 03:31 PM

I'm not sure how you were able to compile the client source code; you are missing the comment terminator on the following line:

Code:

/*Read what the server.c  put in the memory.

Btw, your server should detach from [using shmdt()], and remove [using shmctl()] the shared-memory segment, just before it completes.

And a few other miscellaneous comments...

1) Don't assume (from within the client) that a termination NULL character is at the end of your data stream (unless you specifically put it there).

2) You are missing the include file for <unistd.h> for the usage of sleep() in your server code.

3) Use proper indentation in your code.

NevemTeve · 12-01-2013, 11:37 PM

You don't need shared memories for your problem, see my previous answer. But, you have to learn how to debug your program. Most likely it had terminated by the time you 'entered alphabets'

sundialsvcs · 12-02-2013, 01:48 PM

I'll quickly second the notion that ... "this is what named-pipes, or FIFOs or any one of several well-understood existing IPC-mechanisms in Unix/Linux are for."

(And, gosh, if you peek under the covers of their already fully-debugged implementations, you find things like shared memory segments being used ... by somebody else's code.)

"Never imagine that you can actually do better by doing things yourself. You usually can't. Most often, you're just wasting time and (somebody else's) money. Choose your battles."

Spect73 · 12-03-2013, 02:28 PM

Quote:

Originally Posted by pratham29

Hello,
How to write a code for communication between two programs eg:A program "Read.c" takes input from user and other program "Write.c" prints the characters.
How to execute these in Ubuntu.
Thanks for your time and consideration.

Regards,
Pratham

"Advanced Programming in the UNIX Environment, 2nd Edition" by W. Richard Stevens and Stephen A. Rago. Best money I've ever spent on programming.

Coordially,